Support GPR Camp!!

Rear Sprocket Question

Discuss technical information.
Post Reply
User avatar
grunbay
Posts: 17
Joined: Thu Oct 04, 2018 7:01 pm
Location: Southern Maine
x 5

Rear Sprocket Question

Post by grunbay » Mon Nov 05, 2018 1:46 pm

I have a 2000 GPR 80 (EBE075). The rear sprocket is in need of replacement. As I look into acquiring a sprocket, there seems to be a model year break at 2004. After 2004, rear sprockets are fairly available. I can't find rear sprockets that purport to fit a 2000 GPR.

Are the rear sprockets really different? Sometimes the parts are the same, but the listings are inaccurate.

My center hole is 108mm, the bolt circle is 123mm, the chain size is 420. My 2000 appears to have a built-in offset or dish. Perhaps that is the difference? Are the later sprockets flat instead of dished?

Any suggestions?
0 x

User avatar
toratora
Posts: 3161
Joined: Wed Oct 11, 2017 1:05 am
Location: San Francisco
Replica: Derbi GPR
Cagiva Mito
Cobra CX65
Aprilia RS50
x 1768
x 1977
Contact:

Rear Sprocket Question: Counting Teeth

Post by toratora » Mon Nov 05, 2018 7:39 pm

Image
Sachs MadAss Sprocket Specs
There are a number of ways to go. Basically you wont easily find the stock sprocket, which is something around a 46 tooth. I actually can't remember the number at the moment. If you could count yours that would be nice. ;)

What I did with my EBE075 bikes is install the 52 tooth sprocket on them that's employed on the 50s. I don't really suggest this though. It mostly happened because I just swapped 17" wheels on the bike, and they had a 52, or I put an EBE075 into a 2004 bike. :shock:

If you decide to go that way you can order either a 52, or 53 from one of the 50cc GPRs. Another option is to get as custom one made. Sprocket Specialists can make one for you. I have a few of their sprockets that came with one of the track bikes I've bought.

Another option is to source a sprocket from a Sachs MadAss because the 50 and 125cc bikes employed the same 16" Grimeca wheels as the older GPRs. These are available as a 46 tooth, which just might be the same size as fitted on your bike—although I'm not sure about the dish factor. :?
0 x

User avatar
jkv357
Posts: 228
Joined: Fri Dec 01, 2017 6:47 am
Location: WI
Replica: '99 GPR EBE-075
x 441
x 118

Re: Rear Sprocket Question

Post by jkv357 » Tue Nov 06, 2018 6:13 am

Pretty sure my 2000 GPR EBE 075 has a stock 42T rear sprocket.

I went up 1T on the countershaft sprocket to drop a few revs, and it's been fine. My bike has a 24mm Flatslide and an Arrow pipe, and once jetted properly can pull the taller gearing without a problem.

There is an offset to it, so a flat sprocket would not line-up.
1 x

User avatar
grunbay
Posts: 17
Joined: Thu Oct 04, 2018 7:01 pm
Location: Southern Maine
x 5

Re: Rear Sprocket Question

Post by grunbay » Wed Nov 07, 2018 4:09 pm

Ah..... there it is again, those bloody 16" wheels. They run a weird sprocket! Until I find a set of 17" wheels, I'll just have to deal.

I have a thought on adjusting for the dish. The dish is +- 3.5mm deep. How about using a flat sprocket, but shimming the counter sprocket inward 3.5mm with a 3.5mm shim just under the sprocket lock. There may just be enough room on the output shaft. I'll try that.

Again, thanks for your input on my racer project.
0 x

User avatar
toratora
Posts: 3161
Joined: Wed Oct 11, 2017 1:05 am
Location: San Francisco
Replica: Derbi GPR
Cagiva Mito
Cobra CX65
Aprilia RS50
x 1768
x 1977
Contact:

Rear Sprocket Question: Width

Post by toratora » Thu Nov 08, 2018 11:39 am

From my understanding the 16" and 17" three spoke wheels from 2004 back are the same in the hub area. So there's no difference from the wheels being 16" in this case. Wheels from 2005 on are different, but they wouldn't fit on your bike as they are wider.
0 x

Post Reply