Drag Bike  Plan B
Moderator: rztom
Drag Bike  Plan B
Last summer I took up drag racing with an RZ that I’ve owned for over 30 years.
That was Drag Bike Plan A. Run what you brung.
I did make a couple of small changes along the way.
Installed a shift light and a set of clubman bars. Calling that Plan A.1.
I want to race an RZ again next year, but I want to be more competitive, (meaning, I want to win).
In order to win races, I need consistent E.T.’s.
Which means that I need consistent launches and 60’ times.
Last season, my launches were were all over the place.
Not enough throttle and the bike would bog for a fraction of as second, which sucked, because races are measured in thousands of a second.
Too much throttle and the bike would sky wheelie, which also sucked.
First off becuase I had to back off the throttle, and intentionally slowing down the bike is no way to win the race.
And secondly, having the front wheel higher than my helmet, is freaking terrifying.
So, over the Winter, I’m going to build myself a new RZ350 that will be used strictly for drag racing. Calling it Plan B.
I going to make it light, and I’ll stretch it out, and then slam it down to earth so that it only has 1” of ground clearance.
(actually 2” is min. ground clearance according to the rules, but I’ll be lucky if I can get the pipes 4” from the ground)
Making the bike lighter would be pretty easy. Drag bikes don’t need:
Turn signals or signal relays
Horns
Ignition switch/steering lock / and key
Passenger grab rails
Speedo and speedo cable
Dash/idiot light cluster
Plastic rad cover
Centerstand
2 front brake rotors and calipers, 1 rotor and caliper (on the left side) should be enough.
That’s a lot of stuff. Maybe 15 or 20 pounds. I don’t know.
Making the bike longer is also not very difficult, because I know a welder.
For $100 or $150 I can have a swingarm modified so that it’s 6” longer, and will be wide enough to run my spare 4” FZR400 rear wheel with a 150/7018 rear tire.
Lowering the bike should also be a relatively straight forward job.
Longer dogbones or a shorter rear shock will lower the back of the bike.
Raising the forks up in the triple trees will lower the front end of the bike.
You’re done.
Unless you’re some kind of fool who likes to overcomplicate things. (more on this subject later)
So, I have a new plan. What I don’t have is an abundance of money.
As the saying goes: How do you make a small fortune drag racing? You start with a large one.
Fortunately, I do have a pretty decent collection of spare parts.
 complete, running motor, carbs, Toomey pipes
 CDI, PV controller, wiring harness, coil, all the electronics
 seat, side panels, tail cowl, fenders, gauges, switchgear
 front forks, wheels, rotors, brakes, swingarm
 boxes full of misc, little parts
 several tins full of nuts, bolts, and hardware
What I don’t have is a 83/84frame, 83/84 top triple tree, 83/84 taillight bracket, and a set of 86+ footpeg brackets.
Been talking to Rory. Really need to go visit him.
That was Drag Bike Plan A. Run what you brung.
I did make a couple of small changes along the way.
Installed a shift light and a set of clubman bars. Calling that Plan A.1.
I want to race an RZ again next year, but I want to be more competitive, (meaning, I want to win).
In order to win races, I need consistent E.T.’s.
Which means that I need consistent launches and 60’ times.
Last season, my launches were were all over the place.
Not enough throttle and the bike would bog for a fraction of as second, which sucked, because races are measured in thousands of a second.
Too much throttle and the bike would sky wheelie, which also sucked.
First off becuase I had to back off the throttle, and intentionally slowing down the bike is no way to win the race.
And secondly, having the front wheel higher than my helmet, is freaking terrifying.
So, over the Winter, I’m going to build myself a new RZ350 that will be used strictly for drag racing. Calling it Plan B.
I going to make it light, and I’ll stretch it out, and then slam it down to earth so that it only has 1” of ground clearance.
(actually 2” is min. ground clearance according to the rules, but I’ll be lucky if I can get the pipes 4” from the ground)
Making the bike lighter would be pretty easy. Drag bikes don’t need:
Turn signals or signal relays
Horns
Ignition switch/steering lock / and key
Passenger grab rails
Speedo and speedo cable
Dash/idiot light cluster
Plastic rad cover
Centerstand
2 front brake rotors and calipers, 1 rotor and caliper (on the left side) should be enough.
That’s a lot of stuff. Maybe 15 or 20 pounds. I don’t know.
Making the bike longer is also not very difficult, because I know a welder.
For $100 or $150 I can have a swingarm modified so that it’s 6” longer, and will be wide enough to run my spare 4” FZR400 rear wheel with a 150/7018 rear tire.
Lowering the bike should also be a relatively straight forward job.
Longer dogbones or a shorter rear shock will lower the back of the bike.
Raising the forks up in the triple trees will lower the front end of the bike.
You’re done.
