I see many different types of setup these days and racers trying to do things that the laws of physics just won’t allow. You don’t have to be an engineer to understand the problems that can come as a result of this. With just a basic understanding of how the dynamics of the race car work, we can all choose our shock rates correctly to complement our setups.
In the modern world of short track racing for both dirt and asphalt competition, shocks have become one of the most important tuning tools we have. They can complement the current setups, especially the radical bump setups if applied correctly.
Just recently I was involved with setting up a Pro Late Model running bump springs on the front. The team had inherited a set of shocks that were valved correctly at the front for the bumps, but all wrong at the back for the rates of springs we were using. I then used the information contained herein to solve my problem and it improved the performance of the car quite a bit.
The following information is useful whether you are running more conventional setups or the more radical ones. Think along with me as I discuss how shocks affect the speed of movement and the load distribution at the four corners of the car as we transition from high speed to minimum mid-turn, and then to up high speed again.
One of the most basic and important things to remember is this: Shocks do not affect the handling of the car if they are not in motion. To say that you can tune your mid-turn, steady state, handling with shock changes is just wrong. Any competent shock expert will tell you that up front.
Shocks Work With Springs – Controlling wheel movement would be much easier if all we had to work with was the shocks. But in reality, our race cars are supported by a set of springs. Basically, we always want to match our shock rates to the spring rates, and/or the bump device you might be using. In all situations, the shocks rebound rate will always be greater than the compression rate for any racing shock because the spring helps resist compression and promotes rebound.
As we install stiffer springs, we would naturally need to increase the rebound resistance and decrease the compression resistance. Stiffer springs would include adding bumps to one or more corners of the car. Each bump device has a spring rate in the range of motion it is operating in. Bump springs have a constant rate that is just added to the equivalent ride spring rate.
So, we may say we have a soft spring setup because we install 150 ppi ride springs at the front in our coil-over car, but when we add the stiff bumps to that, we end up with 1200-1500 ppi or more spring rates. That is why I am avoiding saying, soft spring setups when referring to bumps setups.
Therefore, with those setups, keeping with the idea that the shocks must control the installed spring rate, we must run shocks with a rebound rate upwards of 1200-1500 pounds at 3-5 inches per second of speed.
“Tie-Down” Shock Terminology – This brings us to an important discussion. The use of the term “tie-down” has been around for some time to describe shocks that are high in rebound resistance. The idea initially was that if we install these high rebound shocks, we can tie that corner of the car down and keep the tire tied to the track. This is completely wrong.
First off, a tire is not connected to the track surface, it is free to rise and fall according to what the other three corners of the car are doing at that instant on the track. If our setup wants that corner to move vertically because load has shifted off of it, then the load will come off regardless of how much we “tie down” with shocks. You just won’t see much suspension movement.
So, we might be fooled into thinking that it indeed did work. No it did not. If enough load comes off that corner, or the setup is unbalanced enough, the tire might well come off the track surface even without suspension movement.
This unloading of the tire will occur on the left front or left rear normally. So, we cannot tie the left front tire down and we cannot tie the left rear corner down, although I’ve seen some teams try. Lack of movement of the suspension does not mean the load remains on the tire.
What we can do is control the spring rate of the ride spring plus any bumps we have installed. If your left rear spring is a 175 ppi spring, then you need to use a normal shock rebound rate associated with that spring or risk taking much of the load off that tire on entry into the corners. On some tracks when using too much rebound, not only will you be loose in, but also loose off due to the way the track banking transitions.
Entry Tuning – If we split the front shock compression rates with a RF shock using a stiffer compression rate than the LF shock, then, while the suspension is in motion due to load being transferred to the front on entry, the RF suspension will move slower than the LF suspension. Additional load will be transferred onto the RF and LR tires causing a momentary increase in the cross weight percent in the car.
This obviously tightens the car. The affect is more pronounced with conventional setups that might be used now days in the classes that are restricted from using bumps, or the stock classes. But it can still affect the bump setup cars.
It is important to note here that the load transfers almost immediately when a force is presented to enact that transfer such as applying the brakes. As we apply brakes entering the corner, the load transfer happens quickly. If on entry we transfer 300 pounds from the rear to the front, the 300 pounds goes to the front in an instant.
The distribution of that 300 pounds between the two front tires, while the suspension is in motion, assuming a new attitude that will support the additional load, will depend entirely on differences in stiffness of the suspension systems at all four corners. Stiffness is defined as the resistance to movement influenced by the shocks and springs.
The result of all of the above is this. The slower moving (or stiffer) corner will momentarily retain more of the transferred load while the suspension is in motion traveling to a new attitude.
Cross weight, as we know, is defined as the percent of the combined RF and LR weight divided by the total vehicle weight. If the cross weight percent increases, then the car will be tighter on entry and the car might be faster if that is the desired effect. This is exactly why it has been said that a stiffer RF shock will speed up load transfer to that corner.
Later in the entry event, some of the load that has been transferred onto the RF due to that corner being stiffer in compression will transfer to the LF tire as the car reaches a steady state at mid-turn. Then the normal cross weight you set the car at will apply.
If the car is already tight on entry, after having eliminated common causes of tight entry such as rear miss-alignment, rear steer or brake bias issues, then an opposite effect can be utilized. If we install a LF shock that is stiffer than the RF shock, and/or run a stiffer LF spring than on the RF, then we can effectively reduce the cross weight in the car on entry while the suspension is in transition by loading the opposite diagonal, the LF and RR. As one diagonal goes up in percentage of supported weight the other goes down.
