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Bump rubbers are used when the suspension is fully compressed, and prevent the metal bits from mashing themselves to pieces. There're like reserve, last resort, very hard springs.
When fully compressed, the suspension is useless to you. Worse than useless, in fact, because it happens so suddenly and with such force that you will, momentarily, lose control of the car.
The thing about bump rubbers is that you really don't want to rely on them if at all possible. If you can find a way to stop the suspension running out of travel - by raising the ride height or fitting stiffer springs - then you would usually be advised to do that. However, if you must use bump rubbers, then you want them to be as big as possible. Because bump rubbers are conical, the bigger they get, the more progressive their effect.
Don't confuse 'bottoming out' (when the bottom of the chassis hits the ground, and you hear a metallic scraping noise) with running out of suspension travel (when either the spring compresses fully, or the suspension components hit each other). Bump rubbers only help with the latter.
In GPL, the spring rates of the bump rubbers are measured in the 400 - 800 lbs range, according to Doug Arnao.
One of the reasons why so many people thought that the cars in GPL were hard to drive was that low setups (typically anything less than 3.00", like the -default- ones in version 1.0.0.0) mean you spend a lot of time on the bump stops. Up in the 3.00" to 4.00" static ride height region - the height the cars were designed to run at in 1967 - you only very rarely fully compress the suspension and, lo and behold, the cars become drivable. If only we knew we were compressing the suspension so much, the first few months of GPL might have been a lot less frustrating...
I've been told that during the beta testing of GPL the minimum size of bump rubber was 0.5". This was increased to 1" just before GPL was released to try and stop people using unrealistically low ride heights. This didn't work - just about everyone used 1" ride heights - so a later version of GPL.EXE raised the minimum static ride height to 2.5". Why am I waffling on about this? Because it shows that GPL was originally designed to use 0.5" bump rubbers, and therefore it's presumably ok to use a third-party setup editor to set them to 0.5" now. (You can reduce them even more, but I wouldn't: when metal hits metal in the GPL physics model, GPL.EXE crashes.)
The only circumstance you might want to have bump rubbers larger than 0.5" is to try to stop the suspension going berserk at the Monaco chicane. Because of the nature of the track at this point, you're going to impact the rubbers no matter what ride height or wheel rate you're using. With 0.5", it's very hard to 'recover' the car after having driven flat out through the chicane.
Some folks like to increase them for the 'jumps' at Mosport or the Nuerbergring; if you do, remember that you should land from a jump on your rear wheels (by accelerating as you take off) so you should make the bump rubbers larger at the rear than the front. Personally, I prefer to keep them short so I get maximum suspension travel around the rest of the circuit.
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