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(FWD) Rear wheel lift off

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Hi everyone, as the title suggests . You quite often see fwd race cars and some street cars lifting the inside rear wheel when getting stuck into the corners.

My question is ... Is this a favorable situation ( lifting a wheel) as far as handling and getting the rear to rotate or are you better off having more rebound travel to stop it from happening.

What actually happens to the front wheel load as the rear wheel lifts off the ground, does it increase ?

I'm able to jack the rear of the chassis on my Starlet about 10-12mm up and then the rear wheel starts to lift off the ground. Should I be looking for more rebound travel ?

In short, it's to try and even out the loadings on each of the tyres on the road, as that's the theoretical best tyre grip.

From there, it gets a bit more complicated. With a FWD vehicle, we expect the front tyres to do most of the braking, all of the steering and to apply all of the power to the ground. As such, we try and load them equally, even if that compromises the rear.

As the vehicle goes around a corner, there is a side force on the vehicle and this normally loads the outside tyres more than the inside, so the inner tyres are able to do less work. Depending on the roll resistance at each axle, this can be more to the front that the back, or vice versa. As I said earlier, we want to load the front as evenly as possible, so we will increase the roll resistance to the rear of the vehicle, by adjusting spring and anti-roll (sway to the USoA folks) bar.

All this means is that the rear inner tyre can lift as a result of body roll and rear stiffness. Some suspension setups will actually reduce the rear droop, to maximise the load transfer at the rear as quickly as possible, but this can result in the rear being a bit twitchy as the rear load transfer changes almost instantly when the 'play' is taken up.

There is a LOT of misunderstanding about this, but some basics are - once the inside rear loses contact with the road, the rear is at 100% load transfer and increasing roll resistance makes NO difference! Some front roll resistance (spring or ARB) is still going to be required - especially when the rear lifts, any body roll will be gaining positive tyre camber - which reduces the grip and increases shoulder wear and the rear cannot control it when the inner lifts. Especially when the vehicle is used on bumpy roads/tracks, and most especially when it's light at the back, there has to be some careful thought regarding balancing the spring rates-ARB balance. Compounding this are the damper settings and tyre pressures. The latter is really important because the tyre acts as a second spring (I expect everyone's been caught out at some point by a well inflated tyre bouncing when dropped, and has also noticed the difference in bounce between different tyre pressures?) and it also has to be controlled by the damper. Pehaps the best illustrations for getting this wrong are the FWD rally cars where the rear bounces uncontollably when hitting a bump, or hump, and throws the vehicle off the road. If the vehicle's lost a lot of weight off the rear, it may need a much lower tyre pressure than OEM, just to control the rear - the increased slip angle will also reduce rear 'grip' aiding turning.

What to advise for you isn't really possible without being familiar with the vehicle and it's characteristics, the suspension being used (OEM or aftermarket, and if the latter what?) and how it's being used. With a road car one may have a need for a stable, mildly understeering chassis, for a fast track one may find a slight oversteer bias works best, for autotests/motokhanas/SOLO one may find a strong oversteer bias helps to turn the vehicle.

The reason some FWD cars lift a rear wheel is the same as some earlier (Escort Mk 1/2, Anglia etc) can lift a front wheel. The driven end is very softly sprung compared to the undriven end. So you get a lot of roll at the driven end and not much at the other...hence the tilts over to the opposite wheel lifting.

The reason the car is softer at one end is grip, predominately traction under acceleration.

In the UK was this more common on cars like Pug 205's/Ford XR2's and accentuated by the short wheel base. And RWD cars like Escorts, Anglia's, Imps etc.

Is it a good thing? Not really but if that is how the car is fastest, live with it.

In order to lessen/stop the wheel lifting you would have to even stiffen the driven end or soften the undriven end. If your car is already well balanced, and all other things being equal, this will change it. So a FWD car will probably understeer/lose traction more and a RWD car will probably oversteer/lose traction more.

As for deliberately causing it...some of the front running 205's/XR2's used to do this, especially in the wet, by droop limiting the rear axle!

As far as practical application for your situation, I agree with Gord's last sentence. Speaking from autocrossing experience in my Honda Civic hatchback (similar to your Starlet), the fastest way around the track for a FWD hatch is a setup biased towards oversteer.

To achieve this, you want stiffer rear suspension and a big sway bar in the rear to flatten the load transfer as much as possible and make sliding the rear as predictable as possible. My fastest laps always involved a mild, controlled, 4-wheel drift staying light on the gas and countersteering through the curve.

If you have things too soft in the rear and enter a corner fast and on the brakes, the rear of the car usually rolls into the turn until the point where the outer tire loses grip, resulting in a sudden breakaway of the tires and a violent oversteer that is very hard to predict. And if you go into the corner too slow and the rear tires stick as the body rolls, you'll have massive, frustrating understeer. The softer your rear suspension, the more prone you are to these situations.

The stiffer the suspension, the more prone you are to breaking traction at the slightest bump mid-corner. It's a trade-off, but FWD cars with light rear ends tend to favor the stiffer option, at least in the rear.

Bottom line: You want a planted, sticky front end and a stiff, predictable rear end with no slow weight transfer or bouncing going on. Stiff suspension, big sway bar, no fear of breaking it loose.

This setup must be driven hard to reap the rewards. Don't let up on the gas when the rear comes out! That will transfer weight forward and lighten your already traction-deficient rear and you'll spin. Feather it through the turn and let the front end pull the rear out of the drift. FWD cars can be animals on the track if driven properly.

As Gord said, tire pressures are important too. Experiment with your specific setup and you'll figure out what's best.

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