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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: firstname.lastname@example.org. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
ANTI-ROLL BARS ON FORMULA VEES REVISITED
It has been brought to my attention that Formula Vees as raced in Europe, including the UK and Ireland, do use anti-roll bars at the rear. In my May 2011 newsletter, I was responding to a questioner who wondered why we use anti-roll bars at all. I said that when there is ample geometric roll resistance, and a problem with jacking, as in the swing axle suspension at the rear of a Formula Vee, an anti-roll bar is undesirable. When that newsletter appeared as my column in the August Racecar Engineering, it was accompanied by an illustration showing a European-style Formula Vee that had a rear anti-roll bar, with a caption pointing out that this is standard practice over there. The car also has an external front anti-roll bar, and rear coilovers arranged to act in both ride and roll, as in older US-spec Vees.
In the US, one never sees this on Vees. Modern ones all have rear springing that acts only in ride. One of the multi-leaf torsion bars at the front is replaced with a solid anti-roll bar, but that is still inside the tube. I was under the impression that external bars at the front were illegal in SCCA.
It turns out that that’s not quite true. Here’s what the 2011 General Competition Rules say:
A. The front suspension and steering shall be standard VW Sedan as defined herein, or an exact replica of the same material and dimensionally identical. The following modifications are allowed:
1. Removal or modification of spring packs including the use of ride height adjusters incorporated into the front beam provided they are not adjustable from the cockpit. At least one spring pack shall be retained as the primary spring media for the front suspension.
2. The use of any anti sway bar(s), mounting hardware, and trailing arm spacers.
3. The use of any direct acting, tube type shock absorber(s) mounted in a longitudinal, vertical plane and acting through the standard mounting points. Spring shocks and linkage activated shocks are prohibited.
9. Caster, camber, and toe in/out settings are unrestricted. Clearancing of carrier or trailing arm to eliminate binding is permitted. Offset suspension bushings and alternate locating spacers are permitted.
There is a front track limit of 52.5” maximum. Track is defined as the distance between rim centers. No height is specified, so this would have to mean the center vertically and longitudinally, as well as laterally. That would appear to mean that the front wheels can have considerable negative camber, without running afoul of the track width rule by being too far apart at ground level.
So it actually would be legal to add an external front bar, and make it adjustable as well, if desired. The front bar on the European-spec car in the illustration is non-adjustable, far as I can see in the photos. The rear one is adjustable, via a series of holes for the drop links to attach to.
The rear bar on the Euro-Vee is very slender and willowy, with fairly long arms, and thus serves as a fine-tuning device. The front bar is visibly thicker, with shorter arms, and therefore inevitably higher in rate.
The Euro-Vee is also visibly longer than a US-spec Vee, and has rack and pinion steering, and pushrod-and-rocker actuated front shocks. Those shocks would be illegal in US Formula Vee, and there would be no room for inboard shocks anyway. SCCA rules call for an 81.5” minimum and an 83.5” maximum wheelbase. That’s a bit longer than an Austin Healey Sprite wheelbase, and a bit shorter than a Triumph Spitfire’s: pretty short for a mid-engine car. The Euro-Vee driver still sits back near the engine, so the longer wheelbase implies less front percentage on the Euro-Vee.
The Euro-Vee has different tires than the US-spec car. It runs on treaded radials, and they are the same size front and rear. At least in dry to moderately wet conditions, US cars run on bias-ply slicks (radial slicks are prohibited), and the rears are wider than the fronts.
This means the US car has more rear tire relative to front, and more front weight relative to rear, compared with the Euro-Vee. That would explain why the Euro-Vee needs some extra front bar. If the rules allow the front bar to be adjustable, it’s hard to see the need for the rear bar. On the other hand, if the front bar has to be non-adjustable, it starts to make sense to have a soft rear bar that is adjustable.
The Euro-Vee also does have a rear Z-bar, and it is apparently stiffer than the anti-roll bar. The Z-bar runs inside a frame tube, so it is impossible to see how fat it is, but the arms are short compared to the a/r bar arms, and the a/r bar is really slim. The car has three rear springing systems: a stiff one that acts in ride only; a soft one that acts in roll only; and a soft one that acts in both ride and roll. Really, a car only needs two of those systems, one would think. It doesn’t appear that there is any significant non-linearity designed into any of the three systems.
Would that combination be legal in SCCA? As I read the rules, no. Here’s the relevant wording:
B. The rear axle assembly shall be standard VW sedan as defined herein with axle location provided by a single locating arm on each axle.
1. The rear axle tube may be rotated about its axis.
2. Coil spring(s) shall provide the primary springing medium, with telescopic shock absorber(s) mounted inside the spring(s). Cables, straps, or other positive stops may be used to limit positive camber. An anti-roll bar or camber control device may also be used. When said anti-roll bar or camber control device is removed, the required coil springs shall continue to perform functionally.
