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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: email@example.com. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
SPEEDY AND PUBLIC TRIALS
We build Sporting Trials cars (pic attached). They have a rear mounted transaxle with “parallel” wishbones and a single transverse shock absorber with an anti roll bar. I have noticed that the cars have good grip if momentum is maintained but are very reluctant to dig in and find grip in the mud once forward motion has ceased. Is this possibly due to the lack of anti-squat in the parallel wishbones?
This is the first question I’ve had from a trials competitor. We don’t have this form of motorsport on my side of the Atlantic and I’ve never had a client doing it, but I’ll offer some thoughts nonetheless.
Anti-squat does give a momentary increase in wheel loading when the sprung mass is accelerating upward – that is, for a very brief time interval upon abrupt application of power. This is true whether the car is in motion or not. In some cases, the momentary increase in load will make an important difference. Ideally, in a trials car we’d like to time this to coincide with the momentary load increase from a bounce by the passenger or bouncer (in the US, that’s the person who throws you out of the bar when you get too rowdy).
Anti-squat also makes the rear of the car sit a bit higher under power. This raises the c.g. a little, and slightly increases rearward load transfer when accelerating forward or going uphill.
With independent rear suspension, assuming there aren’t drop gears in the uprights, getting anti-squat depends on making the hub move rearward as the suspension compresses.
Since no drive torque is reacted through the upright, it doesn’t matter how the side-view inclination of the upright changes with suspension displacement. That is, as long as we’re under power and not braking it doesn’t matter how long or short the side view projected swing arm is, or whether the side view instant center (SVIC) is ahead of the wheel, behind it, or undefined.
If the side view instant center is ahead of the wheel, it must be above hub height. This means the side view projected lower control arm must slope upward toward the front. If the side view instant center is behind the wheel, it must be below hub height. This means the side view projected upper control arm must slope upward toward the front. If the side view projected control arms are parallel to each other, the side view instant center is undefined. In that case, both side view projected control arms must slope upward toward the front.
So in all cases, at least one side view projected control arm must slope upward toward the front, and in most cases they both will.
The choice of whether to put the SVIC ahead of the wheel or behind depends on whether one wants more or less anti-lift or pro-lift in braking, assuming outboard rear brakes. At first glance one might not suppose that braking would matter much when the main task is to climb muddy hills. However, reading the very minimal BTRDA rules (http://www.btrda.com/images/uploaded/66_2512249.pdf), I see that although sequential gearboxes are illegal and there is a spec for tires, there is no rule on diffs at all, or on brakes. That would mean you could opt for a spool, but that would make the car hard to turn. Or, you could have an open diff or a non-locking limited-slip, and use a tractor brake (hand brake acting on one rear wheel or the other, either with one lever that you can push two directions, or two levers that you can pull separately or together).
With a tractor brake, some pro-lift in braking might be desirable. When only one rear wheel is spinning, application of the brake would jack that corner up and help load that wheel. This would involve having the SVIC behind the wheel.
The idea of having a single coilover for ride springing and an anti-roll bar for roll springing is interesting. This leaves the roll mode undamped, but it does allow the system to be softer in roll than in ride, which might be useful for attaining softness in warp. A slightly costlier alternative that would provide some damping in roll would be to use individual wheel coilovers as usual, and add either a Z-bar or a third coilover to stiffen ride.
I would expect there would be a compromise regarding how stiff to make the ride springing. Having it very compliant indeed would be best, except for getting maximum benefit from the efforts of the bouncer. More ride stiffness would probably help for that.
UPRIGHT GEOMETRY AND RELATED CONSIDERATIONS
I am in the process of designing a universal front and rear upright. I have designed it for 10mm scrub radius when used at the front.
Then I asked myself the question: is scrub important on the rear wheels? And what should it be?
The car it will be used in is a rear engine space frame type car (front wheel drive engine and gearbox moved to the back) – target weight 600Kg and 250 hp.
Last night I was reading up and with horror discovered you can have negative scrub. So this made me question all the info I thought was cast in concrete and what I was designing into my upright.