Unless you’re some kind of fool who likes to overcomplicate things. (more on this subject later)
So, I have a new plan. What I don’t have is an abundance of money.
As the saying goes: How do you make a small fortune drag racing? You start with a large one.
Fortunately, I do have a pretty decent collection of spare parts.
 complete, running motor, carbs, Toomey pipes
 CDI, PV controller, wiring harness, coil, all the electronics
 seat, side panels, tail cowl, fenders, gauges, switchgear
 front forks, wheels, rotors, brakes, swingarm
 boxes full of misc, little parts
 several tins full of nuts, bolts, and hardware
What I don’t have is a 83/84frame, 83/84 top triple tree, 83/84 taillight bracket, and a set of 86+ footpeg brackets.
Been talking to Rory. Really need to go visit him.
Last edited by Off Road on Thu Dec 13, 2018 1:40 am, edited 1 time in total.
 Questo vecchio rz
 Posts: 854
 Joined: Sun Aug 16, 2015 5:55 pm
 Location: Reconquista of California
Re: Drag Bike  Plan B
Sounds like fun to me!
When you get to the point of more power, I would heavily consider utilizing a 47mm crank along with a
1.) (Banshee) drag ported top end.
OR
2.)drag ported CPI Cheetah cylinder.
3.) Really any brand Banshee top end set up ..CPIs Serval,Super Servals,Super Cubs, Wumpus,TRex. Twister etc. Can be had reasonable from various classifieds.
Either of these done with care will lead you from the approx 60 h.p. to a honest 80100,+h.p.
Search Banshee sites for drag motor cylinders ....
Stock 68mm Yamaha Banshee cylinders with drag port easily can net you 80+ h.p. This option may really the most cost effective Motor build you can do! , you might even be able to modify & fit side exit Banshee Drag pipes too! Never tried it... but seems feasible. If that ain't enough.. Spice it up w with Alky...
Your Toomeys are a decent budget top end pipe..for now.
You can go the route you doing 1st and put together a motor during season next year. Then come back & Surprise folks with a 10 second  ish RZ...
I'm good at spending other people's money.. lol
When you get to the point of more power, I would heavily consider utilizing a 47mm crank along with a
1.) (Banshee) drag ported top end.
OR
2.)drag ported CPI Cheetah cylinder.
3.) Really any brand Banshee top end set up ..CPIs Serval,Super Servals,Super Cubs, Wumpus,TRex. Twister etc. Can be had reasonable from various classifieds.
Either of these done with care will lead you from the approx 60 h.p. to a honest 80100,+h.p.
Search Banshee sites for drag motor cylinders ....
Stock 68mm Yamaha Banshee cylinders with drag port easily can net you 80+ h.p. This option may really the most cost effective Motor build you can do! , you might even be able to modify & fit side exit Banshee Drag pipes too! Never tried it... but seems feasible. If that ain't enough.. Spice it up w with Alky...
Your Toomeys are a decent budget top end pipe..for now.
You can go the route you doing 1st and put together a motor during season next year. Then come back & Surprise folks with a 10 second  ish RZ...
I'm good at spending other people's money.. lol
Banshee (Baja) race bike, Lonestar AArms, dual 9" L.E.Ds, FMF, Toomey,19cc domes, IMS tank, run flats.
96 GSXR SRAD, Future Yoshimura Lucky Strike rep.
85 custom TriZ
RZ/YZR track bike(project)
86 VFR750 RC24 Merkel replica (project)
96 GSXR SRAD, Future Yoshimura Lucky Strike rep.
85 custom TriZ
RZ/YZR track bike(project)
86 VFR750 RC24 Merkel replica (project)
Re: Drag Bike  Plan B
And this fool had some Questions:
What’s the overall effect on the bike?
What are the new rake, trail and wheelbase numbers?
How much lower can the bike get?
I decided that I just had to know, so I got a pen and a piece of paper, opened up an Excel spreadsheet, and got to work.
Before I get into it, I have a quick word to the wise. This is a long ass series of posts.
If you have a bit of an interest in motorcycle geometry, then you might find the following posts mildly interesting.
If you’re not all that interested in the subject, then you’ll probably find that the next few posts will get real boring, real fast.
I wrote the following posts over the course of several days (weeks?).
I spent an hour here and half an hour there and before I knew it, I had written a 7 page novel outlining my journey, and it still wasn’t finished.
I reread what I’d written, and did a bit of editing, but sadly, it’s still a 7 page novel.
I discovered that I kept repeating myself, saying the same thing over and over.
In the end, I decided to pretty much leave it as it was, for a few of reasons.
 If you not very interested in rake and trail, you probably won’t read the post and it won’t matter whether it’s long or short.
 I want to provide a decent amount of detail, so that the people who are interested will have a clear understanding of what is going on.
