Flywheel lightening

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LC Cnd
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Flywheel lightening

#1 Post by LC Cnd » Wed May 16, 2018 11:10 pm

Contacted NK racing (UK) about lightening a flywheel for a new engine rebuild. Learnt I had to ship my original to and frow fm Britain to get it done. Mailing would be killer unfortunately since NK's price was decent. A local machine shop could do the job, The attached photo is fm the NK racing site. Has anyone done this mod and how much was shaved off to get a light yet tolerant/functional flywheel?
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RD LIGHT FLYWHEEL.jpg
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RuZty
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Re: Flywheel lightening

#2 Post by RuZty » Wed May 16, 2018 11:26 pm

I've got one around somewhere that I did like that, don't recall the exact dimensions but I turned it down based on recommendations at the time. I was meaning to look for it this week before I put my engine together, I'll measure it when I find it. I ran it for quite a while but had to change it when my stator died and it was not compatible with the replacement I had at the time. It revved quicker, not sure how much that translated into real performance.

LC Cnd
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Re: Flywheel lightening

#3 Post by LC Cnd » Wed May 16, 2018 11:36 pm

Appreciate it Ruzty. How long ago did you do the mod? Also undertand there is less stress on the crank to boot.

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JonW
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Re: Flywheel lightening

#4 Post by JonW » Wed May 16, 2018 11:40 pm

The problem people have with these is that the weights and the rest of the flywheel is heavy, so removing metal can cause them to fail if you do it wrong. defo dont do any on the front near the rivets, that part is pretty thin for the weight of the outer part :/
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evan_calgary
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Re: Flywheel lightening

#5 Post by evan_calgary » Thu May 17, 2018 4:01 pm

This was very common with dirt bikes. Biggest issue is maintaining balance when removing material. Have never heard of a flywheel actually coming apart or failing rather they end up junk when they are badly balanced.

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Re: Flywheel lightening

#6 Post by (F5) » Thu May 17, 2018 4:57 pm

The picture is how we did mine back in the day. Dont be greedy and no real need to take off front face.
Didn't bother on my big bore but helped the 350 feel more spritely.
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RuZty
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Re: Flywheel lightening

#7 Post by RuZty » Thu May 17, 2018 8:49 pm

LC Cnd wrote:Appreciate it Ruzty. How long ago did you do the mod? Also undertand there is less stress on the crank to boot.
Okay, I found it. I did this probably 15 years ago and used it for several seasons. I took 4 mm off the OD (a 2mm cut) and radiused it at the center ring where the triggers are. I did a straight cut and did not touch the outer radius or front face. I now have a spare stator/rotor assembly that is the same so once I get my engine assembled and broken in I will try a back to back comparison.
Not seeing how there is less stress on the crank, it is a balanced rotating assembly, all you are doing is reducing the inertia of the whole assembly.

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JonW
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Re: Flywheel lightening

#8 Post by JonW » Thu May 17, 2018 9:09 pm

evan_calgary wrote: Biggest issue is maintaining balance when removing material.
The idea is to remove only metal from the area that has no balancing dimples or the trigger (of course), that way it should stay roughly in balance.

I like the idea of it, but have never done it. Id hate to weaken the unit and have it fly apart... would be 'messy' :/
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LC Cnd
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Re: Flywheel lightening

#9 Post by LC Cnd » Fri May 18, 2018 1:54 am

Request, is it possible to see a photo of your flywheel Russ? Curious what your flywheel looks like where the front face meets the outside diameter - is it 90 degrees or tapered like the photo above? Funny you should mention the dimples Jon as the machinist whom I'm using was curious if the removal of material might affect the balance. This vid on lightening a Yamaha blaster flywheel is interesting.
https://www.google.ca/url?sa=t&rct=j&q= ... gIdDKHQnzU" onclick="window.open(this.href);return false;
Here is how rotational inertia on a motorcycle flywheel is effected by horsepower and why Russ you noted the difference:
https://www.youtube.com/watch?v=CxuER0qIWJk" onclick="window.open(this.href);return false; More of a tech tip matter really.

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Re: Flywheel lightening

#10 Post by hondaror » Fri May 18, 2018 3:54 am

From the pic, I would have guessed about 0.020" depth of cut, about 1mm total. The lighter flywheel would create less torque on the crank. The result is a quicker reving engine. Too light is not good. You do need inertia. This is, what I consider, an extreme mod. If you are an awesome rider, which most of us are not, and have a hundred of these to cut and test for best results, go to her. Fun to talk about, more fun if you have experience with them...Russ?! Lol
Rory

RuZty
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Re: Flywheel lightening

#11 Post by RuZty » Fri May 18, 2018 11:58 pm

Here's a pic from when I did it in '02. Looking at it again I recall I didn't turn it down myself, I gave it to a machinist at work as I didn't have use of a lathe at the time. The 2mm dimension came from someone on the old UK forum that had done a number of these, based on cutting one apart. I don't agree with the statement in the video you linked that the 'corner' going into the radius is a stress riser, that is no different than the end of the taper on the crank. It would matter if it was cut into an inside radius. I probably put 10000km on this one, I don't recall it being excessively vibey, but I'll refresh my memory soon. I was a bit concerned that it should have been rebalanced, and I have a crank taper saved to mount it to for that purpose, but never got around to it.
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Re: Flywheel lightening

#12 Post by hondaror » Sat May 19, 2018 3:06 am

Yeah, wow, that one is cut down. That looks like a fun mod. I might have to try that.

As far as balancing goes, drilling small divots where it is heavy, is effective.
Rory

RuZty
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Re: Flywheel lightening

#13 Post by RuZty » Sat May 19, 2018 5:54 am

The amount I removed works out to around 103g or 3.6 ounces.
I used to have an '82 RX7 and swapped in a lightweight flywheel from Racing Beat, 28 pounds down to 12. I also added a heavy duty clutch. Boy did that change the character of the car, more fun when you were having fun but getting stuck in traffic was brutal. Anyone who drove it stalled it 3 times before getting it to move.

LC Cnd
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Re: Flywheel lightening

#14 Post by LC Cnd » Sat May 19, 2018 3:41 pm

I get that balance is key - can this be acheived in a static balance setup, like balancing a motorcycle wheel, or does the flywheel have to be in constant motion to find where to drill to balancing out? Found this photo'd sample on a Harley form. Then there is the question of how much to drill that's the challange. The video, on shaving a flywheel, stated that is not a issue interestingly.
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Bare
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Re: Flywheel lightening

#15 Post by Bare » Sat May 19, 2018 9:19 pm

Why dick about shaving the old lump ? Find and buy a light flywheel assly .. for a genuine weight reduction ?
There are a couple of units available. Not free, but then Nothing worthwhile ever is.

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