Bike vs Car

[Andres125sx]

Since cornering speed depends on too many things I was trying to suggest an scenario where you can compare both easier, and that´s equaling one of the parameters (acceleration) to see if it´s posible and what would happen in that case at the corners

But I agree even so it will be too dificult... they´re too different to equal any parameter, acceleration is affected by weight, total power, traction... so it can´t be equal at all speeds



To me the reply is they are too diferent to compare as a theory, you can´t compare a generic bike vs a generic car because there are too many unknown parameters (weight, tire compound, tire size, speed or corner radius...)

The most real comparison would be between fastest bike cornering vs fastest car cornering, in that case the car will always win, even without downforce, right?


Maybe that´s a good scenario to do some real comparison and get some real numbers

[Tim.Wright]

The differences in the vehicle are largely irrelevant when you are talking about a grip limited situation (i.e. cornering). The vehicle with the better tyres will usually be the fastest.

Weight and power have a relatively small effect when you are not power limited. A comparison of steady state cornering between a bike and a car is not such an impossible comparison...

[Andres125sx]

The differences in the vehicle are largely irrelevant when you are talking about a grip limited situation (i.e. cornering). The vehicle with the better tyres will usually be the fastest.

Obviously, but if the question is bike vs car I guess we should assume same tires/compound

Weight and power have a relatively small effect when you are not power limited.

Don´t understand this, may you explain it?

Power is irrelevant for cornering, but weight is crucial, that´s the reason Moto3 bikes are faster around corners than MotoGP

[Greg Locock]

I wonder why skidpan lateral acceleration numbers are regularly published for cars but not for bikes?

Think about the implications before you publish your answer.

[Phil]

Phil, have you raced against any light bike?

Yeah, I have. But any comparison is difficult to draw, because my car isn't nearly as fast in a straight line to be remotely as quick on just about *any* track - that is over the entirety of a lap. The only conclusion I've been able to draw is that in small tight corners, my car has a lot more grip. Tight corners being below 80kph. At some point, in high speed corners (~>140kph), bikes are extremely fast and have me beat, which isn't that hard either, since my car at 1000kg (with me in it) and semi-slicks isn't setting anything on fire. Now, my friends Caterham R500 is a different matter - they destroy me in any corner and are seriously quick even in a straight line speed. That's my subjective hands-on.

If I was to put any scientific meaning to this discussion on cornering grip between bikes and cars, I'd say you'd come up with some kind of diagram (X axis being corner radius for a particular corner, Y being speed) and a car (defined by weight and contact-patch) would probably be quicker in the low radius corners, but the higher the radius gets, the more the bike would close the gap until eventually outpacing the car thanks to the weight advantage. This would be a somewhat simplified illustration where we are assuming that tyre compound to be identical. Think of the diagram as perhaps a simple test where you test that car against the bike for Vmax on corners, with radius 5m, 10m, 15m, 20m, 25m ...... etc. I'd imagine (assuming perfectly driven vehicles) that the resulting curve would be quite consistent.

[Tim.Wright]

I wonder why skidpan lateral acceleration numbers are regularly published for cars but not for bikes?

Might have something to do with max Ay corresponding to the point where the rider falls off?

The differences in the vehicle are largely irrelevant when you are talking about a grip limited situation (i.e. cornering). The vehicle with the better tyres will usually be the fastest.

Obviously, but if the question is bike vs car I guess we should assume same tires/compound

This isn't really a valid assumption in this case for the reasons I mentioned above. The construction of the tyres are responsible for the fundamental differences in cornering performance. If you assume the tyres' performance are the same - the cornering performance will be the same.

Weight and power have a relatively small effect when you are not power limited.

Don´t understand this, may you explain it?

It branches off from what I explained before but I'll make an example because it can be a little bit abstract to understand. The effect is called "tyre load sensitivity", maybe if you search for that term you might find something clearer than what I have written below...

Consider a non-aero "vehicle" (i.e. 2 or 4 wheels) on an ideal tyre which has a constant coefficient of friction of 1.2 under all conditions. This means that for a given vertical load it can produce 1.2 times that in cornering force. Now given that the only vertical load is only coming from the weight force in a non-aero vehicle then it follows that with a coefficient of friction of 1.2 you are able to do 1.2G of cornering acceleration regardless of the weight. A heavier car produces more vertical force and therefore more grip but it also requires more cornering force per G of lateral acceleration so the 2 effects "cancel" each other and you are left with a cornering performance which is independent of weight.

In reality, a tyre does not have a constant coefficient of friction but instead on that drops with vertical load. Typical values might be 0.01 - 0.05mu/kN (very rough numbers btw). So a change in mass is going to change the peak Ay by something in the range of a few percent. E.g. a 10% reduction in mass on a 1000kg car will net you about 1-5% more grip depending on the tyre.

Once you move into the power limited range, the "F" in your F=ma becomes the limiting factor and your acceleration performance decreases proportionally to the extra mass (instead of just a small percentage of it). In this case, a 10% reduction in mass will give you 10% more longitudinal acceleration.

