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Do Light Bikes Need To Lean More?


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I noticed in a couple of scenes from Twist II that when all the bikes were riding around the track together, the kids on the little sport bikes not only DID lean more, but it looked like they HAD to lean more. The bikes were going about the same speed, so did they have to lean MORE to move enough weight inside to counter balance the centrifugal force? Just curious.

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Having watched GP racing on TV, my observation is completely opposite; we know the 125s corner faster than the 250s (of yesteryear) that again corner faster than MotoGP bikes, yet it seems that the amount of lean is the most for MotoGP and the least for 125s. My theory, which may be wrong, is that a) the narrower tyres and shorter wheelbase of the small bikes means less lean is required for any given cornering speed and B) the rider's weight makes up for more of the total bike/machine mass and hence hanging off will have a greater influence on moving the combined CoG to the inside of the corner, allowing less lean.

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Eirik is correct, light bikes need less lean angle for the same speed, hence why they can carry much more corner speed and get remarkably close to the bigger bikes laptimes on very twisty circuits. In actual fact, the total mass of rider and bike as a combination is what matters, hence the fight for low mass bikes and riders are all small. Now you understand why Pedrosa in theory should have such an advantage over larger riders.

 

Bullet

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Pedrosa hold several advantages:

 

1) He is lighter, so there is less mass to accelerate = quicker acceleration

2) He is smaller, so there is less drag = better high speed acceleration and top speed

3) Moving his body/bike combination require less energy than let's say Spies = he can use more power from the engine without running out of fuel

 

So with more power, less mass and less drag it's no wonder he can outgun most other riders down longer straights and also from a standstill.

 

But Pedrosa doesn't only have benfits; you would expect a smaller, lighter body to have less power with what to control the bike, for instance. And he has less weight to shift around to counter wheelies and stoppies.

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I noticed in a couple of scenes from Twist II that when all the bikes were riding around the track together, the kids on the little sport bikes not only DID lean more, but it looked like they HAD to lean more. The bikes were going about the same speed, so did they have to lean MORE to move enough weight inside to counter balance the centrifugal force? Just curious.

 

Having just gotten a light bike after riding a 600, I noticed that is it much easier to lean the lighter bike way over, and I am a lot more confident doing it, so I tend to lean it a bit more and hang off it less than I did on the 600. My bike is similar to the 125s you see in Twist, and it feels very solid even when leaned way over, because you have a whole lot of tire grip relative to the weight and power of the bike. Also it's harder to hang way off because the bike is really narrow and there is less to hang onto, plus you can't afford a lot of air drag caused by sticking body parts out, AND the bike reacts a lot more dramatically to body changes so I upset the bike if I move too far or not smoothly enough.

 

So I'm thinking maybe in the Twist DVD the BIKES don't need to lean more, but maybe the riders are hanging off less, and also it's easy and just plain fun to lean the little bikes way over. They could have been hamming it up for the camera a little, too. :)

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  • 4 weeks later...

I think I'm right in saying that the height of the centre of gravity makes the difference. If it's high then for a given lean angle, it's making the bike fall inwards more hence more cornering force and a higher corner speed. Moving your weight to the inside does the same thing i.e. provides more "falling in" (centripetal force) as the C of G is moved farther (sideways) from the pivot point (tyre contact patch on the tarmac). Something like a 125 probably has a higher C of G as the rider is a bigger portion of the total.

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  • 3 weeks later...

I've spent a bit of time looking into the maths on this after Bullet said that light bikes don't need as much lean angle as heavy bikes for the same corner speed and I just don't get it.

As far as I can see 2 bikes with the same centre of gravity etc but one twice as heavy as the other need the same lean angle to generate the same rate of turn.

The 2 main differences as far as I can see are: 1) lateral forces on the tyre and 2) the effect of the rider i.e. the lighter the bike the more effect the rider has hanging off etc

 

With our 2 theoretical bikes with the (identical) rider hanging off the same amount the lighter bike would require less lean angle than the heavy bike for the same corner speed because the rider has more relative effect on the combined centre of gravity.

 

As, essentially, lighter bikes can carry more corner speed because the effect of the rider hanging off is much greater and although the contact patch tends to be smaller they generate less lateral force through the tyre raising the level at which the tyre will break traction.

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