Poll! I am curious whether Club Fords run stiffer springs in front or rear. Mine will race vintage, if that's important, and has inboard in front. No need to reveal secrets on spring rates, just the basic front vs. rear.
Thanks,
David
Front springs are stiffer than rear
Rear springs are stiffer than front
Same spring rate all around
Changes depending on track
Do you run stiffer springs in front or rear?
Poll! I am curious whether Club Fords run stiffer springs in front or rear. Mine will race vintage, if that's important, and has inboard in front. No need to reveal secrets on spring rates, just the basic front vs. rear.
Thanks,
David
Spring rate doesn't matter much, what matters is the installed rate after the suspension geometry gets involved. I set up a number of vintage and club fords and I always started around with wheel rates around 50% to 60% of corner weight (for the old Dunlop tires). Since the front corners are almost always lighter than the rear corners, they had lighter wheel rates. Oddly, since the motion ratio at the front was usually lower than the rear, that meant that I often ran very close to the same spring rates front and rear. I usually ended up with springs between 350 and 450 inch lbs, fwiw.
Spring rate or wheel rate? Many CFs have quite a layover angle difference between the front and rear springs.
If I remember correctly when I initially did the calculations on my Crossle (doubtful), a 400# spring rate on the front and a 350# spring rate on the rear gave about 250# wheel rate at both ends.
TroyT - SFR SCCA, VARA, CSRG, SVRA
www.ThingsThatGoVroom.net
'00 Carbir CS2, P2 #60
'79 Lola T492, S2 #61
The advise you are getting here is spot on.
You have to work from wheel rates. And you have to take into account the weight on that wheel.
For my calculations, I use the spring rate at the tire contact patch. That takes the spring rate of the tire plus the spring rate of the wheel. Most of the time, adding in the tire spring rate does not change this much. But there have been tires where the spring rate for the front and rear tires are a lot different and it is necessary to take that into account.
When I start doing a setup for driver and a track, I start changing spring rates very early in the process. The balance of springs rates is the most fundamental tuning tool in chassis setup. And when I change spring rates, I always change the static ride height to account for that spring rate change, stiffer springs lower ride height and softer higher ride height.
If you really want to be anal, you should be changing ride height with a tire pressure change because you are changing the spring rate at the tire contact patch.
Are you sorry you asked the question now?
Not at all!
Glad for more data points. Steve, you have been very gracious with time for those of us with Zinks and Citations. I will say that I, and maybe many others, just don't have the funds or track time to experiment with and swap around springs as much as some do. I'm going to be limited to maybe two or three spring rate options. And, if I'm only getting a couple of practice/qualifying sessions per race weekend, I simply don't have the track time to experiment with springs. I hear you on the reasons why, and they are all solid, obviously, but practical limitations - time and money - have a vote. Seat time in a consistently performing car is probably my best bang for the buck right now as I've only got four races total under my belt and those were in 2013.
What I have gleaned from a few months of asking and listening, for my situation, is that I should run considerably stiffer springs than were on the car, that the bias Dunlops can handle stiffer springs than softer-sided radials so I should know what tires the advice-givers are running, that stiffening the rear adds grip to the front and induces oversteer - and vice versa, that most people running a Z-16 have front springs 100-200lbs softer than the front (more often 200), and that I can be more confident in a lower ride height now that I'm going to springs 2 to 3 times stiffer than were on the car when I bought it. Because I'm building it from a slider, the car has yet to be on four wheels on the ground with engine, gearbox, fuel, fluid and me, so the actual corner weights and ride heights are still in question. The next 2-3 weeks will reveal all that.
I plan to start with 800/1000 at Wild Hare, and 1.5"/2.0". I may not do a change that weekend because I think there is only one practice and then two qualifiers. I just don't know that I'll have the racing sense needed to properly evaluate handling with different springs. However, in May I'll be doing the VRG driver school as well as the race and may be able to experiment with 800/900 and 700/900, maybe even the 1000/1000 option. But, that's going to be the limit. Maybe I'll ask one of the instructors to take 'er for a spin and give me some feedback.