When the bike is rotating around the front axle or the rear axle, and you’re trying to alter dimensions or lengths or angles, it can get real confusing, real fast. (ask me how I know)
Hopefully, repeating the steps that I was following will help to keep things clear.
 I’ve never done this before, and while it seems straightforward, there’s always the possibility that I’ve made some incorrect assumptions.
And if I have made some mistakes, hopefully the smart math guys can point out where I went wrong.
Again, sorry for the length of this puppy, but like they say, Why write a Sentence when an essay will do.
Re: Drag Bike  Plan B
I started out by knowing the following information about the RZ.
Rake – Factory Spec
Wheelbase – Factory Spec
Fork Length – Measured
Triple Tree Offset – Measured
Fr. Tire Diameter – Online info
Rr. Tire Diameter – Online info
Swingarm Length – Measured
Swingarm Angle – Calculated (from an earlier spreadsheet)
Being the genius that I am, I started by looking at how changing the swingarm angle affects the steering geometry and height of the bike.
I looked at what happens when the swingarm angle changes from 12* (fully extended), to 0* (horizontal to the ground), and started making some calculations.
 find the change in wheelbase
 find how high the rear axle moves up
 find the angle between front axle and rear axle ,and horizontal
 rotate the bike (so that the front and rear Axles are horizontal) and find the new rake
 find the new wheelbase again
 find the vertical height between front axle and top of the fork tubes
 find distance between rear axle and top of fork tubes
 find height at the steering neck
 find distance between rear axle and neck
By this time I had a really cumbersome 30 column spreadsheet that gave me the same information that I can get from the RB Design rake and trail calculator that’s online.
And it still didn’t give me the information that I wanted to know.
Well, it gave me some of the info that I wanted, but only at 2 specific points.
The swingarm at 12* and the swingarm at 0*.
It didn’t allow me to change any variables (fork length, offset, wheels).
The calculations were good. The layout of the spreadsheet sucked. And it sucked big time.
Way to go, genius.
I needed a break.
Well, it was Late in The Evening, and even though I never learned to play guitar, I have been underage in a funky bar, so I went outside to…
Rake – Factory Spec
Wheelbase – Factory Spec
Fork Length – Measured
Triple Tree Offset – Measured
Fr. Tire Diameter – Online info
Rr. Tire Diameter – Online info
Swingarm Length – Measured
Swingarm Angle – Calculated (from an earlier spreadsheet)
Being the genius that I am, I started by looking at how changing the swingarm angle affects the steering geometry and height of the bike.
I looked at what happens when the swingarm angle changes from 12* (fully extended), to 0* (horizontal to the ground), and started making some calculations.
 find the change in wheelbase
 find how high the rear axle moves up
 find the angle between front axle and rear axle ,and horizontal
 rotate the bike (so that the front and rear Axles are horizontal) and find the new rake
 find the new wheelbase again
 find the vertical height between front axle and top of the fork tubes
 find distance between rear axle and top of fork tubes
 find height at the steering neck
 find distance between rear axle and neck
By this time I had a really cumbersome 30 column spreadsheet that gave me the same information that I can get from the RB Design rake and trail calculator that’s online.
And it still didn’t give me the information that I wanted to know.
Well, it gave me some of the info that I wanted, but only at 2 specific points.
The swingarm at 12* and the swingarm at 0*.
It didn’t allow me to change any variables (fork length, offset, wheels).
The calculations were good. The layout of the spreadsheet sucked. And it sucked big time.
Way to go, genius.
I needed a break.
Well, it was Late in The Evening, and even though I never learned to play guitar, I have been underage in a funky bar, so I went outside to…
Re: Drag Bike  Plan B
…and when I came back to the room, I started over at the beginning, with a new sheet of paper.
For the purpose of my analysis, the motorcycle starts out as a drawing on a piece of paper, in the shape of a triangle.
The 3 corners of the triangle are the front axle, the rear axle, and the top of the fork tubes.
A line drawn between the front and rear axles is horizontal (x axis).
After that, it’s just math.
 Since I know the Wheelbase, Rake and Fork Length, I can find the vertical height between the top of the fork tube and the axles.
 Then I can find the distance between the rear axle and the fork tube.
 I know the offset and can find the height between the top of the neck and the axles.
 With this info I can find the distance between the rear axle and the neck.
 Since I have the tire diameters, I can find the height of the neck from ground.
 And finally I calculated the trail. Which I could have done 1st, because all you need is Rake, Offset, and Front Tire Diameter.
So, a couple of hours, and 17 calculations, later, I had a spreadsheet that gave me the following info:
 Distance between the Rear Axle and the Neck
 Height of the Neck above ground
 Trail
After I put in the values for:
Rake
Wheelbase
Fork Length
Triple Tree Offset
Fr. Tire Diameter
Rr. Tire Diameter
Swingarm Length
Swingarm Angle
Once again, the same information that I can get online. However, I now have 2 pretty important pieces of info.
