How fast could a 125cc motorcycle go ?

[louis250994]

Hi, I am new to riding and I just bought a new 125cc motorcycle (AEON My125) about two weeks ago, and I was wondering if it is normal that the fastest I can get is 70 km/h at 5th gear ? Is it supposed to be this slow ?

Thanks.

[Doovy]

It's a 125cc, and a relatively unknown model one at that. But that does sound quite slow.
____________________
Yamaha RXS 100 > Honda CD 250 > Honda Hornet 600 > Honda CBR 600RR > Yamaha RXS 100 > Kawasaki ZX6R J2 > Yamaha FZ1

[arry]

That's 40 ish mph isn't it?

Unlikely it's running well. Unless you're an amorphous fattie.

[Azoth]

Usually, you have to thrash the nuts off a 125 in every gear before changing up. If it has a rev counter, change up only when it's about to go into red. It it doesn't, listen for the rev limiter.
____________________
Safety in numbers

[louis250994]

It's a second hand bike, and I am afraid it could be a problem causing it to be slow. I am 55 kg BTW so I guess that shouldn't affect the bike that much.

[louis250994]

[arry]

Punish it.

[Azoth]

[Rogerborg]

Cute little ride.

That's a CG-125 copy engine. Very basic, pushrods, but it's robust and it should - in good tune - be able to make enough power to get you to 100kph with the right gearing, or 90kph at the very least.

First, get it in good condition. Check the tyre pressures, chain tension and lubrication, and that the brakes aren't binding and the clutch isn't slipping. Consider changing the oil, it only needs 1 litre of any cheap 10W40 oil. If the engine is clattery, check and adjust the tappet clearances. Run it hard and pull the spark plug, check that it's not sooty or white - the previous owner may have tried to "improve" it. Actually... is that a fuel injector rather than a carb? Nice.

The rev counter goes to 12,000 and it's good for at least 10,000 of those. Rev it hard in every gear before changing up. You may find that it'll actually go faster in 4th gear depending on how it's geared.

If it's reaching 10,000rpm in 4th or 5th gear but still only doing 70kph then you could try lengthening the gearing. Try a 17 tooth front sprocket - a Honda CG sprocket will probably fit, but check the dimensions. Conversely, if you rev right it out in each gear but then it struggles and loses speed when you change up, then you may need to lower the gearing and narrow the ratios - try a 15 tooth front sprocket. Give it some abuse first though.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[louis250994]

Thank you for taking the time for your answer, I didn't know motorcycles worked like that. So basically, I've been doing it wrong since the beginning.
I'm gonna try that on my way to school.

[Teflon-Mike]

A four-Stroke 125 single, will generally make something in the order of 10bhp; in a 'normal' style motorcycle that should be good for 'about' 60mph or about 90Km/h. A little less if its a dirt style bike with high wide handlebars spreading the rider out like a parachute, or a cruiser with similarly wide bars and lots or heavy bodywork to drag along. Chinese built 125's though are usually restricted to 55mph for local market regulations; usually by down gearing them.

Either way, most 125 four-stroke singles, have a 'quirk', max power will be delivered at almost max revs; you need all the available power to reach max speed; but at half revs where they only have about half the power they usually have much more power than is needed to go half the speed.

This is because of the 'exponential' of wind resistance; Takes about 3bhp to go 30mph, takes about 9bhp to go 60, takes about 27bhp to go 90, takes about 81bhp to go 120mph...
Ie to go 30mph faster you need three times the power each time.

So, little 125 single, will pull well to perhaps 45mph, even in top gear, but to go any faster, it needs to turn faster RPM to find more power; BUT, at 45mph, it only has enough power at the revs it's turning to go 45mph, so wont accelerate and wont find more power, and you have to change down a gear to get the revs up and find more power to make it go faster...

Which seems counter intuitive... to go faster you need a higher gear, changing down makes you go slower..... surely? Well, it does, if you only stay in the one gear.. but changing down, like you would to go up a hill, can let you get the revs up, get at the power to accelerate, and get some speed up, and THEN you change up in to top, to hold it of eek a little more around that peak power you now have found.

And this niggle is encountered by many new 125 riders, who find that they gave a gear-box to play with, and the engine seems so willing at lower speeds that they change up too many gears far too early so that they dont have any acceleration in top to go any faster when they want it, and are loath to change down, in misguided belief that that's going to slow them down more, not help them go faster.

