Forum

[PeteF]

My maths is not up to this, I get confused with ratios and percentages.
I have a 250 Yam which is a bit under geared. I always seem to be reaching for an extra gear above top but first gear is plenty low enough. I reckon I need about 400 revs less at 60 mph. Front sprocket is 15 teeth, rear is 42. She turns 6000 rpm at 60mph approx. Bigger front sprocket is not available but I can get a smaller rear.
Question is, how many teeth less? Would two less be about right?

[Bullet Whisperer]

Yes.

[MartinB]

I used to own a 250 LC and an aircooled RD400 and found the forums specifically for those bikes very helpful.If you're not already a member i'd suggest googling the UK one's and putting a post on and am sure you will (as here) get some helpful advice.

[PeteF]

Thanks BW.
By the way, good job on Karl's bottom end! I gave him a hand to get the engine back in the frame and the top end on last week.

[PeteF]

Martin, I can't find a forum for the YBR 250.

[Thack]

A quick calculation says that going from a 42 to a 40 rear will give you lower your revs from 6000rpm to 5700rpm (near enough). Is that enough of a change?

[Thack]

Knocking three teeth off gives 5580rpm at 60.



HOWEVER: will someone else please check my maths? Thanks.

[PeteF]

300 would probably make all the difference Thack.

[PeteF]

Mmmm, the only RE I've had with a side stand switch was a Sixty 5. That behaved as I described. Could have been rewired I suppose.

[Bullet Whisperer]

Hi Pete F - sorry for the short reply earlier, I just saw your question as I was off to bed and I cut to the chase in answering. I have a 200cc Suzuki X5, as I did way back when I was 17 years old and they are geared for 10mph per 1000rpm in top gear, so a 250 should be a little higher geared, I would think. If it is a 2 stroke twin, try not to over gear it, or you will make the gaps between the gears too large and acceleration will suffer badly. Two teeth less on the rear should be about right. Thanks for the compliment re Karl's engine - it should be a good one now! Cheers, Paul.