I know the distance between the Rear Axle and the Fork Neck.
This is now a “constant” in any further calculations.
This distance won’t change when you change forks, wheels or trees.
It will only change when there are changes made to the swingarm.
I also know the Height at the Neck, which is the whole point of this exercise.
Now that I had a spreadsheet that told me the neck height, I was ready to start changing some of the variables, to see how they affected the height.
I decided to start by changing the fork length because I could verify my results using the online calculators.
Ummm…Ummm…Ummm.
I originally started my spreadsheet knowing 3 things: Rake, Wheelbase and Fork Length.
Hell, Rake and Wheelbase are columns #1 and #2 in a 20 column spreadsheet.
All my other calculations are based on knowing the rake and wheelbase.
When the forks get longer, (or shorter), the rake and wheelbase both change.
The only 2 constants that I have left are the fork length and the distance between the rear axle and the neck.
And that’s not enough information to solve the problem.
Ummm…Ummm…Ummm.
By this time, it was really late in the evening, and…
For the purpose of my analysis, the motorcycle starts out as a drawing on a piece of paper, in the shape of a triangle.
The 3 corners of the triangle are the front axle, the rear axle, and the top of the fork tubes.
A line drawn between the front and rear axles is horizontal (x axis).
After that, it’s just math.
 Since I know the Wheelbase, Rake and Fork Length, I can find the vertical height between the top of the fork tube and the axles.
 Then I can find the distance between the rear axle and the fork tube.
 I know the offset and can find the height between the top of the neck and the axles.
 With this info I can find the distance between the rear axle and the neck.
 Since I have the tire diameters, I can find the height of the neck from ground.
 And finally I calculated the trail. Which I could have done 1st, because all you need is Rake, Offset, and Front Tire Diameter.
So, a couple of hours, and 17 calculations, later, I had a spreadsheet that gave me the following info:
 Distance between the Rear Axle and the Neck
 Height of the Neck above ground
 Trail
After I put in the values for:
Rake
Wheelbase
Fork Length
Triple Tree Offset
Fr. Tire Diameter
Rr. Tire Diameter
Swingarm Length
Swingarm Angle
Once again, the same information that I can get online. However, I now have 2 pretty important pieces of info.
I know the distance between the Rear Axle and the Fork Neck.
This is now a “constant” in any further calculations.
This distance won’t change when you change forks, wheels or trees.
It will only change when there are changes made to the swingarm.
I also know the Height at the Neck, which is the whole point of this exercise.
Now that I had a spreadsheet that told me the neck height, I was ready to start changing some of the variables, to see how they affected the height.
I decided to start by changing the fork length because I could verify my results using the online calculators.
Ummm…Ummm…Ummm.
I originally started my spreadsheet knowing 3 things: Rake, Wheelbase and Fork Length.
Hell, Rake and Wheelbase are columns #1 and #2 in a 20 column spreadsheet.
All my other calculations are based on knowing the rake and wheelbase.
When the forks get longer, (or shorter), the rake and wheelbase both change.
The only 2 constants that I have left are the fork length and the distance between the rear axle and the neck.
And that’s not enough information to solve the problem.
Ummm…Ummm…Ummm.
By this time, it was really late in the evening, and…
Re: Drag Bike  Plan B
…when I returned, I took a different approach to the bike / (triangle on a piece of paper).
I assumed that the Rear Axle and the Neck are fixed so that they can’t move.
If the forks are made longer, then the front axle will have to move forward and down.
Since the neck didn’t move, then the rake didn’t change, and the axle will have traveled in the same direction as the rake angle.
Bingo.
With this info I can calculate how far down, and how far forward the axle will move.
After that it was pretty straightforward to find the new wheelbase (straight line distance between the rear axle and the new location of the font axle), and new rake (the old rake +/ the angle between the new axle location and the old axle location).
Once I had my new Rake, Wheelbase and Fork Length numbers, the spreadsheet calculated the new neck height and trail.
Finally, a bit of progress.
Then I looked at what happens when you change the neck rake of the frame.
Calculated how far forward and up the front axle moves when the rake is increased.
Found the angle that it moved, found the new wheelbase, the overall rake, and the new neck height of the bike.
After that I evaluated what happens when you change the offset in the trees.
For some reason, I had a really hard time visualizing what happened when the offset was changed.
I was looking at the trees and the total offset from the neck and it wasn’t making a lot of sense.
Eventually, the light came on and I realized that the axle moves in the same direction, and by the same amount, as the top of the fork tube. Duh.
Then it was easy enough to find the new values for rake, wheelbase and neck height.
Next on the list was changing the Front Tire Diameter.
When the front tire gets smaller, the front axle moves down and back.
I know how far down it moves, and I know the original wheelbase.
