Bumpstop Setups for a Rough and Bumpy Track
First, I would like to say I'm a big fan, and it was a pleasure to meet you at I-25 Speedway in Colorado. We were the group with the 45 car that won the feature that night.
I'm getting ready to run a new Lefthander chassis at Colorado National Speedway in Denver and was wondering if you could give me some advice on running a bumpstop setup for the track, or would you recommend staying away from that setup since it is a little rough?
I've always been pretty good at balancing a car with the SSBB setup, but haven't worked a lot on the bump stuff, but would love to start working with that style of setup. Any advice would be appreciative. I'm a shock guy, so what style of bumps should I use?
Thanks for all the hard work,
Good to hear from you. You ran a great race that night. As for running any type of bumpstop setup at CNS, you should know that track and how bumpy it is. It's not so much the roughness of the track as it is the extreme dips.
It would be marginally helpful to run bumps there, especially the rubber or plastic types. Various companies are experimenting with spring type of bumps and we did a test on a carbon-fiber disc type of spring that we used as a bump spring.
These provide more travel and are more forgiving when running over rough surfaces and tracks that have large bumps. I don't think I heard anyone say they ran bump setups at CNS when I was there. But that doesn't mean it can't be done. Just don't be too disappointed if it makes the car unstable due to the surface of that track. After all, CNS is a unique track in that respect.
As for the shocks, you need to run the rebound on the left front shock at around the same rate as the bump at 3 inches per second speed, which in most cases runs around 1,000 to 1,200 pounds per inch. And you need to have a “nose,” which is like a pre-load in a shock to where at very low speeds of less than 1 inch per second the rate is 200 to 300 pounds. On the right front, you can just use high rebound, but not as high as the LF and no nose.
Metric Four Link Rear Steer Tricks
I must say you do a great job in CT magazine and I think it's great what you do with the AMSOIL Great American Circle Track Tour.
I'd like to ask you a tech question. I saw your last article where you noticed at the Dells and Grundy there is a new trend about rear steer. You mentioned the RR wheel goes back on braking then goes to a neutral state in the center of the corner, then RR goes forward when accelerating out of the corner.
I too have noticed this on some competitor's cars and race a Sportsman asphalt car with a Metric four-link. How exactly would you get the rear end to do this?
I have never worked with this type of setup stuff, but I have heard rumors. Some teams will drill holes in the rubber bushings (one or more) in the right side of the Metric four-link rear suspension to make those bushings softer and compress more so than the left-side bushings.
If you can find different hardness (or softness) rear bushings for the car, or use neoprene bushings (usually stiffer) on the left side and leave the rubber bushings on the right side, you will also achieve the same results.
Here's how it works. When we brake, the rearend wants to move rearward, and if the right-side bushings are softer, then that side will move farther than the left side causing rear steer to the right and loosening the car on entry. This rotates the car in the direction of the turn.
At mid-turn, there are no fore or aft forces working to move the rearend and so it stays in its neutral position the same as it was at ride height, or hopefully square in the car. This part of the corner is usually of a very short duration.
Under acceleration, the rear end wants to move forward and if the right side bushings are softer, then the RR will move farther forward than the left side causing rear steer to the left which tightens the car coming off the corner.
What a team must do to exact the proper amount of rear steer (you can over do that), is to experiment with different softnesses for the bushings in the right side links to regulate the amount of rear steer. I have seen it work and it works quite well when tuned properly.