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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.
ON THE MARCH AGAIN
A few months ago I was asking you two questions, and you have been very helpful! New question:
with my Formula 2 March 712 at Dijon (France) I have on 2 corners at the beginning and end of main straight in 3rd gear. In these turns I have UNDERSTEER and I feel I have to try first with stronger aft spring or with stronger aft ARB, not only to worsen the grip in the rear but also to improve the grip in the front.
I am also considering wedge adjustment. Is it also useable on road circuit like we have in Europe or just for ovals? How about if I put more corner weight on the LH aft and or RH fwd wheel – or is this a bad compromise?
Before I get into answering the main question here, I thought I'd pass along some information submitted by reader Robert Koch.
You were helping a person who drives a March 712. In 1972 und 1973 I worked on such a car in Switzerland. This car was very softly sprung. It had a very soft chassis which meant that increasing the anti-roll bar or the springs the chassis just flexed. Now the reason why I am writing you: the March 712 racecar had two weak points. One of them was the inserts in the back wall of the monocoque where you fasten the tubular frame with the engine and the transmission. They are not such that they live a long time. One has to replace them quite often. The problem is, you do not see that they are loose! Point 2 is: the upper squared-tube where the differential and/or the gearbox is mounted cracks on the underside, close to the two points where the springs are mounted. This is also something you do not see just like that.
It is important to make sure you eliminate any problems like these, rather than tune around them. I would also make sure the rear wheel pair is not aimed to the left or the right. Both rear wheels should generally have the same toe-in relative to the vehicle centerline. Also, make sure that both rear tires are the same size. Measure their circumference with a tape measure.
Looking at a track map of Dijon (see http://en.wikipedia.org/wiki/Image:Circuit_Dijon-Prenois.png), I see that the first of these turns, called Courbe de Pouas, or Turn 8, is the largest-radius one on the track, and therefore probably the fastest. The one at the end of the straight, Double-droite de
Villerois, or Turn 1, is a slower one, but not really slow. Both appear to be double-apex turns. Turn 1, as its name suggests, is really a double-right, with two fairly small-radius segments joined by a markedly larger-radius segment. Turn 8 has a short, somewhat tight-radius segment at its entry, followed by a large constant radius over most of its length. The circuit is run clockwise, so most of the turns are right-handers. There are only two left-handers of any significant duration.
The main straight, called Ligne Droite de la Fouine, connecting turns 8 and 1, is long. From the map, its length must be well over half a mile – perhaps a kilometer, perhaps three quarters of a mile. The back section of the circuit is almost all turns. There are four straightaways, but none of them is anywhere near the length of the front straight. So the questioner is correct that speed in these two turns, especially exit speed from turn 8, is important to lap time.
For a course like this, I don't generally advise a wedged setup, at least not as a starting point. I do advise a right-heavy setup, for cars that have ballast, which the questioner's car probably does not.
Cars with flexible frames or sprung structures do respond to suspension adjustment. They just respond less to a given adjustment than cars with good torsional stiffness. You need more change in the springs and bars to get a given change in dynamic diagonal percentage or in cornering balance. You need to put more turns into the spring seats to get a given change in static diagonal percentage.
When attacking a handling problem that occurs only in certain turns, I always advise looking for characteristics in those turns that are not shared with the turns where the problem doesn't show up.
If the problem shows up in fast turns but not slow ones, that points to a problem with aerodynamic balance. If the car has understeer only at high speeds, that is best cured by adding front downforce, or reducing rear downforce. In some cases the rules will prevent us from making the changes required to achieve this, but it is the best approach when we can do it.
If the car has understeer only in right turns, and the car is to all appearances symmetrical, that points to an alignment problem, or something loose or flexing that only affects turns in one direction. A car designed for road racing shouldn't act different in right and left turns, if the wheel alignment is symmetrical, the right and left springs are identical, the right and left tire sizes are the same, and there are no other asymmetries. If it does act different in right and left turns, we can crutch that by running more or less than 50% static diagonal, but chances are that we are tuning around some other problem that we have yet to discover.
I do recommend running some negative wedge (usually meaning less than 50% on the right front and left rear) on live-axle road racing cars, especially for courses with lots of right turns. This compensates for the effect of driveshaft torque on wheel loads. However, this would not apply to a rear-engined formula car with independent rear suspension.
RADIAL TIRE CAMBER
I have just started racing an MG Midget. The weight is about 50/50 front/rear and at present it runs 8in crossply slicks all round. Front is basically double wishbone and the rear is a live axle with Panhard rod.
I have considered running radial tyres but have been told that I would need a lot more negative camber (at present about -1 deg).
As far as I can tell most modern (up to F1) cars seem to run quite a lot of negative on the front but little if any on the rear.
So, do radial slicks need more negative camber than crossplies, and if so, why? If it is needed on the front why not the rear (though difficult with a live axle!)? With a live rear axle and a lot of negative on the front, wouldn't the rear be relatively lacking in grip?
It is not necessarily true that radials always want more negative camber than bias-plies, but it is valid as a generalization. Many racing radials are not truly radials, because the sidewall plies do cross each other, although not at as great an angle as in true bias-plies.
It is very common to see more than a degree of static negative camber on independent systems of all kinds, on bias-plies or radials.
Why do radials tend to want more camber? Mainly, I think because the sidewalls are more flexible. This makes them tolerate more camber, and want more.
If the tires want more negative camber, and you can only get this on the front, will the car tend more toward oversteer? Maybe. Remember, your independent front suspension allows the front camber to change with roll, while the rear beam axle produces no camber change with roll (except a bit due to tire deflection). Most independent suspensions provide some camber recovery in roll, but generally not 100%, because this would create excessive camber change in ride. You may currently have a degree of negative at the front statically, but that would mean you barely are keeping the outside tire upright, if that, once the car has reached full roll in a turn.
In any case, if you find you have more front grip compared to rear grip with any new tire or front camber setting, but the new tire or setting gives better overall grip, stay with the new tire or setting and work with the roll resistance distribution to get the balance back. If the car oversteers, the proper fix is to add more roll resistance at the front, or take some away at the rear, using springs and anti-roll bars.
Why do you see so much negative camber at the front on F1 cars, and so little on the rear, even though the car has independent suspension at both ends? I don't know. I can say with certainty that
this helps propulsion and hurts braking. It may be that the engineers have decided that forward acceleration out of the turns is worth more than rearward acceleration at the ends of the straights. Rules permitting, I would be tempted to ask the tire supplier for bigger front tires, and run them at less camber, or add more camber at the rear.
A RADICAL RADIAL NOTION
Instead of relying on low profile tyres with short sidewalls for handling response, why not utilize high profile types (for good surface following capability) and employ internal bracing to resist lateral loads introduced by cornering? One scheme would be to have tension cables or “strings” running from the bead to the intersection of the tread and sidewall on the opposite side of the tyre. That is, tension members would be diagonally disposed relative to tread plane. The system would look analogous to a wire wheel but be inside the tyre itself. Such a tyre would have a high sidewall for good ride and terrain following capability but would also be stiff laterally. Would this work?
I do think this would increase lateral stiffness, and maybe increase response, while maintaining vertical softness. My worry would be that the tire would act more like a bias-ply with respect to the tendency to lift or unload the inside portion of the contact patch. In a conventional radial, the tread can move laterally relatively freely, yet stay planted relatively flat while doing so. I would think the cables or strings would tend to lift the inside shoulder instead.
Another concern might be damage to the cables when mounting the tire. Two-piece rims might be necessary to overcome that.