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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.
KAFKAESQUE F1 SUSPENSION RULES?
What do you think of what the FIA is telling the Formula 1 teams about what suspension systems will be allowed in 2017, as reported here: https://www.motorsport.com/f1/news/fia-warns-f1-teams-over-suspension-designs-878197/
I don’t want to be unfair to the FIA, or go off half-cocked based on a possibly erroneous report. I have asked the FIA for comment on the accuracy of this report. I am still waiting for a response. It’s been a week at this writing. If I find out that the report was inaccurate, I will amend my comments as needed. For now, I will comment on the FIA’s actions as reported. Here are some excerpts (I have corrected a couple of obvious typos):
The FIA picked out five key characteristics or components that it will deem non-compliant:
The FIA publishes the F1 regulations. The most recent revision is dated Jan. 24, 2017.
It doesn’t include any of the above points. It does include the FRIC ban:
10.1.2 Any suspension system fitted to the front wheels must be so arranged that its response results only from changes in load applied to the front wheels.
10.1.3 Any suspension system fitted to the rear wheels must be so arranged that its response results only from changes in load applied to the rear wheels.
I don’t see any ambiguity in the regulations as currently published. I definitely would have problems with the reported policies, however.
First of all, it is very often impossible to say whether aerodynamic gain is the “primary purpose” of a suspension design. One of the fundamental facts of suspension design for at least the last three decades has been that the parts of the car close to the ground need to be held in as nearly a fixed relationship to the ground as possible, to prevent uncontrolled variations in downforce and downforce distribution. This inherently flies in the face of the need to let the suspension move to minimize tire load variations due to road irregularities. Modern tracks are pretty smooth, and it helps the aero guys if the car is a go-kart, but the track still has bumps, crests, dips, and curbs, so some compromise is necessary.
It is simple to single-mindedly produce a suspension whose “primary purpose” is aerodynamic advantage: just make the springs ridiculously stiff. But if we’re intelligent, we try for a better blend of aerodynamic consistency and wheel load consistency over bumps. If we come up with a clever way to do that, is our idea’s “primary purpose” improvement of aerodynamic consistency for a given level of wheel load consistency, or is it improvement of wheel load consistency for a given level of aerodynamic consistency? It’s impossible to say. The question is fundamentally nonsensical.
Any system is prohibited that changes how the car responds to body accelerations? Compared to what? Doesn’t anything we do to the suspension affect how the car responds to accelerations? Isn’t the whole idea to optimize that?
Direct coupling between the roll and heave parts of the suspension is prohibited? Maybe that’s a typo. Absence of direct coupling would be unconventional. Ordinary suspension springs create coupling of the suspension’s roll and heave properties: they resist both roll and heave. Any rising-rate springing or any anti-roll bar with rising or falling motion ratios creates ride height changes with roll. A system with completely separate springing and damping for synchronous and oppositional motions of the two wheels, as recently used in LMP by Porsche and Audi, would be unusual. That is sometimes called decoupling of the modes.
No direct coupling between ride height and steering? Does that mean the push rods or pull rods can’t connect to the uprights? Does it mean there can’t be any steering offset (scrub radius), steering axis inclination, caster, or trail? With most combinations of these things, the car goes up and down when the front wheels steer.
So we have reported rules that are nonsensical. And we also see that reportedly the burden of proof is to be on the entrant to prove compliance with nonsensical rules – with those who wrote the rules as the arbiters of whether the burden of proof is met. If this is factual, it’s worthy of Franz Kafka or Lewis Carroll. If it turns out to be fake news, I’ll let you know.
INDEPENDENT REAR SUSPENSION FOR DIRT OVAL TRACK
My father has been racing short track dirt oval for the past 20 years in South Africa, and I have just recently built him a new car running single piece wishbone independent front and rear.
Could you please assist with some views on suspension design?
As I understand, anti-squat is like longitudinal anti-roll, which we can use to control rear bite. However, “power anti-squat” comes from the differential centre and “geometric anti-squat” comes from the locating linkage angles. I would guess some power anti-squat is also transmitted via the stub axles.
On an independent suspension the two are separate; we only have geometric anti-squat to adjust.
I designed the car with zero anti-squat & anti-dive. Anti-squat will create very unpredictable wheel movement in the sense that caster, camber & toe will change as the wheel moves through bump and droop travel.