David
As you said, Steve Lathrop is very generous with his time, and sharing his set up knowledge with Zinks and Citations. Dave W. is also a wealth of knowledge. If I were you I would call Steve, tell him what tires you are planning to run, and ask for his starting point recommendations. You may have to adjust based on your driving style, but I would be willing to bet you will be in the ball park.
Maybe changing springs over the first weekend might be asking a bit much.
But I going to suggest that every time you go on the track, you change something. And I would start by lowering the rear of the car 1/16 to 1/8 inch after your first session. If nothing else, this will tell you how low you can go. Until the race, always change something to get a feel for how things change. Keep changing the rake of the car until you optimize that setting. Next try changing the air pressure in the front or rear tires about 2 psi. If the handling is better, you know that you want to change springs. Changing air pressure on many tires will change the tire spring rate about 40 pounds per inch for 1 change in psi. If the handling is not better, go the other way with air pressure.
In short keep making little changes like this and over time you will have your car tuned up. A well balanced car will also get the best performance out of the tires and that includes endurance.
I had a funny rule of thumb for setting front ride height on a club ford type car, but it worked on every car I built, including a Lola T492. We always raced most events at Mosport, which has a big braking zone going uphill into turn 5, and the car would ground hard there if the ride height was too low, so that became the low ride height limit. It worked out that if I set up the car with the springs I was going to use, if I stood on the front of the car in line with the front axle line and jumped up and down, if I could just barely get it to hit the ground that was the right ride height. Hitting easy with a bang was too low, and if a moderate bounce up and down didn't get it down to the floor, it was too high...
Highly scientific, but what the heck, it worked!
Then you went and gained 20# and had to find some kid to do the jumping.
Originally Posted by Brian
I had a funny rule of thumb for setting front ride height on a club ford type car, but it worked on every car I built, including a Lola T492. We always raced most events at Mosport, which has a big braking zone going uphill into turn 5, and the car would ground hard there if the ride height was too low, so that became the low ride height limit. It worked out that if I set up the car with the springs I was going to use, if I stood on the front of the car in line with the front axle line and jumped up and down, if I could just barely get it to hit the ground that was the right ride height. Hitting easy with a bang was too low, and if a moderate bounce up and down didn't get it down to the floor, it was too high...
Highly scientific, but what the heck, it worked!That was going to be my question, PF:Then you went and gained 20# and had to find some kid to do the jumping.
How much do you weigh, Brian? Your technique probably won't work for someone much lighter (or heavier), although there is some self-correction built into the technique.
I have used Brian's standard for front ride height.That was going to be my question, PF:
How much do you weigh, Brian? Your technique probably won't work for someone much lighter (or heavier), although there is some self-correction built into the technique.
What Brian weighs is not important because his weight is in the car. What is important is that you are the weight in the car and that is all that matters.
I think I understand what I wrote.
I have it from authorities that should know... Steve is not a confidant of Captain Morgan.
OBTW, a 200# person works for Road Atlanta.
But, if we could ever get Steve and Mr. Pare to put on another setup seminar as they did in 2002 - 2003 .... there are a WHOLE bunch of folks that need to sign up!
The best money I ever spent on racing!
Screw all those videos and data you are collecting. You won't know what to do with it until Lathrop and Pare spend two days explaining what the hell is really happening to that little tire patch that is your friend.
We really need to get them back on the stage before they start spending all their time on horses or airplanes.
Seminar
I'd be very interested!
Sorry to be ignorant, Steve, but can you give me a vector on the air pressure and spring rate change? If, for example, I go from 15F/16R lbs, to 13F/14R lbs, and it feels better, then what direction do I go in spring rate and do I then put the tires back to what they were and just feel the spring difference? Or....?
