Anti-Roll Bars Part 2. *Suspension Stiffness in Roll. *Get More Grip With Stiffer Springs and ARBs

Uncategorized Jan 10, 2020

The details of anti-roll bar operation are discussed in our blog article, “Anti-Roll Bars. A Good or a Bad Thing?” 

As indicated in that article, there’s much more to anti-roll bars than simply reducing body roll.

In this blog I want to focus on two super important characteristics of anti-roll bars in suspension set-up for racing…Suspension Stiffness in Roll and How You Get More Grip With Stiffer Springs and ARBs.

Suspension Stiffness in Roll

The anti-roll bar adds to suspension stiffness in roll i.e. In cornering, both the spring and the anti-roll bar contribute to the suspension stiffness as seen at the tyre contact patch.

I’m going to quote a vehicle dynamics expert here, so you don’t think I am making this up:

“Typically, a bigger bar on the front improves the perception of nimbleness, (agility), as it makes the outer tire work harder in the first half a second or so (of initial turn-in).” Greg Locock – Engineering Tips Internet Forum. Greg is a Vehicle Dynamics Engineer. In recent years he worked with Ford Australia.

Many of us have difficulty with this statement, especially if we’re familiar with the “roll stiffness distribution” concept. Our general understanding is that Increasing roll stiffness at the front will increase steady state understeer and therefore the response of the car will be less. The car will be less agile.

What Greg is talking about is how the increased stiffness at the tyre contact patch gets the weight transfer at the front happening quicker, getting the car to respond faster, gets the car to initially turn harder.

Once the car has settled in the corner, then of course “steady state balance” - roll stiffness distribution, front roll stiffness vs rear roll stiffness - takes over.

My difficulty with the perceived improved agility from fitting a bigger front anti-roll bar goes back more than twenty years.

In the 90’s, we operated a Pedders Suspension franchise in Batemans Bay. At the time, we were modifying road car suspensions – fitting up aftermarket springs, anti-roll bars and shocks to 70’s to 90’s popular models. When we finished doing the mods on those cars, the improvement in the crispness of the handling was immediately noticeable. I was back to back testing cars and customers were very aware of the improvement as well.

In general terms, we would have attributed the improvement to many things and so had no difficulty with what was happening.

But as I started to improve my understanding of roll stiffness distribution, the fitting of large diameter front anti-roll bars to RWD Kingswood/Commodore and Falcon cars, and the resulting improvement, did trouble me somewhat. The cars should have more understeer and therefore lesser turn in. I doubted the improvement and wondered if there was some sort of placebo affect here. Was I imagining this?  

Then, as we understood more about roll stiffness and the effect of the initial weight transfer at the tyre contact patch then it starts to make sense. At fast touring speeds within the linear grip range of the tyres, the extra response from faster lateral weight transfer is the predominant element. As we approach the limit of grip, if indeed the car is a strong understeerer, then understeer will dominate.

How You Get More Grip with Stiffer Springs and Anti-Roll Bars

At Racing Car Technology, we have determined through testing that mechanical grip (and lap time) improves with increasing spring stiffness up to a point where the gains level off and some of the downsides may have a greater influence. If too stiff for instance, tyre wear and tyre overheating may be a problem. Power down and braking may be compromised on rougher sections of the circuit.

Optimizing suspension stiffness is generally the most important improvement we can make to the race car. For many of our clients, increased spring stiffness has been the major factor in improving performance, sometimes by seconds per lap.

For racing cars, we recommend a ride stiffness around for 150 cycles per minute (2.5 Htz). For production cars, this will be 2 to 4 times stiffer than the standard spring fitted to the production car. Less stiffness is recommended for historic cars and cars with compromised lateral stiffness.

For more on how stiff suspension works to generate more grip see our blog:
https://www.suspensionsetup.info/blog/making-best-grip-at-the-tyres-7-post-shaker-rig-testing

The next step is to realise how stiff anti-roll bars can help you make more grip.

In the old days racing the Bowin P4a Formula Ford (1973), I used to think that if I was to reduce the overall roll stiffness (while maintaining the existing balance) that I would find more grip and be faster. My benchmark for this thinking was the previous owner and Championship winner in 1972, Bob Skelton. He drove with less overall roll stiffness than I was comfortable with.  