Modern bump setups use much lower front compression settings due to the high spring rate of the bumps. We can still utilize this method by creating a compression split. The effect will be less than with a conventional setup, but still somewhat effective.
Once the shocks are firmly on the bumps, the load distribution will equalize and the advantage of shock compression split will be nullified. If the shock rebound settings are high enough, the shock will never leave the bump and no amount of compression split will work because there will be very little shock movement.
Exit Tuning Using Split Valve Shocks – Corner exit performance can be improved by utilizing the shocks. This is done by either splitting the compression settings in the rear shocks and/or utilizing the rebound settings in the front shocks. In a car with equal rear spring rates, a stiffer compression setting in the LR shock than in the RR shock will load the LR and RF corners. Load is transferred to the rear under acceleration and while the rear suspension is in motion, this split will tighten the car by increasing the cross weight percent.
For non-bump setups, a shock with a stiffer rebound rate at the LF corner can help accomplish the same effect by causing a slower movement of that suspension and a more rapid transfer of load off of that corner which in turn increases the percentage of load supported by the RF and LR tires.
With the bump setups, the rear compression settings can really help a car that is otherwise limited in adjustments. If the RR spring is somewhat stiffer than the LR spring, and this is very common with bump setups, there will be a loosening affect on acceleration when load transfers to the rear. The stiffer RR spring causes load to be placed on the RR and LF corners reducing the cross weight percent.
By installing LR shock with a much stiffer compression rate, and a RR shock with a much softer compression rate, you can nullify the negative effect of the stiffer RR spring. This serves to equalize the unequal resistance to compression due to the dissimilar spring rates and helps keep the car tight on exit.
Putting All Of This to Use – In order to utilize the configurations we have discussed here, we must be able to use a range of different rates of shocks in order to find the right combination for our car at a particular race track for a particular setup. For a team that races at only one track, the process is fairly simple.
You would experiment to find the fastest set of shocks and ones that suit the driver’s style and stick to those staying within the boundaries of physics. For teams that travel to different tracks, some changes might be necessary if the setup (read as spring rates) needs to change and/or the track layout is different from track to track.
Most shock experts agree with certain basics, such as:
1) The shock package should be softer overall when racing on dirt and when the track is flatter when on asphalt for the conventional setups.
2) Get your basic setup close to being balanced before trying to tune with shocks. Shocks cannot solve basic handling balance problems.
3) Higher banked tracks require a higher overall rate of shocks and springs as opposed to flat tracks. This is because of the higher speeds and the extreme amount of down force.
4) Shocks that are mounted farther from the ball joint should be stiffer than if they were mounted close to the ball joint. That is because with each inch of travel of the wheel, the shock mounted farther away will move at a slower speed which means less resistance in both rebound and compression. This is also true of shocks mounted at high angles to the direction of motion.
5) Of the two transitions, tune entry performance first. If there are no entry problems, make small changes if you want to experiment to see if entry speeds can be improved. Entry problems include a tight or a loose car. By far the worst problem would be the loose-in condition. This can involve an alignment problem, but far too many times, I have discovered a LR shock that is too stiff in rebound.
6) Tune exit performance last. If there are no exit problems, don’t make any significant changes. Exit problems can include a car that pushes under acceleration or one that goes loose under power. Be sure that you do not have a Tight / Loose condition where the car is basically tight in the middle and goes loose just past mid-turn. This is fixed with spring rate and/or panhard bar adjustment, etc.
7) On dirt race tracks, reduce rebound settings on the left side and decrease the compression rates on the right side for dry slick surfaces to promote more chassis movement. This helps to maintain grip as the car goes through the transitional phases of entry and exit.
8) For the bump setups on asphalt, the whole shock package must be much different than when running conventional or soft conventional setups. The bump spring rates (either bump rubbers, bump stops, or bump springs) will be very high and so the shock rebound rates must match those high rates in order to control the ride spring and bump device.
Using a bump that is rated in the 1000 pound per inch spring rate range will need a shock that is rated at around 1000 ppi at 3 – 5 inches of movement per second. Usually the low speed rate of the shock will be comparatively high too and we often see a “nose” rate of between 500 and 800 ppi or more at less than one inch per second speed of movement.
A Closing Caution – The suggestions provided here are representative of trends that can enhance your handling package. Before any of this can work, the setup must be balanced, the steering characteristics must be ideal and the car must be aligned properly. If not, you will probably chase the setup and experience a lot of frustration and expense.
Shock tuning is the last thing to experiment with in order to try to increase your race cars performance, but it is nonetheless a necessary step in finding the ideal total handling package. That said, before you setup your car and chose your shocks, evaluate what you will need as far as shock rates that will match the spring rates you will run.
If you are experimenting with the bump setups, consult your shock expert so that you can match your shocks to the spring rates you will be using. Most racing shock companies have technicians who are very familiar with those setups and can advise you as to the best rates to use to match your bumps.
Sources:
AFCO Racing
www.afcoracing.com
(800) 632-2320
Integra Shocks and Springs
www.integrashocksandsprings.com
(800) 472-3464
Penske Racing Shocks
www.penskeshocks.com
(610) 375-6180
RE Suspension
www.resuspension.com
(704) 664-2277
QA1
www.qa1.net
(800) 721-7761
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