With this info I can find the new rake, trail, wheelbase, and neck height.
Piece of cake. After spending a week with this spreadsheet, it was starting to get easier.
Moved on to changing the Rear Tire Diameter.
Ummm…Ummm…Ummm.
I assumed that the Rear Axle and the Neck are fixed so that they can’t move.
If the forks are made longer, then the front axle will have to move forward and down.
Since the neck didn’t move, then the rake didn’t change, and the axle will have traveled in the same direction as the rake angle.
Bingo.
With this info I can calculate how far down, and how far forward the axle will move.
After that it was pretty straightforward to find the new wheelbase (straight line distance between the rear axle and the new location of the font axle), and new rake (the old rake +/ the angle between the new axle location and the old axle location).
Once I had my new Rake, Wheelbase and Fork Length numbers, the spreadsheet calculated the new neck height and trail.
Finally, a bit of progress.
Then I looked at what happens when you change the neck rake of the frame.
Calculated how far forward and up the front axle moves when the rake is increased.
Found the angle that it moved, found the new wheelbase, the overall rake, and the new neck height of the bike.
After that I evaluated what happens when you change the offset in the trees.
For some reason, I had a really hard time visualizing what happened when the offset was changed.
I was looking at the trees and the total offset from the neck and it wasn’t making a lot of sense.
Eventually, the light came on and I realized that the axle moves in the same direction, and by the same amount, as the top of the fork tube. Duh.
Then it was easy enough to find the new values for rake, wheelbase and neck height.
Next on the list was changing the Front Tire Diameter.
When the front tire gets smaller, the front axle moves down and back.
I know how far down it moves, and I know the original wheelbase.
With this info I can find the new rake, trail, wheelbase, and neck height.
Piece of cake. After spending a week with this spreadsheet, it was starting to get easier.
Moved on to changing the Rear Tire Diameter.
Ummm…Ummm…Ummm.
Re: Drag Bike  Plan B
OK. I got me a problem.
My spreadsheet is set up so that the Rear Axle, and the Neck, are fixed points. They don’t move.
So, if you lengthen the fork tubes by 8”, the front axle is going to move down and forward.
If you rake the neck from the stock 26* to 45*, the front axle will move up and forward.
If you add 6” of offset to your triple trees, then again, the front axle will move up and forward.
In all these cases, I can draw a line from the rear axle, to the New location of the front axle.
I can find the angle between this line and horizontal. This angle is the amount that the rake will change.
And I can find the straight line distance between the front and rear axles This distance is the new wheelbase/
Pretty straightforward.
Now, we come back to changing tire size.
It’s easy enough to find out how far the front axle moves when you change out your 21” front rim for a 12” wheel off of a Honda Grom.
From there, it’s no problem to find the angle between the new front axle location and horizontal.
That angle is the amount that your rake is going to increase.
Finding the new wheelbase is a different story.
When I change Forks, Trees or Neck Rake, the new wheelbase is always the straight line distance between the front and rear axles because the bike rotates when it’s put back on the ground, and both axles are horizontal.
But, when the wheel size changes, the axles are no longer horizontal, and the wheelbase is not the straight line distance between the axles, it’s a bit shorter and requires different formulas to calculate it.
OK, I can do that.
Then I had my spreadsheet working so that when I changed front wheel size, it would give me new values for Rake and Wheelbase.
These values were then put into the section of the spreadsheet that calculates; height at the neck, distance from rear axle to neck, and trail.
And this is where it all falls apart.
When I’m calculating neck height, I draw a vertical line down through the neck until it intersects the horizontal line running between the axles.
These 2 lines MUST intersect a 90* or the trig formulas that I’m using to calculate my numbers, don’t work.
When the tires are a different size, a line drawn between the axles is not horizontal, and it does not intersect a vertical line at 90*, and the spreadsheet falls apart.
Now this information did not come as a surprise.
I knew from the start that the front and rear tires were a different diameter and that the axles were not horizontal.
But, in my infinite wisdom, I chose to ignore this information.
I figured that I could solve this problem at a later time, and then insert the corrections back into the beginning of the spreadsheet.
(Since the calculations from my spreadsheet and the Online rake and trail calculators are the same, it looks like the online sites also ignore the fact that the axles are not horizontal)
I spent a few hours on this little anomaly and then I had a revelation. F*ck it. Move on. Do something else.
Great idea, and since it was late in the evening…
My spreadsheet is set up so that the Rear Axle, and the Neck, are fixed points. They don’t move.
So, if you lengthen the fork tubes by 8”, the front axle is going to move down and forward.
If you rake the neck from the stock 26* to 45*, the front axle will move up and forward.
If you add 6” of offset to your triple trees, then again, the front axle will move up and forward.
In all these cases, I can draw a line from the rear axle, to the New location of the front axle.
