Weight and Acceleration?

[andyto]

How do you work out the effect of different weights on the performance of a car. For example, given the identical driving style and conditions of 2 drivers, in the same car going round Snetterton, one driver weighs 65Kg the other 95Kg. What will be the difference in lap time?

Show workings. This question is worth 10 marks.

[Adam43]

It depends on the car (weight and power and...). If you are looking for a simple model then I would start by concentrating on the acceleration (under power and braking) and just use simple Newtonian physics. Leave the in-corner stuff till later.

The professional teams have very sophisticated modeling to predict this. Which probably consider all aspects including a change of weight distribution and what the driver had for breakfast.

[AU N EGL]

and what is that mathematical formula?


http://en.wikipedia.org/wiki/Acceleration

[andyto]

[quote=AdamAshmore]It depends on the car (weight and power and...). If you are looking for a simple model then I would start by concentrating on the acceleration (under power and braking) and just use simple Newtonian physics. Leave the in-corner stuff till later.[quote]

It must be quite a conplex application of 'simple Newtonian physics'. The weight (mass) of the car and the power are equal for both cars, so you can normalise these in both cases. So

Acceleration = Force/Mass (force is equivalent to power at say 200bhp)(let's say the cars mass is 500Kg)

Car with light driver: a = 200bph/265Kg = 0.75

Car with heavy driver: a = 200bph/295Kg = 0.68

That's the easy bit. Now how do I take into account all the braking and acceleration zones around the track (eg Snetterton)? Note: the nature of the track has a significant impact on the difference.

[AU N EGL]

Originally Posted by andyto

Acceleration = Force/Mass (force is equivalent to power at say 200bhp)(let's say the cars mass is 500Kg)

Car with light driver: a = 200bph/265Kg = 0.75

Car with heavy driver: a = 200bph/295Kg = 0.68

That's the easy bit. Now how do I take into account all the braking and acceleration zones around the track (eg Snetterton)? Note: the nature of the track has a significant impact on the difference.

Dont worry about it. The heavier driver will be slower, if their skill level is the same.

Why do you think most race car drivers and jet fighter pilots are short and light wt?

So for qualifying use the lighter driver to be faster.

[jonners]

or how about 2 different cars - one light with low power, one heavier with more power...

...but the power to weight ratio is the same

Which is faster?

Does driveability win the day??

Sorry for the hijack but have always wondered about this one

[AU N EGL]

Originally Posted by jonners

Which is faster?

Does driveability win the day??

Driver skill almost always wins over speed.

[Mak]

Think the idea of the argument is with the "same driver" but extra kilos how much diff is there in secondes on a lap.. Basically needed for race strategy. As a race driver you should aim for keeping your weight to minimum without sacrificing fitness to drive consistently.

[AU N EGL]

This what we figure for our cars

"1 second / lap = 100 horsepower or 100 pounds"

Then again wt and timing are very important. However, traffic has so much of an effect that ppl can not measure.

Lap traffic hurts lap times more then one can compensate for. However, 'using' lapped traffic to pick and roll' or block your competition is vital. ie pick the proper place where YOU cans safely pass lapped traffic, but your opponent who is right behind you can not get by for a partial lap.

[JamesH]

[quote=andyto][quote=AdamAshmore]It depends on the car (weight and power and...). If you are looking for a simple model then I would start by concentrating on the acceleration (under power and braking) and just use simple Newtonian physics. Leave the in-corner stuff till later.

It must be quite a conplex application of 'simple Newtonian physics'. The weight (mass) of the car and the power are equal for both cars, so you can normalise these in both cases. So

Acceleration = Force/Mass (force is equivalent to power at say 200bhp)(let's say the cars mass is 500Kg)

Car with light driver: a = 200bph/265Kg = 0.75

Car with heavy driver: a = 200bph/295Kg = 0.68

That's the easy bit. Now how do I take into account all the braking and acceleration zones around the track (eg Snetterton)? Note: the nature of the track has a significant impact on the difference.

It's all down to acceleration and force. Deceleration is a -ve accerelation. Your tyres have a maximum they can take before they slide. If you know that you can work out forces, and therefor work out max acceleration and max deceleration in a straight line using the formulas above. You can also use the equations for forces in a circle (can't remember offhand - look up centripetal force) to work out maximum speed around a constant radius corner. To work out for a particular corner with varying radius would involve some pretty hairy differential calculus. You then need to do some more hairy calculus to combine all these individual figures to work in out for the whole circuit. This would give a perfect figure for a perfect line around the circuit driven by a perfect driver with no mistakes. So you could work out the theoretical difference the mass of the drivers make.


Or, you could run both drivers, and see which is the faster.

James

[phoenix]

Before getting deeply involved in calculations, be sure to use the correct values!

Force in your calculations should be units of Torque, not BHP - and note that the value for torque should be that at the tyre contact patch - not the engine output. To complicate matters, this value will vary with engine speed and gearing. In the calculations shown here you have used all-up weights of 265Kg and 295Kg - which if the car weighs 500Kg should be 565Kg and 595Kg I think.

The difference in performance between the drivers due to their weight alone will depend on the overall weight of the car. In a 500 Kg car, the total weight with the lighter driver would be 5% less than with the heavier driver. In a 1000Kg car the total weight with the lighter driver would be 2.7% less than with the heavier driver, so you can see that the difference in driver weight is less significant in a heavier car.

