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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.
4 VS. 3 LINK AND TORQUE WEDGE IN HIGH POWERED RWD ROAD COURSE PAVEMENT CARS
Hello Mark, greetings from
Australia. Thanks for the putting the newsletters out there. I'm enjoying
expanding my knowledge on the 4 wheeled stuff. Thanks also for the opportunity
to put the following question to you.
We run a car here based on an earlier generation V8 Supercar chassis, with equal length four link and a watts linkage. Usual setup is parallel arm geometry, with a slight amount of nose down which gives a small amount of pro squat. The car has approximately 640lb/ft at the flywheel, and runs a 330 rear tyre.
I have a question in 3 parts for you....
A: We race against some Trans Am cars which almost all run a 3 link, with quite long lower arms. What are the relative advantages of 4 vs 3 link in this pavement application?
B: Second part of the
question is about minimising torque wedge. We have been experimenting
with running a stiffer rear anti roll bar with a lower roll centre, based on
what I read in one of your previous newsletters. Obviously there is a finite
limit for rear roll centre as the rocker will hit the ground. Is there a point
where lowering the roll centre and running a stiffer RARB will start to impact
corner exit, even though the roll stiffness remains the same, more or less?
C: Finally is a 3 link better than a 4 link for minimising torque wedge? What, if anything, can we do with arm angles that may help?
The main advantage of the 3-link over the 4-link is that we can have some anti-squat without having the suspension bind in roll or creating roll oversteer.
The disadvantage of using more elastic roll resistance and a lower roll center is that there is more wheel load change over one-wheel bumps. However, there is less lateral displacement of the contact patches with respect to the frame over one-wheel bumps. This matters more when the tires have a lot of lateral stiffness.
As for effect on inside wheel loading when exiting turns – or any other time – the total load transfer (excluding the unsprung component) is always the total roll resisting moment divided by the track width, regardless of what portion of the moment comes from geometry and what part comes from the springs, bars, and dampers. It also doesn’t matter what portion comes from what kind of spring (an anti-roll bar being a type of spring).
A basic 3-link can completely or partially cancel torque roll and torque wedge. This involves either offsetting the top link to the right or angling the top link in top view so that the front of the link points to the right. If the top link is on center and is not angled in top view, the 3-link does not reduce torque wedge.
What we’re doing with the offset or top-view-angled link is creating a leftward roll moment in the suspension under power, using axle torque. This counters the rightward moment resulting from
driveshaft torque. The usual problem with fully or partially canceling torque wedge that way is that we also create a rightward roll moment in braking, when there is no driveshaft torque that we need to cancel. The geometry then needs to be a compromise, in which we accept some roll and wedge in braking in return for partial cancellation of roll and wedge under power.
The way out of this, rules permitting, is to make braking torque react differently through the linkage than propulsion torque. This usually involves mounting one or both brake calipers on birdcages or brake floaters. A birdcage is a bracket that can rotate on the axle tube, carrying two longitudinal links. A brake floater is the same thing with just one link. There are also some other possibilities involving push bars or pull bars: links that only work in one direction. In some cases these may be legal when birdcages and brake floaters are prohibited.
If torque wedge can be
entirely cancelled, the need to maximize rear elastic roll resistance goes
away. There is no longer any need to get the rear roll center extremely low.
However, if desired, there are ways to get the roll center a bit lower than is
possible with the layout shown, while still using a Watt linkage. It is
possible to turn the rocker ninety degrees and lay it flat. This is common in
Trans Am cars. Usually, the rocker is mounted horizontally to the underside of
the diff. Structural constraints permitting, there is a case for mounting it
to the frame instead, as in the layout shown. That makes the rear roll center
rise and fall with the sprung mass, which is similar to the behavior of an
independent front suspension. It is necessary when using a horizontal rocker
to make sure not to run the rod ends out of travel.
RELATIONSHIP OF SPRINGS, BARS, AND CASTER JACKING
I have a small formula car. It started life as an F-440. I started rebuilding it a few years ago. It had rubber shock absorber type units for suspension. I removed those and fabbed in coil overs. The car has a solid rear axle, no differential. So I run 10 degrees positive castor; lots of positive castor helps transfer the weight such that on cornering the inside rear tire is unloaded, allowing it to slip more easily as it needs to travel a shorter distance than the outside rear tire. My question is, would installing an anti-roll system front and rear be counterproductive? If not, how much would you run in such a setup (same as if the car had a differential, or a percentage less than that)?
If the objective is to maximize the de-wedging effect from caster jacking, anything that makes the suspension stiffer in warp will help. Warp is roll or oppositional displacement in opposite directions at the front and rear. With conventional springs and anti-roll bars, anything that adds elastic roll resistance makes the system stiffer in warp. With more innovative systems that connect front and rear wheels, this does not necessarily hold true.
What part of the roll resistance should come from the bars and what part from the springs? For a road car or a race car with little downforce, as much as half can be from the bars, or maybe even a bit more. This allows the car to ride many types of road irregularities better than with stiff springs, while providing the necessary roll resistance.
When the car has a lot of downforce, and particularly when the under-car aero is very sensitive to ground clearance and pitch angle, it becomes necessary use stiffer springs. The bars then become more of a fine tuning tool.
There is also some advantage to running stiff springs in a racecar without significant downforce: the car can run with lower ride height, when it is limited by scraping the ground on dips. This will come at the cost of increased wheel load changes over bumps, so the setting will be a compromise.
This is less true for a car that has to run on the street. There, ground clearance requirements are primarily determined by the need to clear obstacles at low speed. Assuming we are not limited by available suspension travel or fender clearance, that situation favors soft springs and stiff bars.
On race cars, and sometimes on street cars, we can adjust the anti-roll bars, either by adjusting arm length or something else in the linkage, or with a rotating blade. When the bars are stiff relative to the springs, the car becomes more sensitive to these adjustments. This is usually not a problem – just something to be aware of. In some cases it can be a problem. If the bars are very soft, it may be necessary to change bars to get a sufficient change in car behavior. If the bars are stiff and adjustment can only be made by coarse increments, we may find we’d really like a setting between two available ones. But such situations are fairly rare.
Caster jacking is affected not only by caster angle but also by front-view steering offset or scrub radius. It is fairly common on karts to adjust caster jacking via spacers on the front spindles. This is particularly useful on a vehicle with no suspension, but it works with suspension too.