Choosing Your Springs, Anti-Roll Bars and Shocks....
Work With Our Tuning Recommendations to Optimize Your Set-Up
In all our training in suspension set-up at Racing Car Technology, arguably the best help we can give you is to work with you in selection of the springs and anti-roll bars you need for best grip and balance. A suitable shock valving is chosen at the end of the process.
For all the set-ups we have done at Racing Car Technology, and also the set-ups our clients have done, choosing the suspension stiffness you need for best grip at the tyres has been the single most important part of the set-up in improving lap times, sometimes even seconds in lap time due to the improved turn in that happens with the optimized springs.
We calculate the suspension stiffness via a calculation of the "undamped natural suspension frequency". This is a measure of suspension stiffness that is comparable between all road and race cars. To optimize the grip for any given race race car, we choose a suspension frequency we want, and then calculate and fit the spring rates we need to achieve that.
In a recent issue of Ross Bentley's Speed Secrets Weekly, (SSW) (Issue # 477), Ross posed a question for a few of us he knows who are involved in suspension set-up day to day - authors/teachers in set up and vehicle dynamics, race engineers in pro racing, or as in my case, those that engineer race cars for grassroots racers, (owner/drivers racing on a budget).
“Will stiffer springs and shocks make my car handle better? It seems to me that if my car doesn’t lean as much in corners then I should be able to go around corners faster. Am I right?"
Ross added that we should be aiming to answer the question as if from a racer who is also using their track car on the road.
None of the others on the panel of experts agreed with my presentation on choosing springs based on a calculation of suspension frequency. This is a general reflection, I guess, of how few people in racing subscribe to the idea. So, it is difficult for you, if you're reading about suspension frequency for the first time, to work out really whether the idea could be good in your circumstances or not.
After reading my contribution in the SSW, if you're interested in choosing springs and anti-roll bars for your race car, and you'd like to look further at suspension frequency, then email me, Dale Thompson at [email protected] . Tell me a little about your race car and we can discuss.
I wrote:
"I’d like to answer the question from the point of view of a grassroots club racer. We are talking about cars with little or no aero downforce on the car. The question might be from a track day participant, or a club racer in circuit racing with a track only car.
"Your track only car can be optimized for grip with the required stiff racing springs. If you also drive your track car on the road, you need to comprise and settle for the stiffest springs you find acceptable for a reasonable ride.
"The idea we are putting to you is that we can easily calculate an optimized spring rate for your race car springs that will optimize grip, and that there is no softer spring rate that could make your car faster.
"(We should note here that professional racing is a completely different matter. High downforce race cars require super stiff spring to support the car when the downforce comes on in the high speed corners. At times, the race team might try softer springs to help the tyres live or help grip in low speed corners (something we should never do in race cars with no aero). But the overall suspension stiffness of the softest springs used in the high downforce car will still be way stiffer than what’s required in grassroots racing.)
"The questioner says, “….It seems to me that if my car doesn’t lean as much in corners then I should be able to go around corners faster. Am I right?”
"The simple answer is, yes indeed. The reduced roll from fitting stiffer springs does make your race/track car faster, due to the car generating more grip. The tyre is held more upright, relative to the road, thus keeping the tyre contact patch more evenly loaded inside edge, centre of the tyre and outside edge. We can see if the dynamic camber is OK by taking pyrometer tyre temp readings at the earliest opportunity after the car comes in from the track, and add more negative camber if the outside edge of the tyre is too hot.
Shock Absorbers
"The general expectation from racers is that for racing we should stiffen the shock to make more grip. However the opposite is the case. If we make the shocks too stiff (especially at the rear), this will most likely hurt grip at the tyres.
"We should treat shock adjustment as a fine tuning tool that does not require adjustment in most circumstances. Just run appropriate shocks absorber setting that will allow the car to readily transfer weight in the corner.
"Let’s now turn our attention to the questioner’s expectation of better handling with stiffer springs.
Suspension Stiffness – Optimize Your Spring Stiffness for Best Grip and Handling
"As you increase spring rate from standard, the grip at the tyres increases in pretty much a linear fashion, up to point where the increase in grip starts to level off. A by-product of the grip increase and reduced suspension movement is improved response – better steering feel.
"The extra grip comes via a reduction in what’s known as the “contact patch load (CPL) variation”. The stiffer springs reduce the amplitude of the vibration at the tyre contact patch, so increasing the dynamic load on the tyre. (I know, I know. Difficult to visualize, but the effect of the more even dynamic loading of the tyre is very substantial.)
"With optimized front spring rate, you’ll instantly recognize the better feel. The car feels more responsive. And at the track, you can expect to shave seconds from your lap time, compared to running with standard springs.
"To calculate the spring rate needed for any race car, we work with a measure of the suspension stiffness that’s comparable between all road and race cars – the “undamped natural frequency of the suspension”. The so-called suspension frequency is the bounce rate, or cycling of the suspension up and down, as if there is no damping and no friction. It is measured in cycles per minute (CPM) or cycles per second (Hz).
For best grip in racing, recommended suspension stiffness is a suspension frequency around 150 CPM. This applies for all race cars that are stiff enough in the chassis and suspension. Many historic, vintage and classic race cars have too much compliance in this area, so they may be optimized around 120 – 130 CPM.
"Once we have decided on our desired suspension frequency, we can calculate the spring rate needed for that optimized stiffness, then test at the track to see what gains we have made.
"The optimized spring rate is pretty stiff and often comes as a surprise to racers. To give you a feel for the numbers on this, here is an example everyone can identify with.
"In the Spec Miata category in the US, the spring rate of the specified front springs is 700 lbs/inch – around 3 ½ times as stiff as standard springs. Anybody who has driven the standard car vs the Spec Miata can verify that the handling is hugely improved with the 700 lbs/inch springs. When Mazda Motorsports recently developed NC cars to compete in the new Spec MX5 category, the car’s suspension is more modified, including Penske single adjustable race shocks, but the front coils are the same spring rate, 700 lbs/per inch springs.
"So, as demonstrated in the Mazda example, choosing your spring stiffness via the suspension frequency calculation is your first step.
"For most late model production race cars, including the NA, NB and NC Mazda Miata/Mx5 models, and sports racing/open wheelers without aero downforce, optimum front suspension stiffness is around 150 CPM. If there is significant compliance in the chassis and suspension, this hurts grip. Therefore older historic and classic sports and race cars may be faster with less suspension frequency e.g. 120 - 130 CPM.
"For racers driving their car on the road, ride quality becomes an issue. Therefore you have to make a compromise between the stiff, track optimized suspension frequency and the softer suspension frequency you find acceptable for ride on the road.
"So, whether your choosing springs for your road occasional track day car, or your track only race car, it’s worth knowing your suspension frequency."
if you're interested in choosing springs and anti-roll bars for your race car, and you'd like to look further at suspension frequency, then email me, Dale Thompson at [email protected] . Tell me a little about your race car and we can discuss.
