Thursday, July 9, 2009

Springs, Part 2

Tuning with springs

If there were any question that springs are a powerful tuning tool, we'd only have to look at the extensive spring inventory in the typical race trailer and at the amount of time the typical race car spends on the setup pad getting springs changed. A complete treatment of spring tuning would probably be a whole chapter of a book. Today, we'll just offer some fundamentals to stimulate thought.

There are two major influences that springs always have:
-Dynamic ride height changes from longitudinal load transfer,banking loads, and aero loads.
-Roll stiffness and the resulting front-to-rear distribution of roll couple
Tackling these first...

Softer springs allow more ride height and pitch change from braking and acceleration, stiffer less. For example, allowing the nose to drop more on braking should increase front downforce percentage and maybe total downforce amount, lower the height of the mass at the front of the car, and lower the front roll center. IF these work, the result is a sharper turn-in response, which in turn can lead to less mid-turn understeer (even after the nose rises) by the simple virtue of getting the turning done earlier in the corner. But, too much drop can destabilize response, shut off the front aero, move the suspension into the bump stops, cause adverse camber changes, blah, blah. A similar but different set of concerns exists for stiffer springs. And, a similar but different set of concerns exists for the rear springs. As always, the key is finding how much is just enough and how much is too much.

In roll, the springs affect both the roll stiffness and the feeling of support perceived by the driver. With front AR bars creating a fairly significant portion of front roll stiffness on most rear-drive cars, the effect of springs on roll stiffness on the front of the car is relatively subtle compared to their effect at the rear, where the AR bar is relatively soft or even disconnected. But subtle may be all you need, if the front-to-rear bar balance (a whole 'nother blog) is already good.

Of course, one reason springs are so powerful for tuning is that you get both ride height and roll stiffness changes in one tuning change. If you choose to run the same static ride height with a stiffer spring, you get more roll stiffness, less vertical movement, and higher dynamic ride height under aero/banking loads. If you choose to run a lower static ride height with a stiffer spring, you still get more roll stiffness and less vertical movement, but the dynamic ride height will depend on where you set the static. You have to choose.

But wait, there's more. We haven't gotten the full $29.95 worth yet.

Springs have to be matched to the tire. I've seen a change in rubber compound and carcass construction require as much as a 20% change in spring rate to retune a car. With serious sim work and full tire test data, we might be able to reduce the "cut and try". Outside of F1 and NASCAR, the average race team simply doesn't have the money or information, though. The tire might have different vertical or lateral stiffness, different contact patch shape, different internal damping, a compound that requires "working" the tire to generate sufficient heat, etc. There's also the issue of the extent that tire performance falls off over the tire's life.

Springs have to be matched to the track. In broad generalities, lower-grip venues require softer springs, within reason. At higher-grip venues, the car may feel sloppy and unresponsive if not supported by some combination of stiffer springs, stiffer bump damping, and/or stiffer bars. Bumpier venues may require softer springs and higher static ride heights, if a solution doesn't exist in damping. Tracks that put a premium on corner-exit traction or tracks with a persistent reputation for understeer may require a different front-to-rear spring proportion.

Springs have to be matched to the driver. Some drivers like softer, some stiffer. Engineering doesn't completely win out. If the driver isn't happy, you aren't fast. I once saw a team make a nearly 100% stiffer spring change to accomodate a new driver. He won the same number of races as the preceding driver, and was equally or more competitive in general. Same car, same tire, same tracks...

The holy grail of 7-post rig testing, the RMS variation of contact patch loads, is generally better as the springs get softer. But, we can't chase this in isolation either, as should be clear by now.

So, how do we choose spring rates and then tune them?
-Thoughtful consideration of the driver's comments
-Detailed review of the the logged data
-Careful analysis of the available tire data
-In-depth review of all the vehicle dynamics and aero data for your car
-Testing, either stand-alone or in the context of a race event
-Assessment of the track's needs

I can't over-emphasize the need to break free from dogmatic thinking. On two different cars in the last 12 months, I've improved front grip by going stiffer on front springs. Both cars were simply moving too much. On one of these, I saw too much front roll in the logged data. Fairly easy call. But, on the other, it came to a "gut level" call. Nothing in the data or the driver's comments pointed the way. On that car, I had to buck the skepticism of the drivers and the car owner. Luckily, one of the drivers was open-minded enough to go "Hey, it worked even though I thought it wouldn't".

And one detail - street car thinking says don't have the same natural frequency front and rear. See Gillespie. While many factors may well lead you away from this on a race car, I personally haven't found it to be a sin when it did happen.

Have fun!


  1. Hi Buddy, great blog!

    In the case of a car where aero has no significant influence, the exceptions to softer springs up front reducing understeer would be reduced even further correct?

    I understand the need to mention the important exceptions, but isn't is true that the vast majority of the time (especially on a non-aero car), stiffer spring up front leads to less mid corner oversteer ?


  2. David,

    The general expectation is that softer front springs will reduce understeer and stiffer will reduce oversteer, with more influence in initial entry and mid-corner than on exit. Much of the time, this is true. But, even on cars without significant aero, it's possible that softer will be worse, if that makes the car too soft. One of the two recent examples that I mentioned in the main blog post was a car with relatively little front downforce.

    It's really important not to fall into the "vast majority of the time" trap. You have to be mentally questioning every change, unless it's within a range you've previously explored and have a lot of confidence about.

  3. Hi Buddy,

    How do you factor in the suspension geometry i.e. "roll centers" and anti-pitch/squat into your spring choices?

    What have you found to be the trade-off with changing geometry as compared to springs?

  4. Your spring choice relates to ride and vertical load issues, as well as pitch/roll control. I often get the gut feel that these predominate over pitch/roll issues, assuming you aren't way out of the ballpark to start. As a practical matter, I rarely find myself making dramatic tuning steps in geometry - many modern cars are already fairly well thought-out in that regard. So, for incremental changes in anti- geometry, spring changes are unlikely. Anti dive, squat, and rear brake antilife can then be used as fairly tightly focussed tuning tools, affecting only certain types of behavior in certain parts of the turn.