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Hi All,

I have been searching the old threads and found much info, but didn't manage to find the answer to my specific question, so wondering f someone can assist, please?

i am aiming to replace the Comstar wheels on my 1978 CX500 with spoke rims and radial tyres.

From another project, I currently have in stock a couple of pristine spoke rims off my other project, a 2008 Thruxton.

Front Rim: 18x2.5 which takes a 100/90/R18 tyre vs the standard Comstar 19x3.25

Rear Rim: 17x3.5 which takes a 130/80/R17 tyre vs the standard Comstar 18x3.75 and has 4.25x85 H 18 tyre

I think I can overcome the hub lacing issues by using the Cognito Moto hubs (rear with disc brake combo) - so great to see that info in the threads - many thanks. Plus I have GSXR front end and forks are set up for 17inch rims, so hopefully can adapt to take the 18inch rim and get the offsets, +/- speedo etc right. I also read that using a 130 tyre with the original swing arm should be ok.

My questions are:
1) Will using a smaller rim on the rear create an issue for the final drive? The OD of the 130/80/R17 is 610mm and the OD of the 4.25x85 H 18 tyre is 635mm being 25mm (1 inch) taller than the new wheel and tyre combination.

2) Will the overall performance of the bike using 18inch front and 17inch rear wheels be impacted in a material way?

I did see the thread on the earlier build using 17inch wheels front and rear and the bike still looked great. I wasn't sure though whether the original final drive was maintained on that bike as he had migrated to a single sided swing arm.

Many thanks
 

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Surprised no-one else has jumped in here.

Using a wheel/tyre with smaller diameter will of course reduce the gearing, so more revs for less speed. If you don't mind that, go for it. Otherwise you could try and find a 'taller' tyre that would increase the 'rolling radius' without going too wide to fit in the Swing-Arm.

I think it's a case of 'suck it and see' as it's all down to personal preference.
 

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The biggest problem I can forsee is the tyre clearance in the swing arm. I doubt you can get that rear tyre to fit in the gap....
 
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