Soft does not equal grip part 3, anti roll bars

Soft Does Not Equal Grip - Part 3 - Anti-Roll Bars (ARBs)

Previously we discussed damping and springs, now we move on to the next bit in the suspension; the anti-roll bar (ARB) or sway bar. Much has been written about its functionality so we won’t beleaguer the point here. The goal of this post is to address the approach in balancing the rate of the ARB with the rate of the springs. As with many elements of race car setup, there can be some subjective points applied to the objective. We’ve found that many of our clients benefit from understanding this balance and have found what works for them best because of it.

Balanced

Balanced

Roll Control

The role of the ARB is to do what it is named for; control the roll of the vehicle when cornering force is applied. With any racing or performance car, we will normally look for a substantial increase in roll control compared to an average road going setup. When we look at a vehicle in pure roll (this almost never happens by the way) the main springs and ARBs are more or less equivalent. You add the contribution of the springs and the bars to the roll stiffness and viola, that is how much your car will roll per unit of g force. Our goal here is to make sure we keep the floor of the car at an acceptable angle and the tires at an acceptable range of camber.

Pretty close to pure roll

Pretty close to pure roll

Throw in Pitch

Roll analysis is easy on paper. We’ve all seen the car roll of the trailer that the engineer proudly says is perfect. This same car has infuriated many a driver. When the car is being driven around a non-perfectly smooth race track there is a constant state of change in roll AND in pitch. Therefore, we must mind our trade-off here carefully. Given a desired amount of roll control, the stiffer your bar is, the less pitch control there is. This means that the front or rear will move up and down more than if you did the job with the main spring. Conversely, with a soft ARB and stiff springs, we have limited pitch movement and limited roll movement.

A C6R in roll & pitch

A C6R in roll & pitch

The Bumps

Just as in the case of pitch, ARBs also present complications with respect to road irregularities, curbs, and general bumps. Since the ARB connects the wheels, the stiffer it is the more connected those wheels are. If you’re running a stiff front bar you may not be able to nail the inside curbs as hard without making some compensation elsewhere. This is not always a bad thing and many creative people have used it to their advantage. The point is that we must keep aware of it so we consider the whole picture when making setup decisions.

No discussion about kerbs is complete without V8 Supercars

No discussion about kerbs is complete without V8 Supercars

The Balancing Act

Getting ARBs right in the big pictures takes time, just like the rest of the setup. We’ve seen big spring soft bar cars win, and others with (what Carroll Smith aptly named) a solid axle conversion kit installed. Much of this comes down to driver preference and style. How each driver approaches the corner, whether they use kerbs, and how they throttle out. As an aside, I have noticed that drivers who come up in dirt cars or sedan classes and drivers who came up in karts are on opposite ends of the spectrum.

We also have the car to consider. Does the front ride height need to be within a small window? Probably can’t use a big front bar. Big V8 with loads of torque? Might not be best to have a big rear bar, but probably loves a stiff front ARB. Front drive touring car? Big rear bar may help you get off the corner very well. The permutations are endless but armed with practical awareness, we can make good decisions.

If you’ve made a bar change that gave positive results in some places and set you back others; look at pitch and bumps. Do your best to isolate each condition and list out the positives and negatives. Talk to your driver and watch what they’re doing in videos. By breaking down the situation into when the bar is working hard and when it is not. Take a lot of notes and be willing to try some counter-intuitive things, you’ll be surprised at what you can learn.

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A practical guide to gas pressure

Canister pressure in dampers is often misunderstood in what it does and does not do. Here, we’ll go over some practical points.

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Generally, all gas pressure does is add to the amount of force it takes to move the suspension from full extension and/or to hold it at any other position. In the world of springs, this is known as preload. Spring preload has been and still is used in myriad applications to manipulate static loading versus dynamic stiffness and/or natural frequency.

