Big Bar Soft Spring set up
Stock Car Suspension Guide - Big Bar Soft Spring Setups
How To Convert To The Popular BBSS Setups
From the March, 2009 issue of Circle Track
By Bob Bolles
read this from Bob at circle track
http://www.circletrack.com/chassistech/ctrp_0903_big_bar_soft_spring_setups/viewall.html#ixzz1rvKJHIqB
http://www.circletrack.com/chassistech/ctrp_0903_big_bar_soft_spring_setups/viewall.html
summarized below
The very first thing to know is that the BBSS setups are not going to be right for all racetracks. There are some tracks where the gains are significant and others where there is either no gain at all or where the more conventional setups are just plain faster.
Most ill-handling cars are traditionally tight and will not turn well. The BBSS setups do help the car to turn better by forcing additional load onto the Left Front (LF) tire.
Handling balance can be accomplished with adjustments to the weight distribution meaning a change in crossweight percent. So, making the car neutral once we have gone to the BBSS setup is usually not a problem.
Aero efficiency is improved, sometimes greatly improved, on longer tracks due to the front valance being lower and the rear spoiler being higher. The soft front springs compress more and the stiffer RR spring forces the LF corner down to where the front of the car is low and parallel to the track in the turns.
There are some interesting results that come from the transition. First of all, if the car is setup "right", meaning all of the way to BBSS as it is applied in most cases, and we'll explain what that means later, the dynamic balance could be way off. The front wants to roll to say 1 to 1.5 degrees and the rear wants to achieve a negative roll angle of from 0.5 to 1.0 degree.
This difference in desires means that a lot of extra load is being put on the RR tire. In older traditional setups that were unbalanced, the rear out-rolled the front and a lot of extra load ended up on the RF tire. That tire soon began to overheat and loose grip. The car either pushed badly or the driver could overcome the tight condition with extra steering input until the car went to a tight-loose condition.
The final result was either a worn out RF tire or a burned out RR tire. One of the two was sure to go. So, the big question is, why doesn't the RR tire give up on the BBSS setups similar to the RF tire that was overloaded with an unbalanced conventional setup?
When the car is unbalanced with the rear out-rolling the front, the RF tire has to not only carry a lot of extra load to keep the car on the track through the turns, but it also works to make the car change direction and turn the car. It's the extra duty of turning the car that overloads the RF tire and causes it to give up so quickly on a conventional setup.
Contrary to the conventional unbalanced situation, with the BBSS unbalanced syndrome, the RR tire does not have to turn the car. It only needs to resist the centripetal loads and keep the rear of the car on the track. It carries a heavier load to help it do that. But, the RR tire will be working harder in the acceleration phase because it will carry most of the rear loading of the car.
Since the BBSS setup is unbalanced with the front tires being more equally loaded side to side than the rear, the front develops more grip. So, there is a need to increase the crossweight percent to tighten the car to a more neutral handling condition. And that is exactly what we find when we do a back-to-back test.
Keeping in mind that there are many variations of the BBSS setups, let's take a look at a common configuration for the straight rail Late Model cars that usually run the touring series. We will offer general directions, so don't run out and put this in your car. Every car is a little different and a slow approach to the transition will keep you from getting in trouble.
The sway bar size runs from a low of 1.375 inch diameter medium wall thickness through a 1.50 inch bar and all of the way to 2.0 inches and more. For most Late Model cars, 1.50 to 1.75 is more common. Some very successful teams have backed off the very large bars and are now running a 1.375 inch bar with either medium or thick walls.
Front spring rates for the BBSS setups vary from a low range of 125 lb/in springs up to 225 and 250 lb/in springs. Again, if you're in the 200 lb/in range, you're leaning more to the soft conventional setup configuration, which is gaining popularity. The RR spring rate is usually increased over conventional rates from 100 to 300 lb/in. This means you would run a minimum of a 250 lb/in spring all of the way up to and beyond a 400 lb/in spring.
The crossweight must be increased along with these changes. Typical increases are from 2 to 4 percent of total weight. It's often better to begin with the lower crossweight range (for a 50 percent front percent car, it's around 51.5 percent cross with a conventional setup) normally used with stiffer springs and smaller sway bars and then add 3 percent or so until the car is neutral in handling.
Since the rear roll angle is a lot less than the front, we also want to lower the Panhard/J-bar about as low as it will go. With most chassis designs, we are limited to going down to 8-9 inches off the ground. Go there!
The RR shock will travel about half as much or less with the BBSS stiff spring in the car, so you need to adjust your RR trailing arm angle so that there won't be any rear steer to the right in the turns. With normal travel of 3.5 to 4 inches for the conventional setups, we usually use around 1.5-2 degrees of trailing arm angle in the right trailing arm. When installing the larger spring in the RR on the BBSS setups, reduce that to half, or 0.75 to 1 degree of angle-front high, of course.
One of the biggest changes that must accompany the BBSS setups is to your shock rates, both compression and rebound. The compression settings generally go up at the RF with the LF compression going down.
The RR will need a little more rebound to control that stiff spring. It's very helpful in the tuning stages of the conversion to BBSS that you use adjustable shocks, preferably double adjustable.