It is likely there's not much wrong with the bike; its probably still pretty slow, but is actually faster than you are getting, just because you aren't riding it right...

Tiddler's you gotta threash'em a bit to make em shift, you really do.

Answering the accademic question of the title, How fast can a 125cc bike go? WELL.... the fastest 125cc production bike, is disputed between the two stroke aprillia RS125 and the similarly two stroke Cagiva Mito 125. Both have set 'flying mile' or 'flying kilometer' times under record condtitions to clock a few tenths over 101MPH, from limited production 'racing' variants of the over-the-counter models, of which the earlier ones were usually the quicker in magazine tests and the like; as successive emmission controls reduced their power. Both of those bikes are notoriously unreliable for road use, but about 30bhp and reasonably aerodynamic 'race' fairings to help them to top speed.

The old two-stroke 125 Grand-Prix bikes of a few years ago, had about 45bhp as they were supplied to the 'teams'... and engine lives measured in running hours; teams might tweek them a bit more if they could afford the rebuilds, and a 'hot-lap' qualifying engine expected to last no more than maybe five laps, ten or fifteen miles, might offer something over 50bhp, and could clock perhaps 150mph on a fast circuit.

For outright speed records; there are a number of classes; and the 'ultimate' classes allow not only slippy cigar streamlining, but supercharged engines and special fuels and stuff, which make the engine displacement something of an irrelevance to a degree.

Last time I looked, the record for a 'normally aspirated' Ie non super-charged, or nitreose oxide injected 125cc engine, running high-octane, (special) fuel, but not 'nitro-' (explosive rocket) fuel, in a full stream-liner bodywork, was something still quite markedly under the 200mph mark, about 170mph I think, using specially tuned two-stroke engine's.

I struggled to find what had been achieved with a 4-stroke 125cc engine, in record breakers; as they didn't seem to differentiate in the non-production classes.

If you built one, you would struggle to get as much power from one as the two-strokes by a long way; if you could get as much power per cc, though as say a race tuned Yamaha R1, that would be about 200bhp per liter, you might get around 25bhp.... so you would be looking to stream-lining to get one to crack a 100mph, and even with that, probably not much more, maybe 120-130 or so, I would suspect.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[carpe_diem]

I got mine to about 55mph indicated today on the A52, but there again I weigh nearly twice as much as you so that's an achievement in my book. Man, 55kg? I've done shits that weigh more than that.
____________________
Currently riding: Suzuki EN125 | Previous rides: Vespa 125PX, Yamaha SR125

[The Shaggy D.A.]

[M.C]

[Teflon-Mike]

[Alawson68]

My 125 Vespa from 1968 tops out at 50mph, but the Yamaha RD 125 LC that I had in the 80's (de-restricted Swiss import), I had the speedo needle on the 'A' in Yamaha.
____________________
1968 Vespa Primavera
2016 KTM Duke 390

[NJD]

Something as slow as my arrow, at last, wheeeey.

Traffic light grand prix OP?
____________________
The do it all, T̶h̶e̶ ̶b̶r̶o̶k̶e̶n̶ ̶o̶n̶e̶,̶ ̶T̶h̶e̶ ̶b̶i̶g̶ ̶l̶u̶m̶p̶,̶ ̶C̶h̶o̶n̶g̶ ̶N̶o̶o̶d̶l̶e̶

[symonh2000]

[Teflon-Mike]

Lol.. oooh the 80's.... FM Galactica hats, Red & white leather jackets, coloured vynal saddles, and a row of variously melted pistons on the garage windowsill.... a mate you needed to borrow a spanner off round the corner who had a GSX250 and always asked "So how fast do you think you were going when it happened this time?" and another, down the pub, who'd tell you the exact same thing happened to him over Digbeth over-pass on his 250 Samuri in 1976!!!

Oh the nostalgia.. what happened to Digbeth over-pass? lol

I had an AR125... I left the bludy motor the fuck alone for three years!!! It wasn't the fastest 125 in the county..... but at least it worked!! I saw that off the clock a few times on big enough hills, but geez, the needle was wobbling all over the place! I think speedo's should have had the same sticker on them as pub fruit machines "For entertainment Purposes Only" on'em! Lol.