I can find the angle between this line and horizontal. This angle is the amount that the rake will change.
And I can find the straight line distance between the front and rear axles This distance is the new wheelbase/
Pretty straightforward.
Now, we come back to changing tire size.
It’s easy enough to find out how far the front axle moves when you change out your 21” front rim for a 12” wheel off of a Honda Grom.
From there, it’s no problem to find the angle between the new front axle location and horizontal.
That angle is the amount that your rake is going to increase.
Finding the new wheelbase is a different story.
When I change Forks, Trees or Neck Rake, the new wheelbase is always the straight line distance between the front and rear axles because the bike rotates when it’s put back on the ground, and both axles are horizontal.
But, when the wheel size changes, the axles are no longer horizontal, and the wheelbase is not the straight line distance between the axles, it’s a bit shorter and requires different formulas to calculate it.
OK, I can do that.
Then I had my spreadsheet working so that when I changed front wheel size, it would give me new values for Rake and Wheelbase.
These values were then put into the section of the spreadsheet that calculates; height at the neck, distance from rear axle to neck, and trail.
And this is where it all falls apart.
When I’m calculating neck height, I draw a vertical line down through the neck until it intersects the horizontal line running between the axles.
These 2 lines MUST intersect a 90* or the trig formulas that I’m using to calculate my numbers, don’t work.
When the tires are a different size, a line drawn between the axles is not horizontal, and it does not intersect a vertical line at 90*, and the spreadsheet falls apart.
Now this information did not come as a surprise.
I knew from the start that the front and rear tires were a different diameter and that the axles were not horizontal.
But, in my infinite wisdom, I chose to ignore this information.
I figured that I could solve this problem at a later time, and then insert the corrections back into the beginning of the spreadsheet.
(Since the calculations from my spreadsheet and the Online rake and trail calculators are the same, it looks like the online sites also ignore the fact that the axles are not horizontal)
I spent a few hours on this little anomaly and then I had a revelation. F*ck it. Move on. Do something else.
Great idea, and since it was late in the evening…
Re: Drag Bike  Plan B
Changes to the swingarm.
For this exercise, I again started with my triangle (front axle, rear axle, and fork tubes), but I also drew in a swingarm (of know length and angle)
This time I assume that the Front Axle and the Neck are fixed, and only the rear axle can move.
If I lengthen the swingarm by 1foot, the rear axle will move down and back.
Draw a line between the front axle and the new location of the rear axle.
Calculate the angle between this line and horizontal. This angle is how much the rake will decrease.
Calculate the distance between the front and rear axles. This is the new wheelbase.
When I change the swingarm angle from 12* to 0*, the rear axle will move up and back.
Draw a line between the front axle and the new location of the rear axle.
Calculate the angle between this line and horizontal. This angle is how much the rake will increase.
Calculate the distance between the front and rear axles. This is the new wheelbase.
When I first started looking at making changes to the swingarm angle, I had another one of those Duh moments.
The Swingarm Angle Always Changes. If the Rake changes, then the Swingarm angle changes.
Had to go back and rework the spreadsheet, before I could continue.
For this exercise, I again started with my triangle (front axle, rear axle, and fork tubes), but I also drew in a swingarm (of know length and angle)
This time I assume that the Front Axle and the Neck are fixed, and only the rear axle can move.
If I lengthen the swingarm by 1foot, the rear axle will move down and back.
Draw a line between the front axle and the new location of the rear axle.
Calculate the angle between this line and horizontal. This angle is how much the rake will decrease.
Calculate the distance between the front and rear axles. This is the new wheelbase.
When I change the swingarm angle from 12* to 0*, the rear axle will move up and back.
Draw a line between the front axle and the new location of the rear axle.
Calculate the angle between this line and horizontal. This angle is how much the rake will increase.
Calculate the distance between the front and rear axles. This is the new wheelbase.
When I first started looking at making changes to the swingarm angle, I had another one of those Duh moments.
The Swingarm Angle Always Changes. If the Rake changes, then the Swingarm angle changes.
Had to go back and rework the spreadsheet, before I could continue.
Re: Drag Bike  Plan B
So, my spreadsheet is getting better.
I can now put the stock specs for any bike into my spreadsheet, and then I can change;
 neck rake
 fork length
 triple tree offset
 swingarm length
 swingarm angle
The spreadsheet will then calculate the following;
 rake
 trail
 wheelbase
 height at the neck
 height at the fork tubes
 rear axle to neck length
For example – Stock RZ
Rake = 26*
Wheelbase = 54.5”
Fork length = 28.25”
Offset = 1.94”
Rear tire Dia. = 24.37”
Front tire Dia. = 24.92”
Distance from Rear Axle to Neck = 47.25” (Online Rake & Trail Calculator)
Neck Height = 36.73” (Online Rake & Trail Calculator)
Trail = 3.78” (Online Rake & Trail Calculator)
When I put the stock RZ info into my spreadsheet, it gives the exact same numbers for Trail, Neck Height, and distance from Rear Axle to Neck.
