Desired Wheel Rates

[ford396]

Getting ready to spring my project. 71 Ventura with coil-over front suspension. Shocks are mounted at ball joint. Roll center is adjustable and can be moved up and down quite easily. Not sure where I will end up, but guessing somewhere in the 1-1.5" range, centered in the chassis. I am curious as to desired wheel rate of suspension in the front. I will be mostly street cruising this car with the occasional off-road romp. I am also wondering how much front wheel compression should a street car have?

Here are the two questions:

1. How much total front suspension wheel rate do I want? Bar and spring included.

2. How much compression should I allow for in the front suspension at the tire centerline.

Thank you
Bob

[Mateospeed]

Bob,
While you've asked a rather simple question, it requires a very complicated answer. It wouldn't be smart to just hand you a set of wheel rates, because a full set of calculations is required to do so, involving making a few compromises that only you can decide on. I recommend starting on Ron Sutton's website (search his name here), and getting a few of the books he recommends on suspension design. Then, you can start to determine how do find your first crack at wheel rates.

I'll give you a set of numbers that I'm using on a '66 Mustang street/ track car. My street setup uses a front wheel FREQUENCY of 72 CPM, yielding a front wheel rate of 110 lb/in. Generally, the spring and bar rates are figured seperately, to give the desired roll angle. Mine uses a bar wheel rate of 627 lb/in. This would be quite heavy for a street car; I just don't feel like changing the roll bar between track events.

Please resist the urge to take these numbers and apply them to your setup. This works for me, but my setup in entirely different from yours. Use with caution!

Good luck!
-Matt

[ford396]

Matt:

Thank you for your reply. I was beginning to wonder if anyone would. :) I have adequate experience in calculating wheel rates and laying out suspension systems. I have built several late model race cars used for circle track racing. Some of these were big bar soft spring, some on bump stops. I have Performance Trends software that I use for my calculations. I say this not to showcase my experience, but rather to explain that I just did not know how much wheel rate to apply to a street car. Some of my cars would compress 5-6 inches on the track, while others didn't move.

With your 110 lb/in wheel rate, how much wheel travel do you see on the street? Is it harsh?

I am concerned with my allowed wheel travel in bump. I only have about 2". I am not sure if this is enough?

Bob

[Mateospeed]

Bob,
Sorry I'm slow to reply - I missed your post. My car isn't finished yet, so I can't give you feedback on how it will behave. I have set my ride height to offer about 4" of bump travel (from ride height) on the street, 2" on track (road course). I'm aiming for a firm, but not harsh ride. I'm also running a BBSS (Big bar/ soft spring) setup on this car, so those bars will lend a LOT of roll resistance on the street, leaving a lot of room for bump travel under load. I should have 1-1.5" of bump travel left at 0.8g of braking (a guess at street tire capability).

If you're looking at 2" of bump travel for a street car, I very seriously doubt you'll be able to get away with a 110 lb/in wheel rate. You'd most likely be all over your bump stops, all the time. You'll probably have to go for ~ 100 CPM wheel frequency. That, however, is just a guess, with no #'s to back it up!

If you're interested in running the numbers for your car, shoot me a PM - I'll get the data I need from you, and can run it through my program. I'd give you the spreadsheet, but I very seriously doubt anyone but me could read the damn thing!!

Best of luck!
-Matt