Brixton BX Top End issue. (Sprockets)

[LatencyXR]

This question is for those that under the match behind sprocket set ups.

Whilst my CB300 is getting the engine replaced I brought a125 to get me to work. I actually own the exact model of bike back in 2018. That previous bike top out at around 58 to 60mph depending on wind and weather conditions. The current bike does 47 to 50mph.

The stock (previous bike) the sprocket set up was F14t and R46t

The current is - F14t and R45t

That's just -1 tooth down, but realistically on a bigger bike there's performance gains, but on a 125 isn't -1 tooth down just annihilated top end speed?

Whilst I know abit about bikes sprockets are difficult to understand for myself!

Really all I wanna know is .. does that -1 make a huge difference seeing as it's lost 9 - 10mph.
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Honda CB300R - More 125's then the heart can handle!

[MarJay]

Dropping teeth on the rear should theoretically increase speed. However, that only works on bikes with... well... power. Doing it on a 125 is just as likely to blunt acceleration AND top speed.

I'd try putting the original drive ratio back... but there might be more wrong too, as in a slightly f*cked engine.
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[LatencyXR]

Yeah I'll buy the original sprocket and give it a go, the engine is probably pretty fucked, goes through the gears (1,2,3,4) perfectly fine. Hell best I've seen on a little 125 but 5th gear.. jesus. If you red line in 4th then change to fifth it'll lost acceleration.. but change from 3rd to 5th .. it'll climb to around 47mph then stays there. To which I kind of understand that last gear on a 125 is pretty much a cruise gear rather then acceleration.

Oddly confusing! Engines been checked by mechanic to which he said it's had a great service history..
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Honda CB300R - More 125's then the heart can handle!

[Easy-X]

Try this:

https://sprocketcalculator.com/
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Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Ste]

According to my book of Tef volume 3 chapter 8:

Bigger the rear wheel sprocket, smaller the gearbox sprocket, 'lower' the gearing.

Smaller the rear wheel sprocket, bigger the gearbox sprocket, 'taller' gearing.

lower gearing = higher acceleration

taller gearing = higher top speed.

IN THEORY.

But, speed is an expression of 'power'.

Power = Force x Speed.

So Speed = Power / Force

Riding along, force you are working against is 'Drag', force you have shoving you is 'Thrust'.

And this is where it gets complicated!

Engine has a power curve. As the engine revs increase, so the power it makes increases, gets to a 'peak. then starts to drop off.

Coming from the engine, Power is Torque x Revs....

Gear up, and you increase the rear wheel revs, but you reduce the 'Torque' which is whats giving you the 'Thrust' to punch your way through 'drag'.

So, if you up-gear, it WONT necessarily make the bike any 'faster'.

Because the 'Thrust' you get from the motor isn't enough to over come drag... so the bike will actually go SLOWER.

Conversely, if you lower the gearing, you will increase the 'torque' and hence thrust at the rear wheel, which COULD give you some entra force to overcome drag and go a bit faster....

BUT depends on the shape of the power curve, where 'peak' power is 'made' and how much is drops off either side of that peak.

If you gear too high, then you will never get the engine to rev up to peak power and deliver full-force

Gear too low, you will run out of revs before you reach a road-speed where you lack 'thrust' to go any faster.

So to get 'best speed' its a question of finding the 'optimum' and finding the exact gear-ratio that gives you exactly the 'speed' that has the 'Drag' you have the power to beat....

Most bikes as standard are USUALLY slightly 'over-geared' so that the engine revs are knocked back for more relaxed 'cruising' at a speed a lot below thier pottential maximum.

So its more often that LOWERING the gearing, not only increases acceleration, but CAN see a few clics of extra speed.

BUT, thing to note is that GEARING doesn't 'set' how fast you can go...... but POWER.....

You can mess with years as much as you like, unless you have the engine power to make the THRUST force to shove you along that fast, you will NEVER go any quicker.

tl;dr your bike doesn't have enough power, go back to the original 46t gearing as MarJay say or if you're bored and want to play with things you could go up one more to see what that does.

[A100man]

[blurredman]

No it wouldn't.
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CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 50k.

[slowasyoulike]

<Googles "Brixton BX">

It's Chinese. If it runs and it's reliable then put it back to stock, leave it alone and be grateful.