Rear ride height

Hi guys ,

Under heavy breaking the front end of the car is compressing, the rear end rebounds and the weight moves to the front. This makes the rear axle unstable due to the less weight on the rear tires resulting into less grip.

Does lowering the rear ride height help to keep the rear axle more stable, or what suspension adjustments are most effective to take care of this problem?

You didn't say what the vehicle is, some have known issues and it may help narrow it down - you could check on-line with relevant marque clubs to see if it's so?

Ride height makes a no significant difference in the static tyre loading, as that is down to the mass distribution and it's centre - it can make a little bit of difference under braking, or acceleration, by altering the height of the mass centre.

Rear end stability can be due to several problems - in no particular order

Incorrect suspension alignment - sometimes this can be a front end problem that feels like the rear.

Suspension deflection due to worn bushings, rod ends or flexing suspension mounting points - this could be at the front as sometimes the vehicle moving around feels like it is from the back, as one has to keep correcting the steering.

Too much rear brake bias and/or uneven braking across the axle- this can be also be at the front as sometimes it is hard to tell from the steering.

Poor damper control of the springs, even a broken spring - it is often triggered by one of the prior points.

Something else.

I'd suggest getting the vehicle up in the air and having a carefull check over all the suspension and chassis, with a pry bar to load it for play, and run a spanner check on it.

Have the alignment checked.

If you can, try disabling the rear brakes and seeing if that makes a difference?

If you have adjustable dampers, try altering the rebound/extension setting?

The trick is to try and find out what the problem actually is, rather than try hap-hazard changes which will confuse things.

I didn't mention the car model because the question is not about a specific car but more in general, so I'm assuming there are no loose bolts or worn out bushes and I'm aware thath every car handles different and reacts different to setup changes.

The reason I'm asking this question is, because I have a discussion with an other person about it and I just wanted to hear some other views. In his opinion every "racecar" needs to be dropped down in the back due to the behavior described in my question.

I think to set the static level of a car to a "reverse wedge" just in order to level the car out under breaking condition doesn't really help because as you mentioned, it is down to the mass distribution (static and dynamic) and it's centre. Lowering the mass centre does help reduce the dynamic mass distribution but just a little bit and the problem here is, that on the most cars the mass centre is in the front so the alteration of the mass centre by just lowering the rear is not worth mentioning. Also as mentioned before the car is only leveled out under braking conditions and on all other conditions you have a reverse wedge, what i think is to avoid in most cases.

In my opinion there are better ways to make the rear axle stable under heavy breaking (some of them also suggested by you):

-Spring rates/damper set up

-toe/camber

-brake distribution

-LSD

Ah, sorry about the granny-egg slant - hard to know where people are coming from sometimes.

TBH, I think your friend is one of those people that has a (very) little knowledge and has the sadly common idea that by lowering the ride height it means the vehicle is automatically going to weigh more that end, and so is talking out his bottom*!

*i still do that sometimes, but I hope I'm getting better ;-)

Hey Marcel

Yeh, like Gord has said, lowering the rear of the car might help a bit, bot not massively. Changes you can make that will definitely help are as follows:

- Rear Toe, you want a rear toe in, or neutral, NOT toe out

- Damper settings, going stiffer on the front, will reduce front end squat and keep the rear on the ground.

- Spring rates, similar to damper, stiffer front, will reduce squat.

- Brake bias, a loose rear can usually be a sign you're running too much front bias and the rear brakes aren't slowing the car.

- Weight distribution, as discussed, moving ballast to the rear of the car can help the amount of 'dive' the car will have.

Matt :)

I would disagree about the rear brake bias - it's too much that generally leads to instability, not too little.

Easiest way to think of it is how a supermarket trolley is much harder to control when it's pushed backwards. When pushed forwards the front wheels can steer and the rears will just follow, like a car with the rears still short of starting to slip. When it's pushed backwards it's like a car with the front (steering) wheels straight ahead and the rear wheels free to steer where they want.

Any deviation will from straight ahead will rapidly increase as the front moves laterally and the rears try and continue straight ahead - if, say, it's being released down a slope the cart will almost immediately do a '180' and roll down in the normal way, casters first, as this is stable.

In short, that's why locked front brakes will be stable and have the car continue straight ahead, whereas yanking on the handbrake will have it trying to spin through 180 and require some careful steering to prevent it. If the vehicle has a rear brake bias just short of causing a lock up, it may still be in the slip transition range and losing some lateral stability. Lock all 4 and the vehicle 'should'continue in the same direction, but may be easily deflected.

It's easily checked by backing it off a little and seeing if it helps.