Project Incognito

[Smoker]

Keeping busy.

Realized another flaw in my thinking. I wasn't thinking about the amount of weight on the fork springs.

That's probably why I was getting a lower number than I expected on the calculators.

Now, when I look at the bike, I think there's a large weight bias toward the rear. Could be good for my numbers. Got a scale from Amazon to check it out.

[Smoker]

I got a scale to check the actual rear weight bias. Didn't do it yet, but plugged in an 80% rear weight bias into the calculator, just to see. Also reduced the sag to 25%. The number was 232 lbs, with a recommended 200 lb spring. I can't even find a spring that soft.

The really bad news is, I found out Ohlins does make the TTXM.2 coil spring in a 250mm length, and now it looks like I can't use it. I did see a website that sells custom titanium coil springs. Could get really crazy, if I had to get more than one spring. I'll check it out before getting an air shock.

[JonW]

Defo looks better with the stem riser, does it feel better to ride more upright?

[Smoker]

Defo looks better with the stem riser, does it feel better to ride more upright?

Much better.

Checked on the custom springs. 200lbs to 800lbs only.

Found an off-the-shelf 200lbs spring in Hong Kong. Will contact them and see if they have anything softer.

[Smoker]

Well, this rear shock situation sucks.
Sent an email to Cannon Racecraft. They sell custom coils for motorcycles. Don't have much hope, though.

I think a linear rate coil shock is right for this application, but I might get stuck with an air shock. (See what I did, there?)

Problem with an air shock is the number of seals used to hold the high air pressure and the stiction they cause. The air pressure also makes them progressive. No way can they provide the plush ride of a well-tuned coil shock. Even worse for me because of my light rider weight and extremely low leverage ratio of the bike.

[Smoker]

I'm learning as I go. Been looking more at air shocks, just in case.

Air shocks should have numbers that equates to a coil spring - but, they don't. For example, the air shock (DB Kitsuma) air pressure range is 50 - 300psi. Does that equate to a 350 - 650lb coil, or a 250 - 1,100lb coil? Tried to get that info, with no luck.

Started looking at leverage ratios for assorted mtb bikes. I measured my rear wheel travel at 80.1mm. The range of leverage ratios at 80mm are over 2.6 to about 3.3, with most in the high 2s. My bike is just over 1, so close to 1/3 of normal.

Got the owner's manual for the Katsuma Air. They recommend starting sag setup at 1psi per 1lb of rider weight. Makes sense that most riders will be 50-300lbs.

So, If my rider weight is 150 - 160lbs, and I need to reduce the pressure to 1/3, I should be okay with 50psi., or a little higher. Low pressure is bad for overcoming that stiction, though.

[Smoker]

Getting closer to a rear shock solution.

Got specs from Ohlins on the coil springs for the TTXM.2 shock. Sent the specs to Cannon. Waiting for a response.

The Ohlins rep I've always used, sent me the specs. He's on the motorcycle side of Ohlins, so I decided to contact someone on the bicycle side. New rep said I should check out Hyperco coil springs.

So, Hyperco has a spring that might work, but the specs aren't the same as the Ohlins spring. The Hyperco spring is only around $50usd, so it's probably worth checking out. They have 150, 175, 200, and 225 lb spring rates, so I hope they work.

If Cannon says they can make a spring with the proper dimensions and rate, I'll probably go for the TTXM.2 shock and try the Hyperco springs first.

[JonW]

I love how things like this on projects send us down rabbit holes to learn all about something we never considered before, its one of the delights for me about building stuff. :)

[Smoker]

I love how things like this on projects send us down rabbit holes to learn all about something we never considered before, its one of the delights for me about building stuff. :)

Me, too. I like to tinker.

Still trying to check my work.

The 150lb coil, recommended by the spring calculators, should compress 1 inch under 150lbs of weight.

The sag on a coil spring with 3 inches of travel could be 33%, or 1 inch, for a plush ride. So, 150lbs (or a little less) should be about right on the rear shock.

Hope I don't go pogo-ing down the street, but we'll see.

Got out the new digital scale. It's 1 inch tall, so put a 1 inch stack of aluminum under the front tire. Scale under the rear tire said 45.1 lbs. Swicheroo- front tire was 34.5 lbs. Total = 70.6 lbs. Jives with the specs.

Took a bit of practice, but got up on the bike and balanced with the scale under the front tire. Got 2 different consistent readings, probably because of fork stiction, or maybe my positioning. Got 85.6 and 87.5 lbs. Used both numbers. My weight is close to 150lbs + bike weight of 79.6lbs = 229.6lbs.

229.6 - 85.6 = 144lbs rear tire weight
229.6 - 87.5 = 142.1 rear tire weight

144lbs = 59.5% rear weight ratio
142.1lbs = 61.5% rear weight ratio
I was using 60% on the calculators, so that is right on.

I'd like to know the sprung weight, and the ft/rr sprung weight ratio. The front will be difficult to check, without my unfinished custom lift.

[Smoker]

Guess I didn't post it, but got a thumbs-up from Cannon on the custom spring.

Looked all over, and couldn't find the post-it with my measurements for the shock mounting hardware. Planning to measure again, anyway. In too much pain today.

Hoping to get my order in on Monday. Very curious how this will play out.

EDIT: The correct shock number is TTX22M.2, non-trunion.

[JonW]

Excited to see how the calcs translate into the real world

[Smoker]

Rear shock is out (again!).

Double-checking measurements on the shock hardware. Also measuring the mounting points on the frame.

Some confusion on the hardware. If the frame mount measures 34.0mm, do I buy 34.0mm mounting hardware, or 33.8mm hardware? Not sure how it's measured & marketed. Will need to make some calls on Monday.

Weighed the rear unsprung weight = 21.4 lbs + 2.51 lbs (the shock). Swingarm was still on the pivot, but hopefully, the numbers are close.

[Smoker]

The frame and swingarm are painted aluminum.

Even on the brand new bike, the paint was slightly messed up around the shock mounting bolts, and worse under the bolt heads. Most likely could have been mimimized if they had used washers. Even worse after removing the shock a few times.

So, the rear bolt is 50mm. Will need a slightly longer one, to use washers. Found a database with more than 100 bikes, and the shock bolt sizes. Nothing over 50mm. I did find some long universal Chinese bolts on aliexpress. Could take 5 to 10 weeks to get them.



Saw some nice titanium bolts. Found out on the database, they're too short.



Now thinking about getting some 8mm titanium rod, cut to length, and drill and tap the ends for M6 bolts. Could red Loktite a Ti bolt in one end, and screw one in the other end. Not sure how well I can tap M6 threads in an 8mm rod, but we'll find out.

[Smoker]

Okay, check out the pic of the titanium bolts above.

Rep got back to me, and said those bolts are long enough. Also let me know that if I cut off much, I'll need to re-tap because the threads aren't very long. Good info!

Ordering those bolts today (in gold, of course). Hoping it works out well.

[Smoker]

Actually, the rep said if I cut much off the female shock bolts, there may not be enough threads. Was my idea to thread them deeper. There's a chance they're not hollow past the existing threads. Just have to give it a try.