Hills and oversteer...

Solari · 31 Jan 2009 at 23:15

Quick question (excuse wrong subject line - argh),

I was wondering whether understeer would be any more likely to manifest itself while driving up a hill if you were booting it. Going uphill would mean some sort of centre of mavity shift the rear wouldn't it? So would it make grip easier to lose at the front?

I was heading up a hill which bends ever so slightly near me quite quickly earlier while following a mate who had got a bit of a lead and noticed the front lost a fair bit of grip and was caught a bit off guard so am curious (and yeah, I know these cars have a rep as understeer monsters anyhow so I don't need reminding

).

PMKeates · 31 Jan 2009 at 23:24

Uphill I'd have thought would be less prone to understeer as the car is effectively pressing in to the hill.

Solari · 31 Jan 2009 at 23:26

PMKeates said:
Uphill I'd have thought would be less prone to understeer as the car is effectively pressing in to the hill.

I was battling over this in my head and am trying to figure out what the hell happened (it *is* damn cold so maybe I hit a patch of something slippery

). I've been up that hill countless times and never had that happen, even when I'm in a bit of a rush

danza · 31 Jan 2009 at 23:27

It'll be worse when you get to the brow of the hill, but as you hit the bottom of the hill and go a way up, I imagine the car would (as said above) be less likely to understeer.

olv · 31 Jan 2009 at 23:28

the car isn't pressing into the hill, it's on the hill and the angle means there is less weight over the front tyres so yeah, you'll experience a bit more understeer.

Felix Da Housecat · 31 Jan 2009 at 23:40

PMKeates said:
Uphill I'd have thought would be less prone to understeer as the car is effectively pressing in to the hill.

The only time the car pushes into the hill is few moments when you hit the base of the hill and the suspension compresses but once you are on the hill you assert no more pressure into the hill as you do on a straight road, after all the car still weighs the same regardless of it being on it's wheels or on it's roof.

______________________

Going back to the OP's question, I would have thought you would have slightly less grip as the weight shifts slightly towards the rear whilst going up hill.

PMKeates · 1 Feb 2009 at 00:02

Yeah when I imagined it in my head I had this version of an upwards curve rather than a hill with a flat surface at an angle

rypt · 1 Feb 2009 at 00:04

olv said:
the car isn't pressing into the hill, it's on the hill and the angle means there is less weight over the front tyres so yeah, you'll experience a bit more understeer.

Indeed, however you'd really need to know the powerlevel of the car, because even if it should make it more likely to understeer than oversteer, if it's some 1000BHP RWD monster and you give it some gas, it will break rear traction and oversteer.

The simple way to work stuff out is each tyre has a certain % of the cars weight on it. The higher the percentage, the higher the amount of "grip" that tyre can provide. Going uphil the weight the car (center of mavity) is shifted towards the back, so there is more grip weight acting through rear tyres, and less through front ones.

laissez-faire · 1 Feb 2009 at 21:02

What matters is not gradient but the derivative of the gradient, i.e. the rate of change of the gradient. If a hill is dropping away from you then there will be less grip, whether going uphill or down (i.e. you're more likely to understeer uphill getting less steep or downhill getting more steep)

atpbx · 1 Feb 2009 at 21:13

It will do it more down hill, not up hill, more weight on the front, more forces on the tyres going into bends, more momentum, the effects of mavity etc etc.

Dogbreath · 1 Feb 2009 at 21:53

You will get some weight shift to the rear, so theoretically it may tend to understeer more going uphill, but unless it a very steep hill the difference will be pretty small.

laissez-faire · 1 Feb 2009 at 22:05

Most of this weight shift stuff is nonsense - only fluids will shift and they are pretty fixed in their zone of the car. perhaps some luggage....

Solari · 1 Feb 2009 at 23:12

laissez-faire said:
Most of this weight shift stuff is nonsense - only fluids will shift and they are pretty fixed in their zone of the car. perhaps some luggage....

The hill I was going up is close to, if not more than 45 degrees, I would say so may have some bearing on it. OK I was probably being a bit of a knob hammering it to keep up, and going around its curve but I was still shocked at how the front end just was carrying on completely ignorant of the steering

Felix Da Housecat · 1 Feb 2009 at 23:25

laissez-faire said:
Most of this weight shift stuff is nonsense - only fluids will shift and they are pretty fixed in their zone of the car. perhaps some luggage....

Wrong

So if you stand a car on it's rear bumper is the weight still being pushed down through the suspension and to the wheels? I think not

After all it's mavity and it will always be pulled down to the same point (ie the center of earth)

Thomas. PLease. · 1 Feb 2009 at 23:33

laissez-faire said:
Most of this weight shift stuff is nonsense - only fluids will shift and they are pretty fixed in their zone of the car. perhaps some luggage....

No, how do you think lift off oversteer works. Its all about the weight shifting on to the front of the car, momentarily leaving the rear lighter and with less grip, hence the oversteer.

Weight transfer has a huge bearing on driving.

FreeStream · 1 Feb 2009 at 23:43

laissez-faire said:
Most of this weight shift stuff is nonsense - only fluids will shift and they are pretty fixed in their zone of the car. perhaps some luggage....

heh.

No its really not.

I recently had to do this question in mechanics.

Up a hill a RWD drive car has more tractional "grip" than a FWD car.
Down a hill a FWD has more tractional "grip" than a RWD car.

Where I say tractional grip, the car with the most grip will be able to put more of its horsies down!

Jonnycoupe · 1 Feb 2009 at 23:57

PMKeates said:
Uphill I'd have thought would be less prone to understeer as the car is effectively pressing in to the hill.

You have more weight transfer to the back to start with = more grip. Understeer is when the front wheels lack grip/traction.

Why would a car press into a hill?

Jonnycoupe · 2 Feb 2009 at 00:00

laissez-faire said:
Most of this weight shift stuff is nonsense - only fluids will shift and they are pretty fixed in their zone of the car. perhaps some luggage....

Get some strain gauges on some springs and datalog what you see.

Look at the characteristics of many cars dive and lift under acceleration and braking.... what else would cause the springs to compress and extend outside their static poistions other than changing forces at each corner of the car?

rypt · 2 Feb 2009 at 00:27

Jonnycoupe said:
Get some strain gauges on some springs and datalog what you see.

Look at the characteristics of many cars dive and lift under acceleration and braking.... what else would cause the springs to compress and extend outside their static poistions other than changing forces at each corner of the car?

I think the dude things that with "weight transfer" something physical actually moves. What is moving is the center of mavity as a result of rotation around the wheels axels of the whole car body. While fluid movement is a part of it, it is not the main part.

Jonnycoupe · 2 Feb 2009 at 08:09

rypt said:
I think the dude things that with "weight transfer" something physical actually moves. What is moving is the center of mavity as a result of rotation around the wheels axels of the whole car body. While fluid movement is a part of it, it is not the main part.

Yeah, but as whats becoming a trend he's still talking out of his ring