Certainly any increase in weight will have an impact on acceleration, braking and cornering. In the case of a 500 Kg car I doubt very much the difference in lap times could be predicted to be 5%, however.

How accurate, and therefore how useful, any calculations could be in predicting lap times is very difficult to say. Having said that, in F1 they seem to know the impact on lap times of a just few kilograms of extra fuel.

[jonners]

Originally Posted by AU N EGL

Driver skill almost always wins over speed.

There's a difference between driveability and driver ability

[GORDON STREETER]

Originally Posted by AU N EGL

Dont worry about it. The heavier driver will be slower, if their skill level is the same.



.

I think Gerry Marshall proved that point !!!!

[AU N EGL]

Originally Posted by jonners

There's a difference between driveability and driver ability

Yes there is

Driveability has to do with the car set up
and

Driver ability is the racecraft or skill of the driver

[Adam43]

Originally Posted by andyto

It must be quite a conplex application of 'simple Newtonian physics'.

Yep. They are still simple laws though.

Note: the nature of the track has a significant impact on the difference.

Yep. Go to it.

You could get some experimental data, instead treating this purely thoeretically. What about the same driver in the car with a minimising fuel load. Or with and without ballast? Even then you would need to worry about changing tyres, conditions, position of the ballast, consistency of the driver, different tracks. However it still could get you to the answer quicker than modelling it.

[AU N EGL]

or just go buy a Tragmate or some other data logger. Run some test laps with each driver and different set ups.

[jonners]

Originally Posted by AU N EGL

Yes there is

Driveability has to do with the car set up
and

Driver ability is the racecraft or skill of the driver

I think driveability has more to it than set up

Just to make it easier - assume 2 driverless cars

one light and less powerful, one heavier and more powerful

but the power to weight ratio is the same

in general terms which is faster??

[JamesH]

The lighter one. In a straight line they would be about the same (ignoring effect of tyre grip changing with mass, which you shouldn't really), but it requires less tyre grip to turn a light car, so you can use more speed in to the corners.

Hold on a second.....hmm.....not sure that first statement is correct, again, all down to tyre grip vs mass. If that relationship is non-lineat then it all gets very hairy.

Oh well. Go out and practice, it won't be as time consuming!!

[Mak]

Not sure that anyone here should really be worring about the small effects of weight on track performance. The point is when racing in a series you should aim to bring the car in at the minimum weight. Also weight alone does not make the difference. I can prepare a car which weights more but is quicker using same engine power. IE where the weight is distributed and where you take weight off, rotating parts. Also keeping all areas of friction to a minimum would have more effect . In an ultra competetive series with strict regs (ie one make with strict regulations and restrictions) especially low power formula will make keeping weight low more important.

[tristancliffe]

Tyre load sensitivity suggests that for two identical cars, but one heavier than the other, the lighter one will be able to corner faster. The overall coefficient of friction will be higher on a light car, so even with a lower tyre load it can generate more cornering force per mass of car.

[AU N EGL]

Originally Posted by jonners

I think driveability has more to it than set up

Just to make it easier - assume 2 driverless cars

one light and less powerful, one heavier and more powerful

but the power to weight ratio is the same

in general terms which is faster??

The lighter one, on a low speed highly technical track. The one with MORE HP or heavier one on medium technical to high speed track.

As far as cornering it would depend on the corner. a high speed carousel or high speed esses the heavier car would be faster, larger tire contact patches

Have done many tests like this myself with two different cars, yet the same HP:WT ratio.

I race in a HP:WT ( 8.7:1) restricted class, so that is identical. The heavier cars with more HP always win unless it is wet and raining, then the lighter cars win.

[phoenix]

Originally Posted by jonners

I think driveability has more to it than set up

Just to make it easier - assume 2 driverless cars

one light and less powerful, one heavier and more powerful

but the power to weight ratio is the same

in general terms which is faster??

With defference to what has been said previously, it depends very much on the nature of the circuit.

If these theoretical cars have the same amount of drag and the same overall gearing:

1) Both cars would accelerate at the same rate, until drag becomes significant when the more powerful car will have an advantage, regardless of weight.

2) the lighter car will be able to brake harder and therefore later

3) the lighter car will need to brake less (see 4 below)

4) the lighter car will be able to corner faster - so faster entry, mid corner and exit speeds than the heavier car

5) the more powerful car will attain a higher terminal speed, if straights are long enough, than the less powerful car because it will be able to overcome more aero drag.

Simply put, the lighter car will have an advantage in corners and the heavier, more powerful car on longer straights where terminal speeds are obtainable. These advantages and disadvantages will therefore have different significance at different tracks.

[Al Weyman]

Originally Posted by AU N EGL

The lighter one, on a low speed highly technical track. The one with MORE HP or heavier one on medium technical to high speed track.

As far as cornering it would depend on the corner. a high speed carousel or high speed esses the heavier car would be faster, larger tire contact patches

Have done many tests like this myself with two different cars, yet the same HP:WT ratio.

I race in a HP:WT ( 8.7:1) restricted class, so that is identical. The heavier cars with more HP always win unless it is wet and raining, then the lighter cars win.

Yeah but what would sound the best?

[bdwoody]

THE V7 one!!!!!!

[Al Weyman]

Oh you only get that when one cylinder goes down, sorry. :-) Mind you this thread has prompted me to ask for drivers weights to be included in minimum racing weight in races with our club as I reckon at 19 stone I am seriously losing out!