Sprocket fun!

[Royalratch]

Need some help on this guys.

I am doing an 810 conversion with all Dyna bits so am expecting a bit more horsepower than that GPZ 750 Unitrak it's based on.

My questions are these:

1) What is the STOCK front and rear sprocket size?

2) (The grade school maths part!) If I have more power, smaller wheels (17") and want to run an extra tooth on the front - what is should my rear sprocket be?

I'm not into changing the character of the bike too much or chasing top speed - that said, I don;t want to run out of revs too early. I guess I just want it as close to stock as possible in ride character after factoring the extra power, wheel size and extra tooth on front.

[Royalratch]

So I'v been messing with gearingcommander.com

I've entered my updates details into it from stock - mainly a smaller rear wheel. Total rear wheel circumference is important, my smaller wheel will rotate slower than the stock 18".

Also, I'm going 14T on the front and am limited to a 36T on the rear as that's the smallest my Talon SuperMoto hubs will accept.

So here are the figures.

[img width=640 height=474]http://farm3.static.flickr.com/2682/5839647610_65516dd86f_b.jpg[/img]


And the figures from stock.

[img width=640 height=234]http://farm6.static.flickr.com/5077/5839647678_cbd9ee0977_b.jpg[/img]

So what I'm trying to do is get a gearing that is slightly taller than stock as my tweaked engine should have quite a bit more power (20% more all told?)

[chrisNI]

If they say they can only go so small on the sprocket it's because of where the bolts are on the hub if they say 36 you need to work with that and go up on the front to make it work. 17" wheels will certainly make a difference. You need to use a bigger front sprocket but you need to measure clearance inside your chain case to make sure it fits. I had a bit of this with my zephyr hub the smallest they could do was 38 they know what they're doing.

If you need more space inside the chain case you may need to go down chain sizes. What pitch are you using 14t sounds like a 630 as most others won't go that small?

The stock rear would be 18 not 19. You don't really need to factor in increased power as the red line will presumably be inn the same place it will just pull through the gears quicker on the standard gearing.

[Royalratch]

Cheers dude.

The rear wheel is an 18" on the stock bike but with a much taller tyre profile - I'm using a 160/160/17 with a circumference of 78.6".

I'm up for modifying the sprocket cover and even moving the sprocket guard that sits in front it should I need to go to a 15T on the front to make it wall work.

I'll be using a 520(?) chain for sure as both sprockets will be custom.

[Royalratch]

You need to use a bigger front sprocket but you need to measure clearance inside your chain case to make sure it fits. I had a bit of this with my zephyr hub the smallest they could do was 38 they know what they're doing.

I thought that bigger front meant smaller on the rear?

I'm going bigger on the rear so wouldn't you logically go smaller on the rear (I know you can't tho).

Or does the smaller rear wheel restore that?

14T or 15T front and 36T rear is looking the way to go.

[chrisNI]

[
I thought that bigger front meant smaller on the rear?

I'm going bigger on the rear so wouldn't you logically go smaller on the rear (I know you can't tho).

14T or 15T front and 36T rear is looking the way to go.

38/15=2.533333333333334

36/14=2.571428571428572

39/15=2.6

[Royalratch]

So working with a 14 / 16 I should be okay and close to the stock 2.55 primary ratio - worry over?

[chrisNI]

You may find you can't get a 15 tooth 520 for a similar reason to anything smaller than a 36 tooth rear. The smaller wheel will mean you need to go down teeth on the rear from stock if you were say 44/17=2.588 on stock wheel size you might need to go 42/17 or something with the 17". I got someone else who knew what he was on about to do all the maths for me with mine. He was right and a lot of the other figures that were being bandied about were off the mark - you need to find someone good at sums and who knows how to use them. The 650 gearing seems to be different to the big Zeds so it's not really possible to extrapolate.

[Royalratch]

810 converison not count as a big Z then?

I'll get all my measurements super tight then have another go on gearingcommander. Won't be the end of the world if I build on a 14T/36T and have to swap it a bit.

At least I know I must start with a 36T rear...

[Royalratch]

Useful article / site...

http://www.motorcycleanchor.com/motorcy ... tmath.html

[Royalratch]

Well, I have ordered a 36T and and will run a 14T or even 15T up front which will get me within 2mph at the same revs as stock. The extra power and torque from my conversion will get all that back and a bit more.

I'm hoping 14T on the front won't need any mods but the 15T will be better for the final ratio and better fpr the chain so I'll look at what I need to do to the engine casings to make it fit if needed.