This was the original goal of the JRZ Suspension designer, Jan Zuijdijk. Jan wanted the large 22mm shaft to (among other things) hold up the car with the gas pressure. If he could use some pressure, then there wouldn’t be as much spring compression. This allowed less energy to be stored in the spring and thus less for the damper to work on. The less the damper had to do, the less the tire had to deal with, so it could make more grip. The graph below shows the concept:

The blue line is a softer spring with gas pressure, the red line a stiffer spring

The blue line is a softer spring with gas pressure, the red line a stiffer spring

Moving on, we have spring rate effects. Depending on the damper, this can range from negligible to significant. Generally, the smaller the gas volume, the more “springy” a damper is. This is because gas pressure increases with inverse proportion to volume. When a damper compresses, the gas volume gets smaller. The more the damper compresses, the higher the pressure goes, the more force you get. So then you have increasing force with travel, which is by definition, a spring. If we start with a small volume, the force could double or triple with a full damper stroke. JRZ makes the dampers with a relatively large canister volume relative to the shaft size. This allows us to neglect the spring effect for the most part.

What about the effects on damping and shock adjustments? Simply put, past a certain point, there are none. A damper will need enough pressure to ensure it will not cavitate where the fluid reaches vacuum and boils. Other than that, the damper pressure doesn’t change a lot hydraulically. The damping forces at the particular forces will remain the same. What the pressure does effect is the seal friction in most dampers, which mathematically is damping. Though with the right seals this is not a major consideration for most users.

So how do I use gas pressure to tune my chassis? Lets finish off with the good old fashioned practical list:

  1. Use gas pressure to adjust ride height, + or - 25 psi at a time to evaluate rake

  2. Reduce pressure on lower grip surfaces to allow for smaller movements of the track to move the suspension, improving traction

  3. Increase pressure on “support” tracks like Watkins Glen or Road Atlanta to hold up the chassis without adding spring rate, increasing grip over bumps

  4. In 50 DA applications, increase pressure to solve “very soft over the bumps” issues

Rebuild intervals - when do you send them in?

We get this question a lot: when should I send my shocks in to get rebuilt? Like most other things we discuss here, the answer is; it depends.

An early 911 strut in for rebuild at the right time, before it's covered in oil!

An early 911 strut in for rebuild at the right time, before it's covered in oil!

Some roads, tracks, and driving styles are easier on dampers than others. Some cars are harder on dampers than others. Installation is a factor too. For example, are the dampers and/or canisters subject to heat from exhausts, radiators, brakes? All of these things can influence wear to the seals, bearings, and fluid. For example, this Cayman front strut did not have a ton of time on it, yet had significant wear and dirty fluid:

This is a good exmaple of dirty strut fluid that wears bearings, seals, and interferes with proper valving function

This is a good exmaple of dirty strut fluid that wears bearings, seals, and interferes with proper valving function

One of the harder parts for judging rebuild needs is the fact that damper performance typically degrades slowly. Just like your vision, sometimes you don't know you need glasses until you try on the right pair! We've had sets sent in that people were generally happy with, only to be blown away when they put the freshly rebuilt set back on. 

Before (right) and after (left) damper parts cleaned in a sonic tank 

Before (right) and after (left) damper parts cleaned in a sonic tank 

Most of the time, we get dampers sent in after they've begun to leak. Generally, this is inadvisable. A parallel: you would not wait to change your engine oil until there's leaking and smoke. This of course means you've missed your opportunity for routine maintenance and end up replacing a bunch of stuff you didn't need to. To avoid all of that, here are our general guidelines for rebuild intervals:

  1. Racing a full season and doing test days, rebuild once a year and perhaps check on the dyno during a mid-season break
  2. Club racing a 3-8 times a year, every two years during the winter is fine
  3. Track cars doing 5-10 weekends a year, every two to three years during the winter
  4. Street/DE cars, every two to four years. Have a look at the suspension once your level of satisfaction starts to wane or you keep having to make adjustments to be happy. 

Please email or call us with detailed questions!

-The Olsen Motorsports team