Thundercat gearing

[stgdz]

So I have stock gearing with a dmc upper gear that has about 5k of miles on it. I've always liked this setup as I cruise at 90 with 1/3 throttle input and tuned.

I'm considering changing the gearing. I put in an order for an 84p chain and was going to go 21/38 but I'm reconsidering it. I saw that 24/45 is used with Scott Taylor gears.

Both 21/38 and 24/45 are the same ratio but is there a better reason to run with one over the other?

 

[Srxspec]

In theory, larger gears means less rolling resistance, less bending of the chain, etc. I doubt on a trail sled you will notice or feel a difference.

 

[Fleecer]

So I have stock gearing with a dmc upper gear that has about 5k of miles on it. I've always liked this setup as I cruise at 90 with 1/3 throttle input and tuned.

I'm considering changing the gearing. I put in an order for an 84p chain and was going to go 21/38 but I'm reconsidering it. I saw that 24/45 is used with Scott Taylor gears.

Both 21/38 and 34/45 are the same ratio but is there a better reason to run with one over the other?

21/38 is 1.80 ratio. 24/45 is 1.87 ratio.

Did you mean 25/45? More efficient, but much more expensive and need 90 pitch chain.

 

[stgdz]

Correct, I would keep the top gear the same. Just drop teeth in the lower gear and buy a new chain.

21/38 requires both new top and lower gears plus chain. Tflash runs that and he said it loads the engine more than the stock gearing, and that's good for our turbos.

Not sure if the 25 loads the engine as much at low speed.

 

[STAIN]

As SRXspec says the bigger gears are more efficient, probably undetectable, BUT I believe that the less of a dog leg you can have in your chain, in my experience, the better the top gear wear will be. It also keeps the chain away from the post in the case and gives you more adjustment.

 

[Upstater57]

I ran 21/38 on my old modstock tuned 1100. Big top end. With that said, it seemed to put more pressure on the belt. Blew a few with the stock clutches. The problem went away changing to a TAPP and Pro 4 secondary. I am not sure why the problem occurred with the stock clutches but I though it worth mentioning. On my tuned Tcat, I currently run the stock gearing, dmc upper gear, TAPP and a Ron Ward torsional conversion with an ultimax belt. The sled runs out of gear on the lake but is really quick. Not sure if I will go back to the 21/38. Most of my riding is on the trails so the quick bursts from 20 to 80 feel better with the shorter gears.

Years ago, I spit a track out at warp speed on Kevlar Lake. A heavy sled, heavily studded, with a heavy rider geared for the moon is not optimal for the track when you get past 130mph. Maybe I am just getting old and have become more risk averse. LOL

 

[Simplespeed]

Just older and wiser … Experience is the best teaching so they say ….

 

[stgdz]

Something doesn't seem rite

The original gearing


Now with a 90p chain and 45 lower gear, note I'm almost out of adjustment.


I can't touch the back case with both these chains when adjusted like so. 90P is what the charts say and I counted out the links to make sure I got the correct one.

 

[DMCTurbo]

Something doesn't seem rite

The original gearing
View attachment 178739

Now with a 90p chain and 45 lower gear, note I'm almost out of adjustment.
View attachment 178738

I can't touch the back case with both these chains when adjusted like so. 90P is what the charts say and I counted out the links to make sure I got the correct one.

Thats about right. The math works out to 89.19. I always try and run a combo that ends in .69... Less dog leg, more efficient.

 

[stgdz]

the dog leg looks worse on the 45 lower than the 50.

Do I need to get a shorter chain? I'm worried I will run out adjustment when it stretches.

 

[DMCTurbo]

the dog leg looks worse on the 45 lower than the 50.

Do I need to get a shorter chain? I'm worried I will run out adjustment when it stretches.

Shorter won't work. Your kind of right in the middle with that gear choice. You can take that spacer out of the adjuster bolt. I haven't had mine in there forever.

 

[justinator]

I

Something doesn't seem rite

The original gearing
View attachment 178739

Now with a 90p chain and 45 lower gear, note I'm almost out of adjustment.
View attachment 178738

I can't touch the back case with both these chains when adjusted like so. 90P is what the charts say and I counted out the links to make sure I got the correct one.

If that is with 24 top and 45 bottom? If so that wont work. The math says 88.69 chain so 90 is too long. 25/45 comes out to 89.19 so 90 chain is perfect.

 

[DMCTurbo]

I

If that is with 24 top and 45 bottom? If so that wont work. The math says 88.69 chain so 90 is too long. 25/45 comes out to 89.19 so 90 chain is perfect.

Pretty sure he said 25 top.

 

[justinator]

Pretty sure he said 25 top.

His picture shows stock gearing and then he says second picture is with 45 bottom and 90 chain? So im assuming he kept 24 top. I know for a fact 25/45 fits beautifully and doesnt have big dogleg

 

[DMCTurbo]

His picture shows stock gearing and then he says second picture is with 45 bottom and 90 chain? So im assuming he kept 24 top. I know for a fact 25/45 fits beautifully and doesnt have big dogleg

Ok i assumed he had 25/45. 89.19.