How To : Lightening A Stock Clutch

[90GTVert]

Lightening The Stock Clutch

I drilled a hole in each clutch shoe to lighten it up a little. This has the same effect as stiffer clutch springs, making the clutch engage later. I have heard a lot of people say that this causes your clutch to be out of balance and it's not worth the time and others endorse it. I have never done a scooter clutch this way, but have used this method on other small engines. Each shoe had a round marking on it in the same location. I used these for the locations of my holes. I don't even know what size drill I used, just whatever looked good to me. In other small bikes this usually makes a small difference in engagement RPM. Others go all out and drill larger and/or more holes in the clutch for more extreme results.


I would have just bought stiffer springs, but haven't seen stiffer springs in the US. If you don't already have 2000 RPM springs, 1000, 1500, and 2000 RPM springs are easy to come by. Another solution is using springs intended for automotive brakes. I've never seen anyone say a specific make/model/year spring kit that will include proper springs for this (or any) style of clutch so it would involve measurements and/or taking your clutch to the store and bugging the parts guy.

Here's the finished product.




I let someone borrow my cam today, so it wasn't around to take a before pic. Here's an old one though, where you can see the markings on each clutch shoe that I used as my drill spots.




This clutch ended up engaging around 5,000RPM using 2,000RPM clutch springs.

[90GTVert]

An excerpt from Project 49 :

Next up, I wanted to play with the clutch a little. I had originally intended to just leave it as a sidenote with the clutch tuning that you could opt to drill the shoes. After forum member byte bringing up lightening the clutch to help it slip in and possibly see a better launch, I thought it was worth another look. I have lightened clutches in the past, but I've really only done it to get later engagement for high revving setups. I can't say I've noticed a real difference in engagement with what I've done in the past. I assumed you'd need to go much ligher, like an aftermarket clutch or perhaps so much material removed from the clutch that you may risk a failure. Even if it worked no better than springs, it would be an opportunity to save $10 on a set of springs. Worth a look.

I already have a clutch around in good condition that I had lightened in the past. I used this to save me a little work and so I could keep the stock clutch, should I decide to use it later or need it for any reason.




I started out by removing the shoes and weighing them. They were all 165-166 grams. I had drilled holes using the stock "JOG" markings to try and keep the weight removal in roughly the same spot on each shoe.




I wanted to remove a little more weight to see if I could get a little slip that I may notice. Removing weight farther from the pivot point (at the pin the shoe rides on) has a greater affect than removing weight closer to the pin. Always look at each side of a shoe before deciding where you want to modify it. You can see on the rear of the shoe there is a sort of groove on the far left of the pic below, marked with an arrow. That's where the shoe rides on a rubber bushing on the back plate of the clutch. I did not want to drill there. While it may be fine, I thought it could cause the clutch to hang up or damage the rubber bushing. I chose to drill where marked with a red circle in the image, so I had enough to reference that I could keep the weight removal in a similar spot on each shoe. The hole just to it's right is where one side of the clutch spring hooks in on the other side and the other hole is where I had lightened the shoe already.




All 3 shoes drilled.




I weighed each shoe. The lightest was 159.5g. I drilled just a little more out of each hole in the other two shoes until they all weighed the same.




Once all of the shoes were the same weight, I reassembled the clutch using 1,000RPM springs and put the CVT back together. I was using 1,500RPM clutch springs in the stock clutch with no weight removal. I knew the weight would make engagement later, and that's why I started with the softer 1,000RPM springs.




Even 1,000RPM springs were stiff enough that the belt was moving quite a bit before the clutch tried to engage on the stand. I did a few launches in the garage and once on the road. I'd say it was roughly comparable to the stock clutch with 2,000RPM springs. Not terrible, but I'd prefer to keep the belt from moving too much for the best takeoff.

I went to stock springs. The belt moved just a little, but no more than with the 1,500RPM springs I chose for best acceleration in the bone stock clutch. Some launches in the garage and driveway seemed promising so I took it out on the road. The clutch was engaging at 5,300-5,400 RPM just like the stocker with 1,500RPM springs installed. It seemed to slip enough to see the upper 5,000s to 6,000RPM sooner though. Not a big difference, but at very least it seemed to do as well as the aftermarket springs. I was curious to see if I'd see a difference in time and also wondering if the weaker stock springs would fade much, so I took a 33 mile ride this morning for a good test.




16.75 seconds 0-35MPH is the fastest time I've seen so far in the project. It's close enough to the last best on video of 17.07 seconds that I can't call it a great improvement. With room for error, I'd say the difference is minor.

The biggest difference that I saw was one I didn't expect. It seems that the stock clutch springs and lightened shoes actually don't fade as much as just aftermarket springs. In fact, even at 85 degrees in the morning sun and riding around town a bit, I never saw a difference in launch.

I'd say if you are willing to take the extra time, this is a mod worth doing. It'll save you a little money, when done right. (Remember, if you mess up the stock clutch you'll spend a lot more than springs cost to replace it.) Improved performance, with seemingly no fade (also remember I have a vented CVT cover), for just time as long as you have a drill and bits and the tools to disassemble the CVT. You'd need the same tools other than the drill and bits to do clutch springs anyway.