The Mark Ortiz Automotive
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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.
MUMFORD LINKAGE IN COBRA KIT CAR
I have been researching ways to lower the roll center on a live axle for a Cobra kit car to solo race and I have looked at the Mumford link. Are the claims that it can set the roll center as low as you want true?
Basically, yes. You can set the roll center as low as you’re likely to want to, anyway. More specifically, you can set it below any point on the hardware.
It’s questionable whether this is significantly better than setting it three or four inches off the ground, as you can with a Panhard bar or Watt linkage in a low-slung solo car.
For those who have never heard of the Mumford linkage, it is a form of straight line motion device similar to the Watt linkage, but slightly more complex. The Watt linkage has a rocker and two links. The Mumford linkage has two rockers and three links, one of which just connects the two rockers. There are various ways of applying either concept. The rockers can be close together near the center of the car, or out close to the wheels. The rockers can be attached to the frame or to the axle.
The July 2011 newsletter discusses at length the various forms of the Mumford linkage and their effects. Back issues are available free upon request.
The car best known for using the Mumford linkage was one of Arthur Mallock’s designs. It used the Mumford linkage to allow a low roll center with a smooth belly pan that swept upward under the axle and led smoothly into a diffuser.
With a very low roll center on a live axle rear, you do get the advantage of being able to run a lot elastic roll resistance at the rear, which helps reduce torque roll and torque wedge. There are perhaps better ways to compensate for driveshaft torque using the longitudinal linkages, but if one is already committed to longitudinal linkage that does not have such properties, having lots of rear elastic roll resistance, and a low rear roll center to go with that, is the next best thing.
A low roll center means less lateral translation of the contact patches in the roll mode. However, with a beam axle, less lateral translation at the bottoms of the tires in roll implies more lateral translation at the tops of the tires. This requires that the bodywork have room for that. That could be a problem in a Cobra kit car.
The original Cobra had independent rear suspension. The rear fenders were widened dramatically from the original AC Ace to accommodate the car’s big tires. The aluminum was closely wrapped around the envelope in which the rear tires moved. With most designs of independent suspension, the tops of the tires swing in slightly as the suspension compresses, never out. With a beam axle, the tops of the tires go straight up as the suspension compresses in pure ride, and can even move outboard in some combinations of ride and roll displacement.
Consequently, kit car manufacturers offering Cobra replicas with beam axles have to widen the fenders a bit more to accommodate the tire movement with that suspension. If one uses a suspension that further increases lateral movement at the top of the tires, it is quite likely that further measures will be necessary to accommodate that. That isn’t necessarily a reason to throw out the whole concept, but it is something to be aware of.
ADD FRONT ANTI-ROLL BAR AND REDUCE UNDERSTEER?
A very long time ago, I raced in the Bilstein Rabbit series and we were finally able to add sway bars to the cars (legally). Despite the cars' tendency to push, and a front sway bar's tendency to make understeer worse, the best thing we did was have the front so stiff it was all but welded. Why?
There are at least two things that can cause an increase in front roll resistance to decrease understeer.
The first is that with independent suspension, any reduction in roll improves camber. If a car has better camber recovery in roll at the rear than at the front, a reduction in roll helps front camber more than rear camber. The most extreme case of this is a car with MacPherson strut front suspension, lowered for racing, and beam axle rear suspension.
The second, which is particularly relevant to front-drive cars, is that as long as the inside rear wheel is off the ground, an increase in front roll resistance does not add any front load transfer at a given lateral acceleration. It will, however, reduce roll, and therefore generally improve front camber. Again, this especially applies to strut suspensions when lowered for racing, as these generally have poor camber recovery in roll.
The Rabbit has little camber recovery in roll at either end. The front is a strut system. The rear has a twist beam that looks a little like an axle, but it’s basically a trailing arm layout geometrically. The twist beam adds roll resistance and lateral rigidity, but it doesn’t provide beam axle camber properties.
My suggestion for a car like that, particularly on a smooth track, is to use lots of anti-roll bar at both ends, with enough at the rear so the car still corners on three wheels – but at a small roll displacement.
The penalty is that some bumps will not be absorbed well. Therefore, as always, the setup is a compromise between the need to minimize roll and camber change and the need to minimize wheel load changes over bumps.
GENERAL ROAD RACING SEDAN SETUP PRINCIPLES
My background is short track asphalt stock car racing. I do chassis setups and custom valve shocks for guys. I am looking at branching out into the road racing area as I am about to buy a NASA American Iron Mustang. What kind of spring/shock setups do you see in these cars? I hear a lot about stiff springs and stiff compression valving. If that is true, that is the exact opposite of stock cars. What is the theory for chassis set up for a fast 3000 lb. sedan?
First of all, there are three NASA American Iron classes, and they all have different rules, including ones regarding tires, ride heights, and chassis modifications. And for each of these, setup will vary depending on the track, the thinking of whoever does the setup, and the driver.
However, certain general principles will apply. As noted in the answer to the last question above, for all cars the decision of how stiff to make the springs and bars is a compromise between the need to reduce roll and the need to make the car ride bumps. That never changes, but some tracks are bumpy and some are smooth.
A difference from oval track setup is that we face limitations in using static settings to compensate for roll when the car has to turn both ways. We can set the wheels with static negative camber on both sides of the car. That helps the outside wheel at the expense of the inside one. Up to a point, there’s a net gain in cornering, but it comes at a cost in braking, propulsive traction, and tire wear.
Damping is very much a driver-specific thing, in oval track or road racing. I tend to like stiffer rebound than compression for most purposes. It gives a better ride than stiffer compression, certainly. However, it tends to unload the tires more over crests, and when really excessive it can make the car jack down over chatter bumps.
I also tend to like stiffer low-speed damping at the rear than at the front, especially for tight turns and chicanes. It helps de-wedge the car on entry and wedge it on exit.
PUSHROD SUSPENSION JUST AT THE REAR
In a car with rear pushrod suspension and front coilover a-arms, how does one go about tuning the suspension after setting ride height, camber and caster to suggested values?
That sounds like a rather unusual design. However, there are not necessarily any particular tuning strategies that are specific to a car that has the coilovers acting through a pushrod and rocker at one end and not at the other. The thing that matters at either end of the car is the forces at the wheel. Pushrod or no pushrod, there is a motion ratio relationship between the coilover and the wheel. The wheel rate is the spring rate times the square of the spring-to-wheel motion ratio.
With rockers, we have greater opportunities to vary the motion ratio as the suspension moves, and we can adjust ride height with the pushrods as well as the rockers. In some cases, that allows us to have the suspension bottom out or top out at different displacements, as in a “zero droop” setup.
Pushrods also facilitate advanced tricks in suspension interconnection. For example, the DeltaWing car has a clever system at the rear involving a second set of links (pull rods) running back from the rockers to a rocking beam. The beam actuates coilovers that are outboard of the secondary links. In ride, the beam displaces fore and aft, like the top of a T-bar anti-roll bar – which essentially is what it is. In roll, the coilovers have a higher motion ratio than in ride; they displace more than the pull rods. This allows not only the elastic forces but also the hydraulic forces to be greater in roll than in ride.
It is not true, as is sometimes supposed, that pushrods and rockers make a suspension transfer wheel loads in some dramatically different manner. The springs and bars still exert displacement-sensitive forces and the dampers still exert velocity-sensitive forces, and the tires don’t know what kind of connection there is between the coilovers and the wheels. They only see the resulting forces at the wheel. The car responds fundamentally the same to spring, bar, and damper changes whether the coilovers act through a pushrod or not.