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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.
FRAME TORSIONAL STIFFNESS
Recently I was asked to
assess a northeastern dirt modified car which has the tendency to consistently
crack one particular welded joint in the tubular plus 2 x 4 mild steel chassis.
Attached are two bitmap image files from the two leading northeastern modified chassis builders [not included here as they are copyrighted catalog images]. The car in question is not one from either of these builders but is essentially the same in design. It is actually designed to be more flexible, or that is the claim of the builder.
I must confess that even though I have worked with these types of cars recently I had never really taken a step back and studied just the 'naked' chassis structure. All I see is a 'wet noodle' that only resists torsional loads on the sheer size of the material used. The 2 x 4 frame rails are required by rule but the rest of the chassis is just a 'stick built house'.
So my question to you is why do the folks who build and race oval track dirt cars still insist, rather strongly, that a flexible chassis is the 'hot ticket' to get the car to turn in to the corner? That is the claim I hear the most in defending flexibility.
I was trained to believe that you go as stiff as you can go up to the point of incurring a weight penalty and adhering to the required safety rules.
The claim is that the flexible chassis is easier to tune. I agree because to me it is to a certain degree un-tunable with an undamped fifth spring doing the work.
The only drawback to going as stiff as possible with a dirt car chassis that is required to use a beam axle front and rear is that, in my opinion, it would narrow your setup window and in effect, yes, be a little harder to zero in on a fast setup with just springs, shocks and anti-roll bars.
One additional piece of information: the engine is not used as a structural member and in fact
besides the engine plate
which fills the bay below the A-pillars, the mounts are designed to promote
engine movement as well.
I really need a sanity check on this one. What is my counter argument to the 'flexi' folks?
The frames shown could be less triangulated than they are, actually.
Dating from carriage building, vehicles with beam axles at both ends have used torsionally flexible frames with decent results. Most trucks are still made this way. They donít even use poorly triangulated space frames; they use channel-section ladder frames. The stresses in the frame rails are such that they are heat-treated, and frequently carry cautionary labeling discouraging welding on them lest the heat treat condition be altered.
Some people may be convinced that there is a performance advantage in a torsionally flexible frame, but another reason frames arenít triangulated better than they are is packaging. The diagonals required are hard to find room for. Things like the engine, the driver, and the exhaust system tend to get in the way. With more diagonals, the car becomes harder to work on and to get in and out of.
I am not convinced that a torsionally flexible frame is an advantage, for turn-in or otherwise, but with softly sprung beam axle suspension at both ends, it doesnít make much difference because the torsional loads on the frame are small.
A torsionally flexible frame does make the total chassis more warp-compliant, but as the questioner notes, the compliance is largely undamped.
Excessive torsional flexibility can give rise to torsional oscillation. One commonly accepted rule of thumb is that for the suspension to work as it should, particularly as regards damping, frame twist should be no more than a tenth of the total compliance in the warp mode, suspension and tires included.
With an existing car, we can test this. We need a flat support to place under one wheel, or alternatively two of these for two diagonally opposite wheels. These should be large enough to fully support the contact patch, and tall enough to create significant suspension displacement, yet not exhaust the suspension travel at any corner of the car. When we set the car on the support(s), we create a known displacement at the contact patches. If we are working in inches, we can get this in angular terms by dividing 57.3 by the track width, then multiplying by the height of the support. Alternatively, we can measure the angle directly with a long straightedge and an angle finder. If we do this at both ends of the car, the angular warp displacement is the sum of the front and rear displacements. We then know how much warp, twist, or cross-axle articulation we have at the contact patches.
We can measure the frame twist by measuring angle from gravitational vertical or horizontal at any reference surfaces at the front and rear of the frame, first with the car on level floor, and then with
the car on the support(s). The change in the front/rear difference is the amount of frame twist under the load imposed. The frame twist should be no more than a tenth of the warp at the contact patches.
It will be apparent that stiffer springs, bars, and tires will call for a stiffer frame, to meet this requirement. A suspension that uses high roll centers to get its roll resistance, rather than relying on the springs and bars, will not require as much frame stiffness.
Is a car with a flexible frame easier to tune? Well, it is less responsive to changes in springs, bars, and even roll center height. It is therefore less prone to being overly touchy to small adjustments, but cars with compliant tires and soft suspensions tend not to have such a problem anyway. Indeed, even pavement cars with really stiff suspensions are also more often under-responsive to changes, due to insufficient frame stiffness, than over-responsive due to excessive stiffness.
