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Downtravel is in my opinion a more critical set up parameter than rideheight. It makes a larger difference to the handling I think. By downtravel I mean the difference between ridegheight, and the maximum height the car can be lifted, with the tyres still touching the ground. One important thing to talk about is how downtravel is measured. Many drivers like to measure the shock length to state downtravel. This is however a misleading way to do it. It is good if you only want to keep the information for yourself, but to other people the information doesn’t mean anything. This is because to figure out the actual downtravel, from shock length, one needs to know which shock positions, and rear hub positions were used, and often this is not possible. Normally only one set up parameter is discussed at a time. So I never measure the shock length, I always measure as I would measure rideheight, but measure up to the point where the wheels just touch the ground. This way it can be compared to other cars whatever their set up is, and even other brands. I like to measure without wheels as it is more precise, and when I want to compare, I measure with wheels. Of course to compare, one has to use the same tyres, as tyre diameters vary.
The more downtravel, the higher you can lift the car. On the track, more downtravel basically means, that the tyres can stay on the ground longer, when going over a bump the wheel can fall into the hole further, and when landing off a jump the wheels will hit sooner.
More downtravel will make the car jump and land better, give more traction, and most of the time it will go better in bumps. Too much downtravel can make the car roll too much, and make it respond slowly, which could make it spin out and be hard to control. This can also depend on the car. For example Mugens are known for being good with a lot of downtravel, and Losis bad with a lot of downtravel.
I like to measure downtravel like this.Shock length doesn’t actually tell you the amount of downtravel, unless you use the exact same shock and hub positions. When measuring downtravel with wheels on, the measurement can be compared to the downtravel of cars with a different set up, or other brands also.
Less downtravel will make the car jump worse, and most of the time handle bumps worse, but the car will be more stable at high speed, and specially on high traction. It will be more respnosive also, and less prone to flipping over.
As for bump handling, it gets slightly complicated, as depending on the track, and driving style, the amount of downtravel for the best handling, can vary, a lot of downtravel may be the best, but it may also make the car go into all the bumps, and thus slow it down, and make it more unstable. Less downtravel may be worse, or it may actually help keep the car on top of the bumps, and actually make it faster and more stable. There really is no magic set up for downtravel as far as bumps are concerned, each racer really needs to test this themselves. In general, less downtravel for smooth high grip tracks, and more for loose, or bumpy tracks, or tracks with many jumps. Downtravel, along with rideheight has a tendency to change, so it is good to check it before each run.