Light/heavy = under/oversteer

martine · Joined: 17 Oct 2005 Posts: 1376 Location: Bristol, UK

Just read the channel 4 'advanced driving' article from another thread and came across this:

"If the steering goes light you're close to understeer, if it goes heavy, you're close to an oversteer..."

Not heard of this before - would anyone like to explain?
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Martin - Bristol IAM Group Senior Observer and Secretary.

7db · Joined: 06 Apr 2006 Posts: 1766 Location: London

I read that as well and had to stop to have a think too.

What you feel at the wheel is the force feedback from the front tyre trying to straighten against the cornering force it is providing (Newton III). If you lose cornering grip at the front then you have no force coming back to you. The wheels turn without increasing resistive force (ie the steering feels light). This is terminal understeer.

The force falls away at slip angles just before it gives up altogether so it is possible to feel the steering go light before the tyres give up grip. This is by far the best clue you will ever have that you are close to the limit of grip without hearing all that squealing (from either tyres or passenger seat). It's only there if you let it tell you and have a sensitive enough grip to feel it when it comes knocking. I notice that in general I have no idea how far my hands have turned on the wheel, but I know exactly how hard they are pushing (I assume this is a biometric thing about force sense vs displacement). The sensation that I get with terminal understeer is "ooh - that's oddly light - it's not pushing back more as I push on it" -- it's like a spring or rubber band that's failing as you stretch it - you get the odd sensation that something somewhere has gone nonlinear. Hard to explain. I also get an oh-shit-we're-too-fast-to-make-it-round-this-corner moment about a second or two earlier.

With terminal oversteer, I'm less clear that I feel it first at the wheel. I think I normally sense that through the yaw rate rather than the wheel. It's the uncomfortable feeling that your arse (ears) is moving where your eyes tell you it shouldn't.

But if you think through...The correction isn't fighting the car, indeed letting go is an option to allow the car to correct the skid itself -- in other wheels the front wheels are trying very hard to straighten the wheel in your hands and take lock off. The more the back steps out (and the harder it steps out, the harder the front wheels are yanked round by the back of the car, and they in turn try to yank the wheel straight in your hand. This is an increased resistive force if you try to resist it (you won't be - you'll be putting in the opposite lock as fast as you can) which will be felt as heaviness at the wheel.

* * *

More simply, if you think of (terminal) understeer as the front wheels losing (and the rear wheels winning) and (terminal) oversteer as the front wheels being all-conquering then this makes intuitive sense -- the feeling you have at the wheel is how strong the front tyres are. Light - they are starting to lose; heavy and they are winning.

waremark · Joined: 15 Apr 2006 Posts: 721

hardboiled · Joined: 16 Jul 2006 Posts: 201

Lady Godiva · Joined: 02 Jun 2006 Posts: 367

Dear All - oh brilliant, 2 drivers who I respect and admire and trust, give me conflicting advice, all of which makes sense!!!!

7db says that understeer will go light, and oversteer will go heavy. Then Hardboiled tells me that understeer will probably go heavy, but that oversteer will go light.

Great. I now know less than I did, and the only thing I know is that I didn't know it.

Regards
Sally

7db · Joined: 06 Apr 2006 Posts: 1766 Location: London

I'd ask StressedDave to arbitrate (and hopefully come up with a third view).

Phil gets near the limit more regularly than me...so I'd listen to him... Twisted Evil