gn sprockets

[spooky]

does anyone know where i can get a 17T front sprocket for my gn250 ? most suppliers just list 14/15/16,  also is an SP370 sprocket the same cos i think i can get one of those,  cheers

[SteveC#222]

Try B&C sprockets - they use to make one off's from blanks.

http://www.bandcexpress.co.uk/shop/transmission/sprockets.htm

[Moto63]

Hi spooky,
I got an ally rear sprocket made up recently by  uk bike parts. Nice quality and only £50 not bad I thought for a one off special. Find them at ukbikeparts.com.   Regards Michael

[Propellor]

Hi spooky.

I can also vouch for B&C based on my own experience with them and I've also seen the work carried out on moto 63's project.

How about looking on JT sprockets own website?

I noticed you said on another post that you are looking to make the bike quicker. You sure raising the gearing will make it quicker? Presumably the larger front sprocket will actually fit without the chain fouling the engine castings? Could that be why it's not offered?

Looking forward to hear how you get on.

Cheers.

Edit:

http://www.jtsprockets.com/catalogue/model/1607

Seems 15/41 tooth is std setup so that's 13% rise in ratio. Quite a bit higher, assuming I've done me sums right.

[Propellor]

........also is an SP370 sprocket the same cos i think i can get one of those,  cheers

http://www.jtsprockets.com/catalogue/model/2044

[spooky]

Thanks for your replys, phoned and asked but no 17T sprocket available and SP doesn`t have 17t either, so only option is a 37 rear (i already have a 16 front)  with 16/37 combination it will give barely 4mph higher top speed, probably not worth the effort but i need a new chain anyway so i will get the new rear sprocket anyway, figures below out of interest:

The Stock 15/41 sprockets give 58 mph at 5600 rpm

The Current 16/39 sprockets give 65 mph at 5600 rpm

The Proposed 16/37 sprockets give 68.5 mph at 5600 rpm

 

So for the arbitrary 5600 rpm you would only gain 3.5 mph by changing the rear to 37 teeth

[spooky]

just found this place, 17T avaialable ! and according to gearing commander if i use a 17 front and 37 rear it should do 91mph at 7000 rpm, hopefully i will find out in the near future if this works in practice

http://www.aliexpress.com/store/product/Suzuki-27511-32401-ENGINE-SPROCKET-Front-Sprocket-GN250-SP250-SP350-SP400-SP500-16T-17T/817202_1592074332.html

[themoudie]

Aye Spooky,

Watch out for the 'hunting tooth effect of both sprockets being even or odd number of teeth. May not be a problem, but could wear the sprockets and chain prematurely. 

Also make sure your clutch plates, springs and pushrod are all in good nick as you may need to "slip and feed "a la Goldie"  to get the bike away from a standstill or in slow traffic. Just a thought!

Keep us posted.

My regards, Bill.

[SteveC#222]

You want to check that there is going to be enough clearance between a 17T sprocket + chain and the generator casing - I seem to remember from mine that there wasn't that much to spare and just 1 extra tooth is going to make the sprocket bigger than you might think. Don't go too high on the gearing or you'll kill the accelleration and make it feel gutless.

[spooky]

The last two posts both make sense, i think i will play safe and just change the rear to a 37, then both are in the `option` range,   cheers

[Propellor]

You want to check that there is going to be enough clearance between a 17T sprocket + chain and the generator casing - I seem to remember from mine that there wasn't that much to spare and just 1 extra tooth is going to make the sprocket bigger than you might think. Don't go too high on the gearing or you'll kill the accelleration and make it feel gutless.

+1 on both those issues.

Assuming the sprocket fits the space, I just can't see the motor achieving the predicted speed, even on flat ground, still conditions. Ask yourself what power you'll generate at the predicted speed. I don't think it'll be enough. Not only will the bike feel lethargic, you'll be forever dropping down a gear in an attempt to hold speed. The bike might not even make a hill start!

That's my prediction at any rate. If you decide to try it, I'm dying to know the result! Good luck.

[SteveC#222]

I seem to remember reading somewhere that, as a rule of thumb, you need at least 30bhp to make 100mph - as I recall a new GN250 made around 22bhp and add a few years of wear and tear you're probably looking closer to mid teens. Mine wasn't fast, but it handled really well so you could keep the throttle pinned to the stop doing 65mph just about anywhere.

[spooky]

A lot to think about, i think i will just change the rear sprocket and accept the fact that i can`t improve the top speed much more than i have already, but it`s been a useful exercise !!  thanks,  as an aside, i had to replace the back wheel last week, the tyre i had on the old bike had a rolling diameter of 5cm less than the replacement and the bike did appear to go a little quicker, could this be correct ? 

[Propellor]

A lot to think about, i think i will just change the rear sprocket and accept the fact that i can`t improve the top speed much more than i have already, but it`s been a useful exercise !!  thanks,  as an aside, i had to replace the back wheel last week, the tyre i had on the old bike had a rolling diameter of 5cm less than the replacement and the bike did appear to go a little quicker, could this be correct ?

Smaller rolling diameter equals lower gearing!

Plus increased rear wheel torque.

I'd stick my neck out and suggest that the best gearing for top speed, with the bike otherwise stock, is 8000 rpm equalling 90 mph.

That's probably how it came!

Cheers

Edit: not increased rear wheel torque but increased thrust reaction at the tyre to road contact patch.

[spooky]

Well as suggested by a couple of you, there is barely any clearance for a bigger front sprocket, looks like i will keep gearing as it is !  thanks anyway...