# Belt Drive Calculators?



## Agwan (Feb 16, 2011)

I'm looking to build up the SG-S7000 when it comes back in stock, hopefully in may. I know that i Will have to use a 24t Gates cog out back, But I'm trying to figure how to use a calculator (theirs or otherwise) to figure out how large I have to go on the chainring to get a high gear equivalent to around 48t or 50t to 11t.

Anyone have any advice?


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## Drew Diller (Jan 4, 2010)

I think Gates has a utility for it, yeah.

What application are you after for such a high gear ratio? For the cogs you're talking about 4:1 rather than the typical 2ish:1 for an Alfine setup?

Are you on a folding bike, or, are you simply one heck of a strong rider?


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## PlutonicPlague (Jan 19, 2014)

Top (8th) gear on the A-8 sg-s501 hub is 1.61.
I think that would mean more than 60t equivalent. 65t equivalent, to be exact.

example: 65t/24t = 2.71 
2.71 X 1.61 = 4.36
48/11 = 4.36


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## Agwan (Feb 16, 2011)

Drew Diller said:


> I think Gates has a utility for it, yeah.
> 
> What application are you after for such a high gear ratio? For the cogs you're talking about 4:1 rather than the typical 2ish:1 for an Alfine setup?
> 
> Are you on a folding bike, or, are you simply one heck of a strong rider?


Strong rider. But I just want similar to the grunt of a road bike in my high end, and I'm willing to sacrifice my lower gears to do it. I know that 46x16 on my cogs worked pretty darn well with my old A8, but a belt drive will require a 22 or 24 t cog.

I'd also like a tiny bit more Oomph than the 46x16 gave me. not a ton more. but it spun out a lot sooner than the wifes traditional drivetrain did in the highest gear.


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## Agwan (Feb 16, 2011)

PlutonicPlague said:


> Top (8th) gear on the A-8 sg-s501 hub is 1.61.
> I think that would mean more than 60t equivalent. 65t equivalent, to be exact.
> 
> example: 65t/24t = 2.71
> ...


I figured out how to use Sheldon browns Calculator to get the numbers I was looking for.

60x22 belt drive on an Alfine 8 will compare to 48t by 11-34 10 speed system thusly.

1- 8.9 8.9
2- 7.5 7.9	
3- 6.5 6.8
4- 5.7 5.5
5- 5.1
6- 4.6 4.7
7- 4.2 4.1
8- 3.9
9- 3.5 3.6
10- 2.9 2.9

pretty stinking close. I see this set up on bike friday folding bikes. So I hope two things.

1. My Soma Wolverine clears a 60t chainring

2. I won't horribly over torque the hub.


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## PlutonicPlague (Jan 19, 2014)

^I wouldn't worry about over torquing your hub with higher gearing.
I'm the one who should worry, running 26/20 (input ratio of 1.3) on my Pugsley. I don't think I'll break anything, though, since I'm sort of a lightweight.


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## wschruba (Apr 13, 2012)

Yah, you don't have to worry about a high input ratio, the issue is with a lower ratio, the input torque is enough to overcome the friction from the roller clutch, leading to skipping and possible damage to the planetaries.

_IF_ you can get the chainring to fit, it'll likely be by spacing the crank out farther. The maximum chainline on the Alfine is something like 47-50mm, depending on the sprocket, and dished towards the outside of the wheel. This will likely be an issue, since the Gates sprockets are dishless. With a standard IGH, that means you'll wind up with a chainline of about 42mm. Honestly? I'd doubt very much any bike would be able to clear that without being expressly built for it, except maybe a track frame. Gates has measurement directions on their website, and a quick perusal says that a 60 tooth sprocket has a diameter of 204mm. The math from all the above information will tell you if it'll fit or not.


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## Agwan (Feb 16, 2011)

That is actually really helpful information. As soon as I can measure out a 42mm chainline and then see if there is 104mm of space from there to my chainstay (Gates wants at least 2mm clearance) I'll know. though looking at how my 46 tooth chain ring fits now, I somehow doubt anything larger than that would fit for a belt drive.

frankly, I don't think 46x22 on a belt will keep up with slower roadies. which is sort of my goal. I may be best served by keeping a traditional chain.


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## Saul Lumikko (Oct 23, 2012)

Calculating equivalent gear ratios is no different from calculating regular chain drives. 

The pitch is smaller, but the ratio of 45/20 - for example - is the same. Doesn't make a difference whether it's a chain or belt drive.


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