How fast should a 9stone person expect to go on a cg125?

[mchaggis]

The overall drag is comprised the aerodynamic drag and the mechanical drag associated with the tyres and transmission. Transmission losses are normally treated as an efficiency and applied to the engine power to find the 'power at the wheels', rather than just the power the engine gives out.

The losses from the tyres is about 90% due to hysteresis losses from the cyclic deformation of the tyre as it passes through the contact patch (have a look at a car tyre whilst it's moving, it's sometimes surprising how flat they look), and the other 10% is accounted for by slippage. Tyre rolling resistance (drag for sake of argument) is influenced by the normal load, inflation pressure, temperature, applied torque, speed and road surface material. The main factor is the speed, which is approximately linear from 0 to about 80mph, after which it increases exponentially. (Hence, it's not particularly important on a CG125, and is only about 1% loss anyway). The increased vehicle weight will make this loss higher, but I doubt it'd have much effect on your top speed.

Aerodynamic drag is a function of vehicle speed (air speed) squared, and weight will have little effect except for likely increasing the frontal area. More important is what type of clothing you wear imo, as the coefficient of drag will vary much more than the frontal area of a rider will. Flow is also hugely affected by the way the rider sits; sheer bulk makes it more difficult to get behind the fairings/ screen (cg125 haha...) This will increase the direct drag on the front of the rider, as more air will go down his front, making a nice high pressure peak on the front of the rider's helmet (I'd think; otherwise it'd be down on their chest somewhere I'd think) as the flow stagnates and splits to go either up, down or around the helmet.

The angle the back of the rider makes will differ the extent of the seperation of the flow from his back, changing the wake size, giving different induced drag. The ideal angle is 0 degrees (horizontal) but that's not exactly achievable.. Huddle down, your back will be at a better angle to reduce seperation and give a smaller wake, and less air will get down your front, so the pressure will be lower on your front too.

The essence of induced drag and the size of the turbulent wake is that the turbulent wake is an area of chaotic, lower pressure air behind the bike and rider, which in effect sucks you backwards.

I was going to draw a picture to explain the angle of the back meaning more or less induced drag, but I give up, MS paint sucks.

Enough of this, there's no-one going to read it, let alone try and understand my witterings.. Did I get off topic?

Basically, what they said. More weight should mean no significant loss to top speed for a 125, just lower acceleration. A half percent difference at 200 odd mph in tyre rolling resistance will mean that extra mph too. Smooth leathers and crouching extra close to the tank and pushing your arse back in the seat should make most difference though. (And don't forget to close the vents in your helmet too, don't even get me started on internal flows and corresponding loss factors... )

[Ninja]

So, on a 'bike at approaching 200mph, these factors amount to approx 2mph difference? With that 2mph arising simply because of the size of the loads at those speeds . . .

So our 65mph CG125 with a 7st bloke will approximate to something like 64.999943mph with a 17st bloke aboard . . .?

Sounds like a bigger factor in this top speed comparison could be attributed to whether or not the guy was farting at the time - this miniscule applicatin of jet propulsion could surely add more than 0.000058mph?


____________________
James
4 wheels transport the Body ... 2 wheels transport the SOUL

[mr.z]

The most i ever got out of the cg (and holy electric gerbils did i try)
was an indicated 75/6*, this was down a VERY long stretch, theres absoloutely no was it could have gone faster.
The question is how fast could YOU go on it, after 65 its will squirm and shake, engine screaming its poor head off, bounceing arround on the joke tyres, things rattleing arround undoubtable ready to fly off at any moment... can't say i'd reccomend it :o

I'd say generaly the most you will get out of it is 60, 50 and lower for hills which you will learn to dread...

I still wish i had not sold my cg, it was practcaly free to run (150mpg!!), NEVER went wrong, i dropped it once and all it cost me was £5 for a new lever (which i could have got cheeper at a breakers no doubt) it got me a couple of hundred miles several times without incident.

*Apart from when useing it for my das for the first day when the speedo broke and it was showing +90
____________________
>RidingSkills<->Tech Tips<->MyBikes<

[Mr C]

Well said McHaggis

a much more eloquent explanation than my fogged memory of lectures long gone could conjure up

hoorah



apart from the bit about helmet vents, I get no airflow at all around my helmet as it it tucked right into the bubble - if it's a bit damp the visor steams up

no airflow at all
____________________
187.3bhp = 201mph

[G]

I usually got 65 out of my cg after a long in enough flat straight.

Managed 75 once with the windbehind me on a downhill .

[billy whizz]

What was the question oh yes i remember that weight had no effect on top speed, well thats a load of bollocks cos i just did an experiment and it did. i just dropped a budgie feather and an apple at the same time and the apple travelled to the floor a shit load faster! Now i may be Irish but.................
____________________
Here comes the summer!

[Marc_Buck]

That is not only a question of weight though, that is also a great difference in drag or what ever it is called, get two items the same shape but different weights eg football and bowling ball, roughly same size and shape but different weight, see what falls faster, none both the same.

[G]

[Mantzy]