What Changes When You Reduce Car Wheel Size by 4%?

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Discussion Overview

The discussion centers around the effects of reducing car wheel size by 4%, specifically examining changes in torque, tractive force, and the implications for vehicle performance. Participants explore theoretical and practical aspects of this modification, including its impact on gear ratios and vehicle dynamics.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant posits that reducing wheel size will result in a 4% increase in torque, questioning the relationship between wheel size and torque.
  • Another participant asserts that torque remains constant regardless of wheel size, suggesting that changes will occur at the contact patch instead.
  • A participant references the formula F = T/r, indicating that a change in wheel size may behave similarly to a gear change.
  • It is noted that the smaller diameter results in a change in RPM due to a reduced circumference.
  • One participant explains that torque is influenced by gear ratios and that tractive effort can increase with smaller tires, providing calculations for tractive effort based on different tire sizes.
  • Calculations are presented showing that with a constant torque of 100 Nm, the tractive effort increases when switching from a 25-inch to a 24-inch tire.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the relationship between wheel size and torque, with some arguing that torque remains constant while others suggest it may change. The discussion includes competing views on how wheel size affects tractive force and vehicle performance.

Contextual Notes

Participants reference various formulas and concepts without fully resolving the implications of changing wheel size on torque and tractive effort. There is an assumption of constant torque from the engine, but the effects of dynamic rolling radius and gear ratios are not fully explored.

Nick666
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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 ?
 
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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 ?
 
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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..
tractive effort= Torque/tyre radius (dynamic rolling radius)
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

so basically your tractive effort increases which can help you to climb gradients..
you can calculate percentages.
 

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