Mateospeed
03-05-2014, 12:01 PM
Folks,
Here's a rather technical question for you. I'm working out the suspension for my Pro Touring Mustang, and am at the point where I need to pick dampers. I need a little help determining what kind of damping coefficients I should be looking for, particularly in roll. I just can't seem to get my calculations to work. So, help!!
My pertinent information is as follows:
-Vehicle Weight: 3000lb, 55% front weight dist.
-sprung weight front: 750 lb (each wheel)
-sprung weight rear: 575 lb (each wheel)
-un-Sprung weight front: 150 lb (each wheel)
-un-Sprung Weight rear: 100lb (each wheel)
-Motion ratio: 0.85 (front)
-wheel rate: 120 cycles/min (front+rear)
-coil lean angle: 80*
This yields front rear wheel rates of 308 and 250 lb/in, respectively, and coil rates of 400 and 250 lb/in, respectively.
I'm getting the following for damping in one-wheel bump:
-low-speed compression: 32 front, 25 rear lbf/ (in/sec)
-high-speed compression: 16 front, 12 rear lbf/ (in/sec)
-low-speed rebound: 73 front, 56 rear lbf/ (in/sec)
-high-speed rebound: 37 front, 28 rear lbf/ (in/sec)
aaaaaand that's where I stop. Help! Are these reasonable? What should I expect in roll and/or pitch?
I got a theoretical dyno graph from my friendly Viking rep( attached), from which I get the following ranges of available damping coefficients:
-low-speed compression: 35-250 lbf/ (in/sec)
-high-speed compression: 3-8 lbf/ (in/sec)
-low-speed rebound: 8-300 lbf/ (in/sec)
-high-speed rebound: 7.2-15 lbf/ (in/sec)
I seem to already be outside the range, without considering roll. Uh-oh!
Thanks in advance!
-Matt McBride
-'66 Mustang Track Car, EVERYTHING custom-fabricated by yours truly.
Ron Sutton
03-07-2014, 07:34 AM
Hi Matt,
I see several issues with the process & the strategy that I'll point out & offer a different process & strategy at the end.
Folks,
Here's a rather technical question for you. I'm working out the suspension for my Pro Touring Mustang, and am at the point where I need to pick dampers. I need a little help determining what kind of damping coefficients I should be looking for, particularly in roll.
Obviously, the shock can't tell if it's being asking to control pitch, roll or combination ... just wheel travel. Not that we don't don't take all those into consideration when designing a shock valving curve, but they overlap & we have to design the best compromise.
I just can't seem to get my calculations to work. So, help!!
I know of no formula that will help us achieve the optimum shock valving curve, because the needs of the valving force resistance is very different at 0 inches per second (IPS), 1/2 IPS, 1 IPS, 3 IPS, 5 IPS, 10IPS ... throughout the curve.
My pertinent information is as follows:
-Vehicle Weight: 3000lb, 55% front weight dist.
-sprung weight front: 750 lb (each wheel)
-sprung weight rear: 575 lb (each wheel)
-un-Sprung weight front: 150 lb (each wheel)
-un-Sprung Weight rear: 100lb (each wheel)
-Motion ratio: 0.85 (front)
-wheel rate: 120 cycles/min (front+rear)
-coil lean angle: 80*
Your weight math doesn't quite add up.
This yields front rear wheel rates of 308 and 250 lb/in, respectively, and coil rates of 400 and 250 lb/in, respectively.
You didn't square the motion ratio. Did you use a Cosine factor for the spring angle?
Your spring motion ratio math should look like this: .85 x .85 x .985 = .712 x 400# spring = 285# front wheel rate.
P.S. On the rear, there are two motion ratios.
One is side view:
1. Dimension from the suspension pivot to the spring CL ÷ dimension from the suspension pivot to the axle CL. This primarily influences the the spring rate in dynamic pitch mode, but plays a role in bump control.
Second is rear view:
2. Dimension from the chassis CL to the spring CL ÷ dimension from the chassis CL to the tire tread CL. This primarily influences the the spring rate in dynamic roll mode, but plays a role in bump control.
As a race car designer & tuner, I look at the net spring rate from motion ratio #1 playing it's biggest role in corner exit grip. The net spring rate from motion ratio #2 is a key to the roll angle, along with the sway bar if there is one.
I'm getting the following for damping in one-wheel bump:
-low-speed compression: 32 front, 25 rear lbf/ (in/sec)
-high-speed compression: 16 front, 12 rear lbf/ (in/sec)
-low-speed rebound: 73 front, 56 rear lbf/ (in/sec)
-high-speed rebound: 37 front, 28 rear lbf/ (in/sec)
aaaaaand that's where I stop. Help! Are these reasonable? What should I expect in roll and/or pitch?
This is not the approach I would suggest for selecting shock valving.