DK
The tire manufacturers will give you a number for the tire spring rate at a given pressure and a number for how much that spring rate changes with a 1 psi change in pressure. The sensitivity to pressure number is usually in the range of 30 to 50 pounds per inch change in spring rate of the tire. A common spring rate for FC type tires is 1100 lbs./in. at 20 psi. At 18 psi the spring rate might be 1010 lbs.
Next, say I have a 600 pound spring on that corner of my car and I have a 0.8 motion ratio of spring movement to wheel movement. The wheel rate for that spring is 384 lbs./in. The spring rate at the contact patch of the tie is 285 lbs./in. with the tire at 20 psi. At 18 psi, the rate is 278 lbs./in. If I change the springs by 50 lbs. to 550, the new rate will be 267 lbs./in. with a tire at 20 psi. While the changes seem small, you can easily feel that change. And that change at one end of a car might just be the change that brings a better balance. The formulas are motion ratio squared times the spring rate gives the wheel rate. The spring rate at the contact patch is wheel rate times tire spring rate divided by the sum of the wheel rate and the tire spring rate. As a general rule, a 10 lbs. change in the forces at a corner is a significant change.
Lets say I am just clearing the ground with the car on the 600 lbs. springs and I want to go to 550 springs. I can't leave the ride height unchanged or I will be pounding the ground. Lets say my ride height was 1.25 inches with the 600 lbs. spring. My new ride height should be 1.33 in. instead of 1.25 in. That is simply a proportional change in the ride height to the change in the spring rate at the tire contact patch, (285/267)*1.25 = 1.33. Notice also, if you are that close on ride height, lowering tire pressures 2 psi will require a ride height adjustment as well.
If you change the tire pressures at both ends of the car, you are looking at the effect of lower spring rates all around. That is a totally different change than lowering the rates at just one end. Now the question you have to answer is the car better with lower spring rates or do the tires just work better at the lower pressures? But you have an idea that something has to be changes. It may be that the car is a little lazy with the soft tire pressures but the tires seem happier. Then you could go up on spring rates and down on ride height (assuming that you were not hitting the ground) and leave the tire pressure at the lower number. This is where you feel as a driver is what is really important. Do those things that give you the car that you are most comfortable with.
Last edited by S Lathrop; 02.28.16 at 1:06 PM.
Depends on the tire construction, but for most of ours, 40-50 pounds of rate per pound of pressure. The manufacturer can tell you the correct rate change.
The seminars were fun to give, but that train rolled out of the station a few years back, unfortunately!
Help....
I'm missing something on the math. Would this be the correct formula for tire contact spring rate?
TCSR= (WR x TSR) / (WR + TSR) ???
I get the same result as you for the 1100 TSR tire but I get a higher TCSR number for the 1010lb TSR tire. That doesn't make sense. I'm making a mistake somewhere.
Scott
I went back and redid my calculations. I originally used a different spring rate and did not make all the changes correctly.
Your formula is correct.
Exactly how are the spring rates effecting the tire performance?
Is it about vertical load compliance and/or tire temperatures?
Brian
OK thannks.
I was getting an ice cream headache trying to figure out what I was missing........
Scott
The way I do springs in series is the same as doing resistors in parallel. 1/Stotal = 1/S1 + 1/S2. Which amounts to the same thing. I just have Ohms law hammered very deeply into my brain...
Brian
Both. But you need to have a feel for how the tire compound works. My experience says that Goodyear is more temperature dependent than say Hoosier. Harder compounds require a different setup than soft compounds.
But to make it more complicates, I am more and more looking at the rate that the tire is loaded when you brake, turn or accelerate. In this analysis , spring rates are only part of the equation. I can have very soft springs but load the tires very quickly or I can have a very stiff spring but load the tires relatively slowly because other variables are different. Some cars require a lot of damping while other don't need vary much. Bottom line, I don't think there is a right or wrong way to setup a car, but the whole package either works or it doesn't. Much of what works is set down in the design of the car and you have to be consistent with the design philosophy of the designer.
I now understand why I'm not an engineer and am truly thankful to those who are.
"An analog man living in a digital world"
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