If I shorten the forks 2”, lengthen the swingarm 6”, and decrease the swingarm angle by 12*, then I get the following info from my spreadsheet.
Rake = 28.1*
Wheelbase = 60.1”
Fork length = 26.25”
Swingarm Angle = 1.84*
Distance from Rear Axle to Neck = 51.07”
Neck height = 34.42”
Trail = 4.32”
When I put my calculated Rake, Wheelbase, and Fork Length into the Online rake & trail calculators;
Rake = 28.1*
Wheelbase = 60.1”
Fork length = 26.25”
Offset = 1.94”
Rear tire Dia. = 24.37”
Front tire Dia. = 24.92”
I get the following information;
Distance from Rear Axle to Neck = 51.12”
Neck height = 34.42”
Trail = 4.32”
The Trail and the Neck Height values from my spreadsheet are identical to the calculations from the Online calculator.
The Rear Axle to Neck Length from my spreadsheet is off by 0.05”.
Not quite sure what’s up with that, I’ll have to recheck my calculations, not that it matters.
I’m certainly not going to be doing anything that requires calculations to 100th of an inch.
The good news is that by flattening out my swingarm and shortening the forks by 2’, I can decrease the height of the bike by a bit over 2 ¼”.
That drops the centerline of the crank from 16’ down to 14” from the ground.
In addition, both the Rake and Trail will increase and keep the bike nice and stable at top speed, which is also a good thing.
I can now put the stock specs for any bike into my spreadsheet, and then I can change;
 neck rake
 fork length
 triple tree offset
 swingarm length
 swingarm angle
The spreadsheet will then calculate the following;
 rake
 trail
 wheelbase
 height at the neck
 height at the fork tubes
 rear axle to neck length
For example – Stock RZ
Rake = 26*
Wheelbase = 54.5”
Fork length = 28.25”
Offset = 1.94”
Rear tire Dia. = 24.37”
Front tire Dia. = 24.92”
Distance from Rear Axle to Neck = 47.25” (Online Rake & Trail Calculator)
Neck Height = 36.73” (Online Rake & Trail Calculator)
Trail = 3.78” (Online Rake & Trail Calculator)
When I put the stock RZ info into my spreadsheet, it gives the exact same numbers for Trail, Neck Height, and distance from Rear Axle to Neck.
If I shorten the forks 2”, lengthen the swingarm 6”, and decrease the swingarm angle by 12*, then I get the following info from my spreadsheet.
Rake = 28.1*
Wheelbase = 60.1”
Fork length = 26.25”
Swingarm Angle = 1.84*
Distance from Rear Axle to Neck = 51.07”
Neck height = 34.42”
Trail = 4.32”
When I put my calculated Rake, Wheelbase, and Fork Length into the Online rake & trail calculators;
Rake = 28.1*
Wheelbase = 60.1”
Fork length = 26.25”
Offset = 1.94”
Rear tire Dia. = 24.37”
Front tire Dia. = 24.92”
I get the following information;
Distance from Rear Axle to Neck = 51.12”
Neck height = 34.42”
Trail = 4.32”
The Trail and the Neck Height values from my spreadsheet are identical to the calculations from the Online calculator.
The Rear Axle to Neck Length from my spreadsheet is off by 0.05”.
Not quite sure what’s up with that, I’ll have to recheck my calculations, not that it matters.
I’m certainly not going to be doing anything that requires calculations to 100th of an inch.
The good news is that by flattening out my swingarm and shortening the forks by 2’, I can decrease the height of the bike by a bit over 2 ¼”.
That drops the centerline of the crank from 16’ down to 14” from the ground.
In addition, both the Rake and Trail will increase and keep the bike nice and stable at top speed, which is also a good thing.
Re: Drag Bike  Plan B
Back to changing tire sizes.
Currently, in my spreadsheet, I can change the front tire size, or the rear tire size, (BUT not both tires), and it will calculate new values for Rake, Trail and Wheelbase.
Also, when I change the front tire size, I’m pretty sure that I’m getting correct values for the Neck Height, but I definitely need to review that section of the spreadsheet to make sure.
The Axle to Neck Length is f*cked.
I know what’s wrong and I can do the math to calculate the correct length without too much difficulty, but then what?
Once I have all this new information, I’m not really sure how to use it when I change the rear tire size.
I can visualize what will happen when both tires are changed, but I haven’t been able to make a drawing that allows me to write an equation that solves the problem.
I predict a few more late evenings before this is done.
Currently, in my spreadsheet, I can change the front tire size, or the rear tire size, (BUT not both tires), and it will calculate new values for Rake, Trail and Wheelbase.