1) For a rear wheel drive car, is it best to use positive scrub (the SAI and the centre line of the wheel crosses above ground)
2) What is the optimal scrub radius to design into the upright? (I always thought 10mm was a happy medium between “feel” and steering wheel force)
3) What should the SAI be that I use. We used to use the Cortina uprights (5 Deg SAI), but these fell out of favour for the Sierra units that had an 11 Deg SAI. Now my understanding is that you create negative camber as you turn, so the bigger the SAI the more severe this camber creation.
4) How important is the Droop = Compression/2 setup for suspension movement (working on 50mm compression and 25mm droop)? In one of your letters you explain it, but focused on oval track cars, that always turn in the same direction, so always load and unload the same wheel). This is a track day car with some hill climbing and gymkhanas thrown in for fun.
5) And this is the last question. Is it best to have your front top wish bone 2/3 of the bottom? In this picture they seem very close to equal – same as on an F1 car.
First, the term “scrub radius” refers not to an actual radius but to the front-view or y-axis offset of the contact patch center from the point where the steering axis meets the ground. The corresponding x-axis offset is called trail. Especially in the UK, the more descriptive term “steering offset” is used rather than “scrub radius”. However, in the US, “scrub radius” is the usual term.
The questioner appears to be a bit confused regarding sign conventions for both scrub radius and camber. Ordinarily, and per SAE convention, scrub radius or front-view steering offset is positive when, in front view, the steering axis intersects the ground inboard of the contact patch center – or when, in front view, the steering axis intercepts the wheel centerplane below ground.
Until the 1970’s, most cars had positive scrub radius per SAE sign convention. Before power steering and ball joints, many designers thought zero scrub radius, or “centerpoint steering” was optimal. Negative scrub radius was popularized by VW and Audi, and was touted as a selling point when the Golf (Rabbit in the US) was introduced in 1974.
Camber is positive when the top of the wheel is outboard of the bottom. Increasing front-view steering axis inclination adds positive camber when the wheels steer, on both the inside wheel and the outside one.
In many cases it is not even possible to define a steering axis for a rear wheel, and therefore it is often not possible to define a scrub radius. When it is possible to define a steering axis for the rear, it only matters to the extent that it may influence the suspension’s deflection steer characteristics. In a race car with no rubber bushings and good rigidity for all components, there shouldn’t be much deflection steer.
What scrub radius is optimal depends on design objectives and various practical considerations. A negative scrub radius as used by VW and others is used to minimize the effects of brake pulsation, bumps, flat tires, and front wheel drive related forces upon the steering. A positive scrub radius is more common in race cars. The bigger it is, the more the driver feels bumps, brake pulsations, and so on. Up to a point at least, that’s good in a race car or performance car; the steering is more communicative.
The bigger the scrub radius is, the more jacking the steering creates when the wheels steer. The combination of scrub radius and caster makes the car roll in the opposite direction to steer. It de-wedges the car (adds load to the inside front and outside rear wheels and unloads the other two) when steering in the direction of the turn, and wedges the car when countersteering. The combination of steering axis inclination and scrub radius makes the entire front end jack up with steer, and creates a centering force in the steering that is independent of ground plane forces at the contact patches. Negative scrub radius reverses all these jacking-related effects.
Given a choice of 5 degree or 11 degree uprights for a race car, I would normally go with the 5 degree. I would shoot for a scrub radius anywhere from one to four inches (25 to 100mm) – more for low-speed tracks, less for high-speed.
For most applications, we don’t want the suspension to either bottom out or top out. Generally, we want to keep bump and droop travel fairly close to equal. Limited droop travel can be used to free up entry and tighten exit, but topped-out suspension doesn’t ride bumps well. If the track is smooth, that may be acceptable. In any case, there is no sacred ratio of bump to droop travel. There are
merely predictable effects from limiting droop travel, which may be good or bad depending on the application.
For control arm lengths in the range seen on most cars, the 2:3 ratio is not a bad rule of thumb but it is not sacred. Having the lengths more unequal keeps the geometric anti-roll more consistent as the suspension moves. Having the lengths more nearly equal keeps the rate of camber change in ride and the rate of camber recovery in roll more consistent as the suspension moves. Making both arms longer helps everything except control arm weight, but normally packaging constraints limit how long we can make them. It is helpful to keep control arm lengths and length ratios similar at both ends of the car.