A flexible car will respond differently to changes in track conditions than a stiff one. That doesnít necessarily mean it will change less, but it wonít change the same as a stiff car. If the car corners on three wheels, like the one we discussed last month, frame stiffness does not affect wheel load distribution, past the point of wheel lift.
LIVE REAR AXLE SUSPENSION DESIGN
I have a few questions for you regarding different suspension types, set-ups, etc. My vehicle in question is a 1970 Pontiac that will be heavily modified for GT 1 / Solo 1 racing. It will also be licensed for the road. My main concern is regarding the rear suspension set-up. I have a full size Winters Quick Change closed tube live rear axle.
There are a few systems that I have looked at: Satchell link, 4-link w. watts linkage, and a criss-cross-under (?) 4-link that I saw awhile ago. Your article on the Satchell link was informative - it would be great to use a Weissman differential, if possible, with this set-up. I have seen someone use this for GT1 regional racing at Mosport, my home track. It looked composed in most corners. Still, one wonders if there is enough lateral restraint with this system. Could another lower link be introduced with the use of a rod end with an additional lug grafted onto it? That way, one link would be at 30 deg., and the other at 60 deg. from the axle line. If all the link lengths and bracketry were geometrically congruent, could this work and provide the additional lateral strength? The 4-link w. watts linkage obviously appears to be the strongest system. Would there be an advantage of laying out a z-linkage on birdcages as opposed to a trailing arm 4-link, also on birdcages? One concern is with the ctr. section of the quick change. It is sand cast aluminum w. a steel retaining ring for the yoke. Affixing a watts linkage to this for lateral control makes me nervous. Finally, I have seen a crisscross-under 4-link that was on a dead axle on a fwd car. Each link crossed over-under each other at the ctr. of the beam and connected to the chassis on the opposing sides. It resembled a shallow angled x-type layout. I realize that there are obvious roadblocks in applying this to a live rear. If it could be done, would this be a viable alternative? Does it have potential for quelling torque reaction of a live rear as well as providing lateral control?
My vehicle has a 118 inch wheelbase, 66 inch rear track and will have either 16x12 or 17x12 wheels. I plan to use the JRZ RS1 twin tube dampers with 2.25 id coilovers. Gross weight will be
around 2900 lbs. w. 50/50 f-r wt. dist`n. Please excuse my rambling on. Your input and suggestions would be greatly appreciated.
A general principle of beam axle linkage design is that if you use more than four links (or single link substitutes, such as a Watt linkage), at least two of them have to be equal length and parallel to each other or you will get binding, unless you use at least one birdcage.
If you have two diagonal semi-trailing links that cross each other, they must either be at different heights or have bends in them, or they will hit each other in some condition of suspension movement. This would be true regardless of whether the axle is powered or dead.
Crisscrossed or not, the ability of diagonal links to react torque depends mainly on their angle. The closer they are to longitudinal, the better able they are to react torque and thrust. The closer they are to transverse, the better they are able to react lateral force.
Crisscrossed links will not cancel driveshaft torque reaction, if that was the meaning of the question.
People do successfully attach lateral locating devices to the noses of quick change rears, and also the undersides.
If you use Z-linkages (longitudinal Watt linkages using birdcages), donít attach the brake calipers to the birdcages. Either attach them to the axle tubes or use brake floaters. Otherwise, the anti-lift will vary excessively as the suspension moves.
To a considerable extent, the choice of a general layout comes down to what roll center height you want, how you want roll center height to vary as the suspension moves, and what packaging and mounting constraints you have. Cost and complexity are also factors for most of us.
I like the idea of non-parallel trailing links on the right side, with enough anti-squat to cancel driveshaft torque, and on the left side a birdcage mounting the caliper, with trailing link geometry similar to the right side. Alternatively, a brake floater on just the left can give comparable effects. The idea is to have asymmetrical anti-squat under power, to eliminate torque roll and torque wedge, yet also have symmetrical anti-lift when braking.
For lateral location, I like the idea of a Watt linkage with the rocker mounted to the frame, not the axle, at least if the front suspension is independent. This makes the rear roll center move compatibly with the front roll center in heave. Unfortunately, packaging and mounting that type of mechanism is not always practical, but in some cases it will be.
One lateral locating device that may be convenient when using a quick change and stock frame rails is a somewhat angled Panhard bar running from one frame rail to a bracket on the opposite axle tube, ahead of the diff, passing above the driveshaft. This may more easily provide a strong anchorage for the bar than hanging a bracket for it behind the axle.
Whatever arrangement is used, it is important to make things strong and rigid, and strive for good load paths. Lateral compliance in the axle locating mechanism will not feel good to the driver, especially in transient maneuvers.