I got a theoretical dyno graph from my friendly Viking rep( attached), from which I get the following ranges of available damping coefficients:
-low-speed compression: 35-250 lbf/ (in/sec)
-high-speed compression: 3-8 lbf/ (in/sec)
-low-speed rebound: 8-300 lbf/ (in/sec)
-high-speed rebound: 7.2-15 lbf/ (in/sec)
I seem to already be outside the range, without considering roll. Uh-oh!
Thanks in advance!
-Matt McBride
-'66 Mustang Track Car, EVERYTHING custom-fabricated by yours truly.
I use formulas for almost everything in designing a race or performance car ... except shock valving. The reasons why not are many. But the biggest is ... in addition to bump control ... the shock's role in performance handling is to influence the rate & timing of events. The optimum rate (speed of compression & rebound) are affected by many factors.
Some could be calculated in a formula ... and many can not. For front shock compression ... for handling, not bump control ... we need to take into account spring rate, optimum or actual travel in dive, optimum or actual rate/speed of dive, how much grip is in the track surface, how hard the driver brakes, where the driver brakes in different corners, human inconsistencies and driver preferences. (AKA what rate gives the driver confidence on corner entry.)
Some of these things don't change, but many do from day to day and even throughout the day. In a street driven car, we would just tolerate the differences, but not in a racing environment.
Not everyone is familiar with this next concept, but it is major. When someone is autocrossing or running a road course track day (or racing) and the driver is applying the brakes hard on corner entry, that braking force is compressing the front suspension. This is good. Racer's call this "dive" and engineers call this "pitch". In dive/pitch, the front suspension has compressed ... lowered the heavy mass CG of the front end of the car, engine, etc by a few or several inches ... increasing the loading & grip on the front tires. The rear tires are seeing decreased loading & grip. Both help the car to turn better without pushing.
Somewhere before the middle of the corner, the driver releases the brakes ... and the stored energy in the compressed front springs want to push the front end back up immediately. Then the pitch angle is lost ... load transfers off the front tires & onto the rear tires ... and viola ... the car pushes. This is obvious on cars that handle good on corner entry where they are braking & push mid corner either instantly or soon after releasing the brake pedal.
The front shock rebound valving, controls how long the shock holds the the nose of the car down through the "roll through zone" of a corner (after releasing the brakes & before throttle pick up) ... and how fast or slow it lets the front end come up. This is key to the handling in this critical part of the corner. The optimum valving for this is influencing by the spring rate, car front weight & the length of the corner in general and length of the roll through zone specifically. This varies too much for an accurate formula to be created.
The bottom line is the shock is a tuning device to optimize the slight imperfections of a suspension package. A specific car might have needed a 487# front spring & 584# sway bar bar ... but those weren't available. The car owner put 500# front springs & 525# sway bar and the shocks can be used to "tune the package" to be closer to optimum. Again, no formula for that, because every situation is different.
My suggestion is work with a shock company or shock professional ... discuss your suspension package ... buy based on their recommendation ... and go tune the shocks until you either find optimum or run of adjustment with their valving curve. If that happens you can have the shocks revalved to better fit your needs.
Mateospeed
03-12-2014, 07:10 AM
Wow, thanks for taking the time to write such a detailed response, Ron. Really look forward to getting my hands on your books when they come out.
I'm going to attempt to reply to each issue you've addressed, and try to make corrections to my calculations as needed.
Weight math: made a typo. Un-sprung weight front is estimated at 75lbs each.
Wheel rates/ motion ratios: for some reason, no, I didn't adjust for the lean angle of the front coils, even though I obviously recognized that as a necessity (I stated its lean angle). I re-did my math, which yields coil rates of 431 lb/in and 235lb/in, and wheel rates of 307lb/in and 235lb/in. I'm keeping the motion ratio at the rear as 1:1 for the moment, until I can pull some dimensions from my model for motion ratios as described.
With that information, I believe I can follow your logic that dampers are devices used for tuning. Thanks to Ramey's help at UMI, I believe the Viking dampers have the range of adjustment needed for the coils/ weights I'm using. I'll give them a shot.
NEXT
I'm attempting to sort out the roll bar specs I'll need, but the formulas I'm using (papers from optimumg.com and Steve Smith's Advanced Race Car Suspension Development are fatally flawed. Units don't work out; units in some places are blatantly wrong (moment is not lb/in), etc. Basically, I don't trust them, and would appreciate a better source. Can anyone help there? Perhaps someone has tested a set of formulas (or developed their own) for roll stiffness that they'd like to bounce off mine? I've derived my own, but would like to double-check them.
Thanks, everyone! Thanks especially to you, Ron, for taking the time to increase all of our understanding. You're certainly doing a great job of marketing your new books - but I'm sure you're aware of that.
:)
Cheers!
-Matt