Let the Experimenting Begin: TAV2 Edition

[TheDullCarbide]

Last night on the throne I thought to myself, "Could I jam a GY6 contra into a TAV?" I ran downstairs, grabbed my spare 6", and tore it apart, almost sending the circlip through a nearby window. I cut a third off my beat up old 1000rpm contra and it fit like a glove. The 6" got slapped into the place where the 7" was, and with the 7" gearing. Since it was already 9PM, the muffler was put into stealth mode and a few backyard moonlit passes were made.

So far the shifting rpm is just a tad higher and the shifts became buttery smooth. The torque was lacking simply because of the high gearing but I felt much less low-end slippage even with the smaller 6".

Tomorrow I will test this combo a bit more then try the same spring in the 7". I'm not sure yet, but I may have tripped over something awesome here.

 

[1971_MB1A]

Sounds to me like you may be on to something there. Hope it works out for you.

 

[TheDullCarbide]

I tested the 6" combo a bit more today and results were as expected. Torque was pretty good for the 6", but then again it's a 6". There was coil bind right near the top ratio so I modified the 7" a bit before I threw it in.

First, I addressed the coil bind issue. Instead of cutting the spring more, I decided to modify the pocket in the cam plate. I machined off the little blocks that held the stock spring because the GY6 spring had to sit on top of them:



I also wasn't too pleased with how far the belt rode down in the driven. The wear pattern didn't come anywhere near the rivets so I took 1/16" off the sheave boss. This gave me a higher top-end ratio considering I run the tighter -90 belt:



I think I need to cut another 1/2 turn off the spring anyway and add a thrust bearing under the cam plate so the spring can rotate with the sheave, much like the GY6.

The low-end torque of the 7" was KILLER with the experimental contra. No joke the front wheels came off the ground from a dead stop on a steep hill. Keep in mind that it had the highest gearing in it (tops-out at 32mph) and this isn't a minibike. Imagine the front wheels lifting on this:


^^^^THIS IS NOT A PICTURE OF MY KART, but you get the idea^^^^

Another interesting side-effect is the engine brake. If you've ever coasted with a stock TAV, you know it doesn't do much and if anything it just makes the belt flop around. Now, because the spring is purely compression, the sheave can rotate backward to the second set of cam buttons and downshift under back-torque. When you let off the throttle, the TAV upshifts and the engine slows to about 2500 for a second, then it downshifts again and spins the engine back up to 3700. If you blip the throttle every once in awhile, you can just coast. I have absolutely fallen in love with this when crawling down hills in the woods.

I've heard some stories of the optional yellow contra playing havoc on the shifting of TAVs. This one maintains a constant rpm throughout the range much like the green contra, yet it has the low-end of the yellow plus some with the 7".

The only drawback is the extra belt tension that can eat some HP in the top-end. The engine has to work harder because the belt is yanked in and out of the pulleys harder. Once again, I need to cut the spring.

 

[TheDullCarbide]

Well I'm back from my turn with the flu.:repuke:

I've been thinking about how the GY6 can make the best of both worlds (torque and efficiency) yet it's a trade-off for TAVs. Then it hit me. I couldn't fit even half the GY6 spring into the TAV (yes I know belt angles, bare with me). The GY6 has a longer portion of spring that doesn't get compressed...no, that's a bad way to describe it.

You have two identical springs except one has a 1" free length and the other is 2". If you compress each of them 3/4", which one will maintain a more constant pressure? That's the difference.

So to put this theory to use, I took my boring bar and cut 0.09375" out of the pocket in the cam plate. I guess we'll see if that's enough to snap it. I also cut the other piece of my GY6 contra to 3 3/4 turns (the first one was 3 turns).

I said about a thrust bearing in the cam plate in my last post. I was thinking about a roller element bearing like the aftermarket GY6 setups but that would just get clogged with dirt. I instead opted for a 1/32" sheet of PTFE. I got lucky and used two hole saws to cut myself the perfectly sized ring to go in the cam plate.

So I tested this combo. It is a big step in the right direction. The torque is there, the efficiency is there, it's nice. It made the engine rev high enough that I went back from 1 white, 1 blue to both blue garters. The effect is like having a slightly stronger Comet green contra without the problems of the Comet yellow contra.

But I want more.