Wednesday, October 17, 2012

A caster diaster?

Another of my rambling from my visit to the track yesterday comes from the Exotek adjustable front end. I finally put my Exotek 104 with this front end on the track after working on the F104V2 for quite a while. I have run a front end made of Xpress parts which is functionally similar to the Exotek pieces, but no where near as nice or durable. So I was familiar with what usually works, but I had some ideas on things to try. Lately, with the Pit Shimizu 0571/0571/Tamiya TCS tires, the cars generally will get a little light on the inside rear coming off the corner. This will sound like diff slippage, but if you watch the cars, you cane see they are a bit rolled over. I have also talked with guys who have had this problem completely independent of us and came to the same conclusion, so that is my theory...LOL. I tend to think it is the fact that the tires are heavier, and have very good grip.

I thought that reducing caster would help this problem, as caster tends to cross jack the weight to the opposite side rear tire. I figured the outside rear was just getting way heavy from too much caster. So I set the front end to 3 degrees of caster, as I had been running 4-5 degrees. Three was actually a very good setting for taking a shot in the dark, as the car stayed pretty flat. There was still diffing, but it seemed to be reduced. Now figuring there was more of a good thing to be had I continued to try to reduce caster, but it seemed as I worked my way to 2 degrees, problems popped up. First off, the car did not like to go through the sweeper anymore. It would turn in and then push to the wall. Secondly, it didn't want to launch coming off the corner either. The mid corner push could possibly be solved with more camber or camber gain, but I didn't like the corner exit issues. Again, maybe softening the car down the middle (center shock spring/oil) could help, but the caster is a 2 for one deal on both of those issues. It may be worth investigating if the diff issue is solved, but I am also not 100% convinced a little diffing is so bad. The main problem is when the car starts to get sideways as you pull throttle, usually as the car is still loaded up from the corner. If you let the car settle before you get on it, it's not a huge problem, but to really be on it is not easy.

Going the opposite direction will generally get you going toward a traction roll. Anything much more than 5* caster on carpet is usually asking for trouble. I'm thinking 3-4* is probably the sweet spot for carpet and rubber tire.

That being said, running too much dynamic caster effect in the upper arm is probably also a contributing factor. This is spacing up the rear ball stud on the upper arm so that the caster is reduced on the loaded side - the kingpin stands up as the suspension is loaded. So now you have the loaded side reducing caster - or reducing the weight jack to the inside tire - and the unloaded side has more caster, jacking weight to the outside rear. Diff spin at it's finest!

I noticed that my F104V2 got better as I reduced the dynamic caster as the grip came up. On the fixed arms of the stock V2, this will also reduce static caster as well. I didn't reduce dynamic caster on the Exotek car, but I think this would have been helpful as well. As traction increases, you generally need less of this inclination of the upper arm. Higher grip usually means the front end is not going to wash out mid corner, and it will help reduce tendencies to traction roll as well. For a situation like asphalt, I would probably run about as much dynamic caster as I could get away with. Carpet, not so much on rubber. Foam, oh yes, but for rubber tire too much will be problematic.

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