This thinking was wrong, as we know today. There is no theoretical limit to the roll stiffness you can run with. (The number one reason why Bob was quicker than me was probably smoothness. He was typical of the good drivers of the day. Like Leo Geoghegan, he was a real momentum driver, driving classic lines and maintaining maximum speed through the corners.)

Burying this thinking for me was made more difficult by the writings of Carroll Smith. Even in 1998, in “Engineer in Your Pocket”, he maintained that too much anti-roll bar overall results in:
“Car will be very sudden in response and will have little feel. Car will tend to slide or skate rather than taking a set, especially in slow and medium speed corners. Car may dart over bumps.”

As a result of my misunderstandings, I remained a proponent of less anti-roll at the rear for a RWD race car right up until around 2014 I think it was. In 2012, I remember thinking it was OK for David Stone’s Mazda MX5 to run without a rear anti-roll bar as long as the balance remained good. The thinking revolves around the idea that the rear anti-roll bar will have a tendency to unload the inside rear wheel.  (Hopefully we blew that idea away for you in the “Anti-Roll Bars. A Good or a Bad Thing” article.)

This is in stark contrast to our current recommendation in our training courses and consulting since 2015. All race cars should have efficiently designed anti-roll bars front and rear with sufficient adjustment for mechanical balance to balance the car between too much understeer and just enough oversteer to require some counter steering. (Counter steering produces slower lap time than the desired optimum balance.)

To check out our advice on springs and anti-roll bars for set-up, see our free training “Suspension Tuning 101 and 102”.

What changed everything for me was an article in the Mark Ortiz Chassis Newsletter. The newsletter, now in its 20th year of publication is highly regarded by many influential people in racing and race car handling. The article I read mentioned those in racing who still have a problem with rear anti-roll bars, particularly in Australia.

Now, I’ve never corresponded with Mark, but I am a huge fan of his work, especially the four part series he did for Race Car Engineering magazine on race car handling in 1998. But I had this feeling. Was he talking about me? Or is there at least one other similarly misguided individual in Australia?

I was stung into action. I immediately thought of the Mygale space frame race cars like the Formula Ford and F4 versions. These cars have very big anti-roll bars front and rear. If I could work out what’s happening with those cars then I would on my way to a better understanding.

In 2010, I got the opportunity to have a quick look at the numbers for Cameron Walter’s Mygale FF when he was starting out in the Victorian State and National Championships. With the numbers I was given for our Weight Transfer Worksheet (WTW)™, the massively stiff anti-roll bars didn’t make any sense to me. I though the bars must be hollow. But then we never got a chance to sort that out as we did not proceed with a consulting arrangement.

Then in 2014, in response to the what I read from Mark Ortiz, I revisited the issue. The stiffness numbers for springs and anti-roll bars and the motion ratios are publicly available in the factory Owner’s Manual for the Mygale F4 car. So, I could assemble what data I needed for a pretty good crack at doing the WTW.  The analysis proved beyond doubt that indeed that the Mygale is super stiff in roll – around .2 degrees of roll per lateral G. Effectively no roll at all in the suspension.

The Mygales were a revelation when they first competed in Australia in the early 2000’s. In the decades since they have won most of the championships. The only car that can compete with them in Formula Ford is the Australian built Spectrum.

The results of the analysis show that the ride stiffness requirement is similar to a regular race car while the roll stiffness is much stiffer, offering virtually no suspension compliance at all while the car is cornering. According to the old ways of thinking, the handling should be evil. Yet drivers experienced or not, do not notice anything unusual. I checked in car videos of Mygales racing in Europe going full tilt over ripple strips. Again, nothing untoward. It’s all about the zero compliance and instant grip available at the outside tyre and the lowest possible contact patch variation to generate maximum grip that makes the cars so quick. Yet most teams and owner driver do not appear to understand the fundamentals of the Mygale the set-up, sometimes to their detriment. It would make no sense, for example, to try removing the rear anti-roll bar.

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