are 125s overgeared

[czakal]

Now that I'm back on the road after a year (and back on 125 again at that) I'm starting to remember some tips from my instructor. He mentioned that the CG125s we were using was overgeared, so 5th gear was often too high in most cases. Given that I've rarely ridden outside city environments I rarely had a chance to see it in practice. Has anyone had experience with this issue with small cc bikes?

[Ben90]

[Teflon-Mike]

Almost all road bikes are 'over geared' in so much that the top gear ratio provides a top speed higher than peak power will allow.

Most 125's, without the amount of power bigger bikes have, tend to be far less over geared, and the top gear ratio fixed much closer to providing the top speed their power will allow.

Matter I suspect is somewhat mute, though, and I suspect that the missing link in what you recall from what your instructor was trying to tell you was "Use the Revs, not the gears"

Forget ratios'... little CG has a 10bhp engine, that is pretty linear in its delivery and power pretty proportional to engine revs, so if t's deverng 10bhp at 10,000 revs its it's delivering about 1bhp per 1000, so 1bhp at tickover, 2 at 2K, 3 at 3K, 4 at 4 etc.

Trouble is, that 'drag' increases in a ramp that's not linear. Takes around 3bhp to go 30mph, 9bhp to go 60, 27bhp to go 90.. ie to go 30mph faster takes 3x the power.

Little CG125, can make 3bhp with the throttle wide open, as low as 3ooo rpm, so go 30.... and it can probably do that in top gear... but try and accelerate.... if you go faster, engine can deliver more power, but takes more power to go faster than the power you get for it... so you have to change down, to get the engine up to revs where there's power to let you go faster.

And the trouble with little learner bikes, is that it seems counter intuatve to most learners to change down to go faster, and round town dong most learner exercises, usually on sub 40mph roads, very easy and feels more natural to short shift and use the higher gears, than to let the thing rev in the lower ones, ad that denies throttle response and a lot of potential 'control'.

Its not so much that the gearing is inherently too tall, it isn't. It's just that they don't have a lot of power, and learners who tend to try ridg them in too tall a gear, and doing too much work on the gears, changing up too much making work to change back down again. in their nievity and enthusiasm and ignorance, and little bikes don't have the excess to give them the flexibility to make then not so important.
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[Rogerborg]

Agreed, 5th gear on my 125 was pointless for top speed. It was only there to keep the revs down a bit on the flat or downhill.

The answer to any followup question is: get a licence, get a bigger bike. Even my Ninja 250 is far better, and the gearing is spot on for its top speed.
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[czakal]

Thanks for the tips, very informative as always. I did have a YBR250 that was very comfortable for open carriageways... until it was badly damaged in an attempted hotwire theft. I sold the remains for scrap and stayed off the road for a year. I decided it's safer to get a bargain basement machine this time.

[Teflon-Mike]

[czakal]

It was for £600, if I use it for less than a year it pays back in savings on season tickets already.

[rhys99]

My Aprilia isn't, can hit the rev limiter easily in 6th gear.
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[czakal]

Isn't it 2 stroke?

[czakal]

As another note, my choice of a cheap 125cc bike was also because of the London low emission zone where anything from before July 2007 will be subject to a daily charge. Many of the cheaper large bikes tend to be quite old.

[mudcow007]

5th on my CG actually made it go slower.

[rhys99]

[M.C]

Hmm... my XR (same engine as the CG but different gearing I believe) used to accelerate in 5th and you needed it to reach maximum velocity, which was normally dependent on the wind that particular day.

I can see 6 gears being overkill on a 125, I wish the MT-03 had a 6th gear just to shut the thing up at high speed.

[kgm]

[mudcow007]

50-55mph on the flat, in 4th with me laying on the tank with legs tucked up was its terminal velocity

5th an the revs would die rapidly

to be fair it was a 1984 CG

[kgm]

My 2001 does about 65 on the flat tucked in but needs 5th to do it. Has no trouble getting to 55 without a special effort.