The rebound settings in both front shocks will need to be increased with the LF rebound needing the most increase. Some teams feel the need to run very high rebound rates at the LF corner. We don't agree with that concept and this trend has caused a lot of cars to develop a mid-corner push that can't be corrected with crossweight adjustment.
The amount of increase and decrease in rebound and compression varies as to the track size. Long, smooth, and flatter tracks can use much more rebound control than on shorter tracks that might be rough. Rough tracks also have a negative effect on the RR when using a very stiff spring. The car tends to bounce at that corner instead of negotiating the bumps smoothly. A reduction in the RR spring rate along with changes to the crossweight percent to bring the car back to neutral handling is necessary.
Most of the high-end racing shock companies make shocks that are double adjustable. We've been using the hlins double adjustable shocks on our Late Model project cars with a lot of success. We can make changes to the overall spring stiffness and still have enough range of adjustment to make each work.
Yes, there have been a lot of problems associated with trying to run the BBSS setups, usually because of incorrect application. There are other areas where we need to make changes to accommodate the BBSS setups. The front geometry must be redesigned in order to properly gain the advantages of the BBSS setups. Moment center location is still very important, and camber change characteristics are totally different with these setups.
The BBSS car will dive more and roll less. That means our camber changes at the front are entirely different than what we saw with the conventional setups. Both front tires will loose lots of camber due to the high dive numbers, 3 to 4 inches in most cases and low roll angles that normally would counter camber loss in more conventional setups.
The bottom line is that the upper control arm angles will need to be reduced. If you had say 18 to 24 degrees of upper control arm angle with your conventional setup, you will now need to reduce those to 10 to 14 degrees, all the while maintaining a decent moment center location.
The static cambers themselves must be altered with the transition. The RF must be reduced from the normal (-) 3.5 to (-) 4.0 degrees to under (-) 1.5 degrees or less in most cases depending on the type of tire you run.
The LF tire camber must be increased from a normal 2.5 to 3.0 degrees positive to 5.0 degrees or more. This tire will loose around 4 degrees of camber in the turns.
Ackermann effect is very detrimental to the BBSS setups. If you are used to using some amount of Ackermann in your conventional setups, you can't run it with the BBSS setups. The reasoning is this, with the BBSS setup, the LF corner is forced down and a lot of the front load is carried by that tire. Since it is doing a lot of work, it will compete with the RF tire.
These two tires must track along their proper arcs, tangent to the curve and perpendicular to the radius. We have computed and proven that a car running on a short 1/4-mile track only needs around 0.100 inch of added toe, or about 0.2 (1/5) degree of additional steering angle in the LF wheel. On half-mile tracks, that number goes down to 0.040 inch of added toe or less than 0.10 (1/10) degree of increased LF steering angle.
We have made the statement in the past that we don't see track records falling from the use of the BBSS setups. OK, some have, but there aren't any cars out there going a half a second faster with the BBSS setups. And that doesn't mean it works everywhere.
We have proven that the BBSS setups are not meant for all racetracks. In general, the higher the banking at the track, above 10 degrees or so, the less effective the BBSS setups will be. If your track is above 12 degrees of banking don't even think about it.
Tracks with a rough surface and/or large transitions in banking angle from the straight-aways to the turns are hard to manage with the BBSS setups and you might be better off, and more consistent, running more conventional.
The tracks where these setups shine are the flatter and smoother tracks and the long "super" speedways like Kentucky or Nashville Super Speedway (not the Fairgrounds). Gateway International Raceway is another good one and I'm sure similar setups have been used successfully at the now defunct track in Lakeland, FL.
Out west, we might see the BBSS setups at tracks like Phoenix or Evergreen Speedway. The longer and faster tracks will benefit from the added aero effect to provide more overall grip adhesion for faster turn speeds. On 3/4- to 1-mile tracks, that added speed can add up to several tenths lower lap times.
I recently heard from a team that had won the last two year's championships with a more conventional setup. The team now wants to go to the BBSS setups for next year. I can't understand that reasoning, other than the fact that racers just can't stand still. I preach the idea that you can never just maintain, but at the same time, don't shoot yourself in the foot either.
The choice of setup is entirely yours, so chose your setup based on need. If you're winning a lot with conventional setups, you can experiment during a test session like we did, but not once the season starts. There's not enough time to properly evaluate the difference.
Watch your shock travels at the RF when using very soft springs. If the spring binds and/or the frame contacts the track, the car will move quickly toward the wall. Make spring changes in a progression rather than a one-step change. Adjust the shocks to control the transition. Add a little crossweight with each change to maintain the neutral handling.
Once the car is neutral, get good lap times up to 15 to 20 laps and then immediately switch back to the conventional setup and make another run. See which one feels better to the driver and which one is faster, especially for longer runs. Compare those times with the usual lap times everyone else runs. Make a choice and go with it.
If you decide to go with the BBSS setup, go all the way. Do the very soft front springs, install a larger sway bar, increase the RR spring rate by at least 200 pounds and adjust your shocks to complement the setup.
