I have changed the car to a Buri E racer with no suspension

I have been looking at motors and am thinking bigger isnt always better.

For a car to move forward it needs traction. If it loses traction the wheels slip and it loses control. A car weighing 2 kg can pull its own weight without losing traction so you can assume it can have 2KG of forward force on the wheels. This means with no resistance it can accelerate at 1G. If the power is available to the wheels the car can accelerate more until the power overcomes the traction and it looses control so too much power isnt a good thing.

You can add downforce by adding a wing and this will create more traction so the car can accelerate faster at higher speeds. This will help control the car but as the speed increases the frontal load or air resistance will increase and the car will reach a lower maximum.

The maximum speed a car can attain is dependent on the resistance of the air. The frontal load increases with speed. The formula to calculate the frontal load is as follows

Force = Drag x Cross sectional area of the car x Velocity (m/sec)^2

So you can see the drag quadruples with a doubling of speed.

At a given point this frontal load will overcome the thrust provided by the wheels. If the motor small it will not be able to spin the wheels so the car will just stop going faster. If the motor is large the wheels will spin and the car looses traction and control. I would guess that this is worse than the former.

So lets look at my car and do some calcs

The car weighs 3.4KG.

I have a lehner 2250/6 in it with the following specs

Current rpm Torque Ncm Output power

244 57,625 115 6,939.20

The gearing is 17/53 and the rear wheels are 75mm diameter

So with the gearing the car can do 165mph at max revs.

The gearing is reduction so the torque is multiplied. Torque at the wheels is 115 / (17/53) = 358ncm = 36.5 KG cm. With the wheel being 75mm diamter we get a thrust at the wheel/road contact of 9.75KG

In reality we have a 3.4KG car trying to push a 9.75KG weight! Whats going to happen? The wheels will spin and lose control. It gets worse.

An electric motor provides more torque at lower revs so trying to get the car up to speed means a small blip on the throttle will provide massive torque and spin the car. Its like trying to drive a Bugatti Veron on grass.

Now you can add a wing to provide about 4-5KG of downforce and that will work but a wing that size will increase the CD from about .35 to .9. lets what effect this has on the final speed.

With a CD of .35 and a traction force of 3.5KG the car can attain 165mph. With a CD of .9 the car will attain a speed of 106mph. BUT the wing does provide down force so can go faster as it provides more traction force.

The frontal load or drag at 165mph with a CD of .9 is 8.35KG

So lets see what a smaller motor can do.

The lehner 1940/8 has the following specs

Current rpm Torque Ncm Output power

111 56,963 52.4 3,128.00

This gives a torque at the back wheel of 4.4KG

So without a wing and NO LIFT this motor will drive the car to 165mph also

So as you can see adding a wing will make the car more stable at high speed but you need double the power to attain the same speed as a smaller motor. The trick is to balance the motor with the speed required.

Here is the good part.

If you increase the gearing of the larger motor with no wing the traction force reduces and the car will become stable and be able to travel at 225mph!!

So there you have it. Adding a wing just increases the power required and adds stability. If you ensure the car shell provides NO LIFT it will go faster with a smaller motor

Cheers