Why do the driven wheels can lock up if you overrev the engine when shifting down?

Updated: 09 Dec 2024

I mean, there is a (more or less) fixed connection between the engine and the wheels through the clutch and the transmission.

I get that you have a huge amount of braking force due to the engine brake when you just dump the clutch, but if the wheels lock up, the engine speed would be zero rpm, too. Does the clutch just slip in this moment?

Thanks in advance for detailled explanations and sorry if my english isnt that good.

Comments

I’ve never heard of this before. Are you sure the driven wheels lock up?

yes, i’ve read this on different websites and heard it in different videos, can’t point one out tho. But it’s always in the obligatory list of “reasons for rev-match”

Lemme try to explain this and not confuse myself because I think I know what you’re talking about

When you slow down, if you downshift and overrev the engine, the revs build too high and the speed that the wheels are turning at changes suddenly, causing the wheels to lock up. Simply put, the wheels can’t keep up with the engine speed if you downshift too much since there’s friction on the surface that the car is traveling on

I don’t know if i understand you correctly, but why would the wheels lock up under such circumstances? I believed they slow down more than the tires are capable of, so the slip, but to get them to 0 rpm, where would have to be another force which slows them down, since the engine just “breaks” fast to below the limiter and then just like normal when you engine brake.. have you read my answer to the other comment?

The rev limiter limits how fast the driven wheels can turn. If you shift down so early that the car is travelling faster than the driven wheels can physically turn, the resulting loss of grip will destabilise whichever end is driven (btw I don’t think the wheels will actually lock, but they will lose traction)

While shifting down, imagine fastening a screw where the screwdriver is your wheels and your hand the engine. If if you would use the same amout of strength to fasten the screw all the way in, your hand woun’t be able to hold on to the screwdriver, which will cause the screwdriver to “lock up”. This is why you need to use more strength to fasten the last bit i.e. match the revs with the wheels speed.