Coming back to the discussion, the reason I was discounting weight from the equation here is that with a totally different construction between bike and car tyres is doesn't make a lot of sense to be discussing the tyre load sensitivity effects because they are small potatoes compared to the difference in the baseline coefficient of friction.

Also, tyre load sensitivity effects are different tyre to tyre which is another reason why you can't say that a bike will have more grip because it is lighter than a car because the load sensitivity of a car tyre is not the same as that on a car tyre.

[Andres125sx]

Thank you for that great explanation Mr Wright! =D>

In reality, a tyre does not have a constant coefficient of friction but instead on that drops with vertical load. Typical values might be 0.01 - 0.05mu/kN (very rough numbers btw). So a change in mass is going to change the peak Ay by something in the range of a few percent. E.g. a 10% reduction in mass on a 1000kg car will net you about 1-5% more grip depending on the tyre.

So I guess this must be the reason Moto3 bikes are faster around corners than MotoGP, here we´re talking about bikes vs bikes so this should be the only, or at least the main difference. MotoGP + rider weight is around 60% heavier than Moto3


I´ll take advantage of the occasion to see if you may solve a contradiction I´ve never managed to solve by myself, and the reason must be related with the discussion. It´s about the reason on MX bikes (not only on MX bikes, but that´s been my pasion for more than 10 years) you need to move your body forward or backward to improve traction on the wheel you need. Move forward to enter the corner and maximice turning perfomance, and move backward at the exit to improve traction and avoid too much sliding. This is a basic technic in motocross world.

My first though was since the total weight (bike + rider) is always the same, but you´re changing the vertical load on each tire, you´re changing the grip.

But then I think the total mass of the bike must be irrelevant for each wheel, because if you´re moving your weight forward/backward you´re also changing the mass each wheel must "hold", so the two effects should cancel each other as you´ve just explained...

But moving your weight forward improve grip on the front wheel or viceversa, even when I can´t explain it

[J.A.W.]

Against a no-downforce car, what would the bike do, in terms of cornering g. Assuming no driver/rider error, and exactly the same tire compound?

Check out the lap times of a fast sweeping natural track such as the Phillip Island G.P. circuit, or Laguna Seca..

Fast bikes, (even fairly low powered 250cc machines) are quicker around there than non-downforce cars..

& obviously, practicable road cars, at most road going speeds, utilize little in the way of downforce.
Tests of current sticky tyre roadgoing superbikes show they can corner quicker than most road cars..

[Erunanethiel]

So far, my conclusion in this thread been that bikes corner the same speed as cars even though they have smaller contact patches thus having less "adhesive" grip from their tires, but that effectis mostly cancelled out by the lack of weigh. Cars stop better not because of their tires but their low CoG. And bikes acclerate faster because of their high power to weight ratios. How am I doing?

[J.A.W.]

One track vehicles have fundamentally different cornering dynamics/characteristics E..

There are bikes which weigh more than 4 wheel karts, & different wheelbases also affect cornering too..
& the fact that bikes bank in turns, & increase potential tyre grip/G-factor with further bank angle, up to ~60%.

Bikes are able to exploit different lines, due to less width, & can 'straight-line s-bends..
Cars are mentally easier to drive fast, but bikes are more rewarding, & esp' if you are skilled in riding..

Most road cars are out-braked by performance bikes, & in repeated stopping/track use this is even more evident.

[J.A.W.]

I wonder why skidpan lateral acceleration numbers are regularly published for cars but not for bikes?

Think about the implications before you publish your answer.

Because bikes are not routinely tested doing 360`turns on skid-pans..

Tyre manufacturers do it ( with out-rigger frames in case of mis-hap) though..

& lateral G - is one of the parameters captured by the sensor/telemetry suites of Moto G.P. bikes ( & others)..

[Tim.Wright]

So far, my conclusion in this thread been that bikes corner the same speed as cars even though they have smaller contact patches thus having less "adhesive" grip from their tires, but that effectis mostly cancelled out by the lack of weigh. Cars stop better not because of their tires but their low CoG. And bikes acclerate faster because of their high power to weight ratios. How am I doing?

Thats not really where I was going. The fundamental difference is the contact patch geometry which is much bigger in a car therefore it typically has more grip and there for more acceleration in a traction limited condition.

The weight effects (the 1%-5% effects I discussed above) are not enough to change this trend.

As much as I don't have much confidence in youtube videos, advertising BS and and unverified magazine track tests, the video posted previously shows this trend perfectly. Traction limited areas - the car is faster. Power limited - the bike.

[Tim.Wright]

If you want some reading material, Milliken, Pacejka, Guiggiani and Mitschke will help you a lot.

[Tim.Wright]

Andres, I will answer your other question regarding the load transfer in braking and traction later - I understand the reason but I need to formulate a concise answer in my head otherwise it will send the discussion off on a tangent.

[Andres125sx]

Andres, I will answer your other question regarding the load transfer in braking and traction later - I understand the reason but I need to formulate a concise answer in my head otherwise it will send the discussion off on a tangent.

Perfect Tim, thanks!