Was pleasantly surprised five years ago when I had renovated the first of the four-stroke 125 twin Super-Dreams; Snowie was still finishing hers but wanted to go over to a meet in Coalville, so we 2upped on the tiddler, and GPS snail trail when we got home showed a recorded 69.8mph over 1/2 mile of the M42.. probably slightly gravity assisted, but still; impressive for a 25 year old four-stroke tiddler, two-up! Speedo had been some-where off the clock wobbling about like an exited dogs tail, though! Guess they don't get better with age!

I have a 125 twin motor in the shed, I rebuilt last winter, waiting for a frame to drop it in; that has the 309 cam from the 'full-power' engine in it; begs suggestion of 16/17bhp over the 'reduced effect' learner-legal version that only had 12... be interesting to see if that can crack a genuine 80.

Curious, none of the modern 125's don't make more than the old 2v twin could; but seems to be a matter of scale.

Power = Cylinder-displacement x Cylinder Pressure x Crank Revs

So if you cant up the cylinder displacement you have to get it to cram more charge in the pot(s) and or rev higher to get more power, but I suspect shrinking the bore and stroke to be able to double-up and make a higher revving twin gives you two cylinders barely moped displacement; and little 'space' to get more than two-valves into the pot and have room for a spark-plug, which is why latest generation of water-cooled, 4v 125's are still singles. The 125 twin can certainly rev; red-lines at 13K and even the 'reduced effect' motor eagerly buzzes well past that; but little valves I presume just don't let them cram any more in'em. So, trying to make a 'four' to find even more revs, would probably be just as self defeating; so a single, is probably nearer an 'optimum' for making charge move, and can still be encouraged to rev high-enough.

Bigger multi's are still struggling to better 200bhp per litre, where even a 600 has pots bigger than a 125's, and the 'scale' means that they can sacrifice low rpm power, and 'flexibility', to tune it more for peak; as they have so much more to play with; on a 4T 125, in the same sort of 'tune' to get maybe 25bhp, the low rpm loss would be as bad or worse than the old two-strokes.

Suggests that a 100mph four-stroke 125 is a theoretical possibility...... but, practically? It would likely be an absolute pig to ride; and likely inordinately short of service life.

I recall the comments about the 1966, I think, Suzuki 4-cylinder water-cooled 2-stroke 125GP bike; I believe it only ever had one GP outing at the end of teh season, as FIM changed the regulations for following year; limiting the number of cylinders, and capping the number of gears they could have! 8 gears had become common on the super-tuned tiddlers, with motors that were almost fixed speed, they were so 'peakily' tuned, they wouldn't even idle much under stupidly high 'peak-power' revs; and the 'four' was even worse, had 12 gears, and I recall, comment it had to be started on rollers to get enough crank speed for it to run, and rider warned not to let it 'conk' cos it couldn't be bump-started!! Made an impressive 40bhp though, and clocked 145mph. Which isn't far off what the last single cylinder 125 2T GP bikes were punting thirty years later.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Hong Kong Phooey]

[carpe_diem]

[Teflon-Mike]

[kgm]

My CG will hit about 60mph on a flat road assuming no wind but you really have to thrash it to get it there. Any hint of a hill and it'll start to slow. 5th gear is only really any use when you get to the stage where you can't go any faster in 4th and much of the time it's not worth trying as with a tiny hill or head wind it'll start to slow down in 5th. 5th is fine for flat cruising but I rarely use it the rest of the time.

[Derivative]

My CBF125 does about 60-65. 70 is a thing that happens occasionally depending on incline and wind.

It can go from 20-60 in 5th gear just fine, it just takes a while.

I generally sit in the 50s because it feels better anyway. Too much turbulence.

[symonh2000]

I find it a bit dissapointing that modern learner machines are generally sloweer than the old ones.

Most of them struggle to get near to the 15bhp limit, some of the popular ones such as YBR's and CG's struggle to make 2/3rds of it.

I know 125's mainly are meant for learning, but unless they give some excitement the rider might just give up on biking altogether without going for bigger and faster machinary. A taste of bigger and better things needs to be offered IMO.

I know the vast majority of bikes these days are 4 strokes, but even in the 80's a good 125 4T would make 15bhp, and an old Yamaha RXS100 2T is still faster and more fun to ride than a modern commuter.

I know progress is progress, but it seems that much of the fun has gone from tiddler bikes.
____________________
Four strokes is two strokes too many.