The reason the car is softer at one end is grip, predominately traction under acceleration. 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 driven end is very softly sprung compared to the undriven end. 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. 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. 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. 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. 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. 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. Compounding this are the damper settings and tyre pressures. 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. 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. 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. 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.Īll this means is that the rear inner tyre can lift as a result of body roll and rear stiffness. Depending on the roll resistance at each axle, this can be more to the front that the back, or vice versa. As such, we try and load them equally, even if that compromises the rear.Īs 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. 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. 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.įrom there, it gets a bit more complicated.
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