When looking at our car from the side the chassis mountings do permit for adjustment, but if I start mounting the upper A-Arms at angles, this will cause bind, unless I used a 5-link type setup. (The a-arm rose joint mounting bolts run parallel to the longitudinal centre line of the car. I did this to prevent roll steer.)
I used a toe link at the rear of the lower A-Arm, which is equal in length to the lower A-Arm. Some documentation recommends that I should have used a longer toe link to induce dynamic toe-in during bump & rebound as toe-in at the rear is more stable. What do you think?
I think independent rear suspension can work great on dirt. It certainly has in off-road racing, rallying, rallycross, and the Pikes Peak Hillclimb. There has been little opportunity to explore its potential in American oval track racing because for many years there has been no class that allows it.
In the 1980’s there were a few attempts to build sprint cars with independent suspension, but these were done by people with limited education who mostly just copied pavement car suspensions of the day, without really understanding the dynamics. Before anybody could really exploit the potential of the concept, independent suspension was outlawed. Most dirt Late Model sanctioning bodies have explicitly outlawed independent suspension at the rear of the car, while requiring it at the front. For a long time, WISSOTA, which runs in the northern Midwest and Great Plains, had no such rule but nobody tried IRS. For 2017, they have a whole new suspension section in their rules. It doesn’t
explicitly prohibit IRS, but it does include a catch-all provision that anything unusual in the entire suspension system has to be approved in advance. This is just barely short of a NASCAR-style rule that anything not explicitly authorized is prohibited.
If we are designing an IRS system, rather than working within the limitations of one that already exists, we can obtain any amount of anti-squat desired, with just about any bump steer properties desired. We can do this with wishbones or with individual links. The questioner is correct that we have only thrust (geometric) anti-squat to work with, but we can get as much of that as we want. An exception may occur if we have drop gears in the upright, as on Humvees and old VW Transporters. In that case we can get lots of torque (power) anti-squat. But unless we need drop gears for reasons of ground clearance, we are better off without them, so an oval track car would not have them.
With any suspension, the longitudinal jacking coefficient at any wheel depends on how much the contact patch moves longitudinally as the suspension displaces. With independent suspension, when the engine is off, the brakes are off, and the car is in gear, when testing the car statically the wheel does not rotate in side view as the suspension displaces, even if the upright does rotate. The contact patch displaces longitudinally at the same rate as the wheel center. Anti-squat then requires the wheel center to move rearward as the suspension compresses and forward as it extends.
The main advantage of IRS on dirt tracks is reduction of unsprung mass, improving the ability of the wheels to follow road irregularities. This advantage is enhanced if the brakes are inboard. If the brakes are inboard, it doesn’t matter whether the upright rotates in side view as the suspension moves. The anti-lift in braking only depends on how the wheel center moves. The jacking coefficient for braking will be similar to that for propulsion. The contact patch will move with the wheel center if the suspension is displaced with the transmission in neutral and the brakes locked, same as with the brakes off and the trans in gear.
However, if the brakes are outboard, when the brakes are on and the trans is in neutral the wheel rotates if the upright rotates. If the upright rotates in side view as the suspension displaces, the jacking coefficient for braking is different than for propulsion. This can be an advantage in a dirt car. It is common for dirt cars to run a lot more rear brake than pavement cars. If we have a lot of anti-lift, this can result in wheel hop in braking when the track is tacky. We can avoid this by having the side view instant center behind and below the wheel center: the side view projected control arms both slope up toward the front, the upper more steeply than the lower.
But again, this only works with outboard brakes, so we have to decide whether we want to be able to do this badly enough to accept the unsprung weight penalty.
The questioner does not mention what sort of car the system is going on, or what sort of bodywork or bodywork rules it has. Some dirt cars definitely have suspension systems intended to influence their aerodynamics. Whether this is desirable depends on the bodywork and the related rules. Dirt Late Models have large, slab-sided bodies that can generate considerable aerodynamic lateral force when running at large aerodynamic yaw angles. The rear suspensions are designed to create large
amounts of roll oversteer and make the left rear corner jack severely. This strategy works because of the bodywork rules, combined with rules that some sanctioning bodies have regarding how much the right and left wheelbase measurements can differ statically. Sprint cars are not designed to do the same thing.
With independent suspension, the rear wheels can be aimed any way we want statically, without any need to lead or trail either rear wheel. We can also make either rear corner jack up or down as desired, in response to cornering and/or braking and/or propulsion. Whether we want to do these things will depend on the rules.