Also, when I change the front tire size, I’m pretty sure that I’m getting correct values for the Neck Height, but I definitely need to review that section of the spreadsheet to make sure.
The Axle to Neck Length is f*cked.
I know what’s wrong and I can do the math to calculate the correct length without too much difficulty, but then what?
Once I have all this new information, I’m not really sure how to use it when I change the rear tire size.
I can visualize what will happen when both tires are changed, but I haven’t been able to make a drawing that allows me to write an equation that solves the problem.
I predict a few more late evenings before this is done.
Re: Drag Bike  Plan B
End this all with some funny shit
Spreadsheet still sucks big time.
I’m getting the information that I was looking for, but it’s not very eloquent or user friendly.
I’m having a problem when change variables, and the spreadsheet is calculating the new rake angle.
Which way did the bike rotate? Was the angle change + or  ? Is that angle added to the original rake, or subtracted from it?
When the fork length increases, the wheelbase increases, and the rake INCREASES.
When the swingarm length increases, the wheelbase increases, and the rake DEREASES.
So, sometimes I add the change to the rake angle, and in other parts of the spreadsheet I subtract the change from the rake.
I need to look at the formulas and see if there is one equation that works in all cases.
Then there was a math issue.
I can’t use the Sine function when calculating the new wheelbase.
The new wheelbase is calculated by dividing the old wheelbase by the Sin of the angle change.
If the rake doesn’t change then the angle is 0* and the Sine of 0* is 0. And you can’t divide by 0.
So, I have to find the length of the 3rd side of my triangle, and then use the Cosine function.
The Cosine of 0* is 1 and the math works again, but it adds another column to the spreadsheet.
I don’t really need to know what happens when you change both the front and rear wheels, because I only plan on changing the rear wheel.
Still, I’m going to figure out what happens when you change both wheels. I’ve come too far to not finish.
Now it’s personal. Me vs. The Rotating Two Dimensional Object.
PS: The Sentence would have been,
“I made my own rake and trail calculator because the online calculators didn’t give me the information that I wanted.”
Spreadsheet still sucks big time.
I’m getting the information that I was looking for, but it’s not very eloquent or user friendly.
I’m having a problem when change variables, and the spreadsheet is calculating the new rake angle.
Which way did the bike rotate? Was the angle change + or  ? Is that angle added to the original rake, or subtracted from it?
When the fork length increases, the wheelbase increases, and the rake INCREASES.
When the swingarm length increases, the wheelbase increases, and the rake DEREASES.
So, sometimes I add the change to the rake angle, and in other parts of the spreadsheet I subtract the change from the rake.
I need to look at the formulas and see if there is one equation that works in all cases.
Then there was a math issue.
I can’t use the Sine function when calculating the new wheelbase.
The new wheelbase is calculated by dividing the old wheelbase by the Sin of the angle change.
If the rake doesn’t change then the angle is 0* and the Sine of 0* is 0. And you can’t divide by 0.
So, I have to find the length of the 3rd side of my triangle, and then use the Cosine function.
The Cosine of 0* is 1 and the math works again, but it adds another column to the spreadsheet.
I don’t really need to know what happens when you change both the front and rear wheels, because I only plan on changing the rear wheel.
Still, I’m going to figure out what happens when you change both wheels. I’ve come too far to not finish.
Now it’s personal. Me vs. The Rotating Two Dimensional Object.
PS: The Sentence would have been,
“I made my own rake and trail calculator because the online calculators didn’t give me the information that I wanted.”
 Attachments

 r065a.JPG (179.1 KiB) Viewed 170 times
Re: Drag Bike  Plan B
you need to get one of those fancy engineering design programs that make all the mistakes for you. That and a bottle of Whiskey!
A modest "high" can be obtained by listening carefully to the sound of a 2stroke hitting the fat part of the powerband while inhaling cool fall air tainted by the exhaust scent of VPC12 and Golden Spectro, mixed at 24:1.
Re: Drag Bike  Plan B
Good news. I have not worked on my spreadsheet since my last post; therefore I won’t be rambling on about a bunch of useless, boring crap.
On second thought, maybe I will, but it will be different, useless, boring crap.
I got a frame from Rory, and the new drag bike is starting to take shape.
Last week, I took it to the shop and put it on the dyno.
At the moment, it’s not making very much power.
I pretty sure it will run better after I tune the carbs and finish setting up the suspension.
On second thought, maybe I will, but it will be different, useless, boring crap.
I got a frame from Rory, and the new drag bike is starting to take shape.
Last week, I took it to the shop and put it on the dyno.
At the moment, it’s not making very much power.
I pretty sure it will run better after I tune the carbs and finish setting up the suspension.
 Attachments

 r066a.JPG (193.42 KiB) Viewed 50 times