# Torque and wheel size of a car

• Automotive
• Nick666
In summary, changing the wheel size of a car from 25 inches to 24 inches will not affect the amount of torque at the wheel. However, there will be changes at the contact patch and the RPM of the car. The formula for torque is F=T/r, and the circumference of the wheel will also change. When considering changing tire dimensions, assume it is constant and the tractive force will increase. The tractive effort is calculated by dividing torque by the tire radius. With a constant torque of 100 Nm, the tractive effort will increase from 314N to 328.08N when using a 24-inch diameter tire, which can aid in climbing gradients.

#### Nick666

So let's assume I have 100 Nm at the wheel of a car, with a wheel size of 25 in diameter

If I put wheels that are 24 in diameter, so 4% smaller, what exactly will change ? Will I have 4% more torque ?

Waaat.

You'll have exactly the same amount of torque. Something will change at the contact patch through.

Isnt there some formula that might be applied here ? ... F = T*r ? Doesnt a change in wheel size of a car act like a gear change ?

billy_joule
Ohhh...its F=T/r ... I get it now.

If F is the ground force on the wheel then T- F.r = IG

Also your rpm changes. The circumference (pi*D) is less on the smaller diameter.

no you won't have more torque .. torque is a function of gear ratios at wheel torque = engine torque*gear ratios*differential ratio
So when you are considering change in tyre dimensions assume it constant. so for changing wheel selection following things will occur.
in this case your tractive force will increase..
say you have 100Nm of torque which is constant
now you used 25inch tyre= radius=0.3175m
so tractive effort = 100/.3175=314N

Now you used 24inch dia tyre=radius=.3048m
so tractive effort = 328.08N