3. The shock absorber mounts may be modified.
A “camber control device”, as used here, is a device which is functionally equivalent to the “camber compensators” sold in the early 1960’s for swing axle suspensions: devices that limit jacking by increasing wheel rate in ride without increasing wheel rate in roll. A Z-bar qualifies as such a device. I have seen Formula Vee rear suspensions where a torsional Z-bar was the only springing device. Apparently that is no longer legal. There has to be at least one coil spring; it has to hold the car up; and it has to have a shock concentric with it. But there can be just one; you don’t have to have two. I have seen a Formula Vee rear suspension with two additional shocks to damp, but not spring, roll. Apparently that is legal.
So you can have a single rear ride spring, which gives you the same effect as a “camber compensator” – except it’s not one legally, because it’s the required coilover that holds the car up. You can then add a “camber control device” OR an anti-roll bar – but not both, because the rule is an “or” statement. You cannot have the same combination as a Euro-Vee, but you can have a coilover that acts only in ride, additional shocks that act only in roll, and an additional torsion bar that acts only in roll.
Under what conditions would the anti-roll bar be desirable? It becomes desirable when it is possible to put enough elastic roll resistance on the front to make the car understeer.
Current US-spec Vees corner reasonably neutrally, but their front camber is not optimal. The front grip would benefit from more static negative camber, and/or less roll. The trailing arm front suspension has zero camber recovery in roll. If the outside front tire can be kept more upright, more load transfer at the front can be allowed. That gives us less load transfer at the rear, and therefore more cornering power at the rear as well. If we encounter either inside front wheel lifting or more understeer than we want, we then might benefit from a rear anti-roll bar.
The objective should be to run the front wheels at optimal camber, not degrade their cornering power with poor static camber; minimize roll, as permitted by track roughness, to minimize front camber loss due to roll; then add rear roll resistance as needed to keep the inside front wheel at the point of incipient lift most of the time, or as close to that as front grip will allow without excessive understeer. At least on smooth surfaces, a setup with lots of roll stiffness will have more overall grip due to improved front camber, and some of the front grip can then be traded away as needed to improve rear grip.
However, even though such a setup might involve using a rear anti-roll bar, the rear suspension will still have a substantially greater wheel rate in ride than in roll, unlike other independent suspensions with anti-roll bars.
Taking the whole thing a step further, if the car is not lifting a front wheel when front camber is optimized and the understeer gradient is to our liking, it might be worthwhile to try a set of rear wheel width slicks on the front. They’re a bit wide for the required 4” rims, and the steering probably would feel less precise, but the front would probably have more cornering power than with the narrow tires, which could once again be partially redistributed to the rear tires by adding more front roll resistance. Whether this would be worth the drag penalty in such a power-limited class would probably depend on the track.
GENERAL ADVICE ON SETTING DAMPERS
I race a stock car in the U.K. on short quarter mile left hand flat oval asphalt tracks. The car weighs 1440lbs. and currently sprung as follows: i/f 250lbs; o/f 275lbs; i/r200lbs; o/r200lbs. The dampers are double adjusting (bump and rebound). Please can you give me any advice on setting these dampers?
Not sure what a “stock car” in the UK looks like, per your rules. 1440 pounds would be something smaller than what we call a four-cylinder mini-stock over here. I’ve seen pictures of things from the UK that were called stock cars that were more like what we call modifieds in the US: no fenders on the front, and rear bodywork that only partly covers the rear tires.
Judging by the spring rates, however, this car must have coilovers. The springing would be considered on the stiff side for an oval track car that light.
I cannot offer much specific advice from so little information, but I will offer some general suggestions.
First would be to understand how the adjustments work. Not all double-adjusting shocks are the same. In some cases the “rebound” adjustment is a bleed that actually affects both directions, but affects the rebound more noticeably because the rebound shim stack is stiffer. You really want to get a good set of shock dyno traces for your shocks. Preferably, you should have your own actual shocks dynoed. Failing that, maybe you can get results from the builder showing what they’re supposed to act like.
Next general rule would be to avoid using the shocks as a way of tuning handling balance or understeer gradient. You want to do that with springs, tire stagger, and alignment settings, and have the shocks valved to minimize load variation at the contact patches – in other words, to keep the wheels on the ground. For this, you want the shocks as soft as possible without making the driver uncomfortable with the car, or having uncontrolled oscillation over any washboard ripples the track
may have. This means you set the shocks so the bump, especially at high shaft velocity, is softer than the rebound (this may not mean fewer clicks on that side – that’s one reason you need dyno traces) by a factor in the range of about 1.5 to 3.0, and then you uniformly soften them until the car does something you don’t like, or that the driver doesn’t like, then back up a little.