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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.
DIRT LATE MODEL RIGHT FRONT “PINNING” AND HEAVY AXLE TUBES
I have a question about the theories of “pinning” the RF on a dirt late model. I stopped racing in 2006 and just being a spectator now has me curious. I’ve asked a handful of people about this and really have gotten nowhere. It appears many guys are running a really soft RF spring to allow car to fall on to some form of a bump stop (bump spring, bump rubber) and then they have a very high rebound shock to hold the car there. My question is (or theory, I guess) why all the shock technology? Could you not just lower the static ride height and run what would be considered an overly stiff spring to get ending wheel load without all the movement? I feel it is the dynamic ride height (where the car stops rolling) that is important and not the movement to get it there. In other words why aren’t they just setting the car up in a dynamic state of roll/hike and leaving/keeping it there? Am I misunderstanding something?
Second question, from a different person:
Lately I have been reading that many dirt late model and modified racers have been adding non-rotating weight to the left rear axle. For years people have been trying to get unsprung weight as light as possible, aluminum axle tubes and spindles and aluminum radius rod brackets, etc. Now there are steel axle tubes as thick as can be without interfering with the spinning axle. I would like to hear your understanding of this.
When you see suspension strategies that really don’t seem to make sense by ordinary reasoning but seem to be popular, there are two possible explanations. First is “trick of the month effect”: people are blindly copying something that is rumored to be the hot setup, without understanding. Second is that the seemingly anomalous strategy allows us to get around a rule, or work a rule, often for aerodynamic advantage.
Sometimes we see a combination of the two reasons: a technique is useful due to a rule in one series but gets adopted imitatively in another series that doesn’t have the rule.
ARCA cars use very soft springs and bump rubbers or small coil bump springs, with hold-down shocks, at all four corners. This used to be done in the upper NASCAR divisions. The reason there is (was) to get around a ground clearance rule. NASCAR did away with the ground clearance rule, so we don’t see such setups there anymore. ARCA still has the ground clearance rule so we still see soft spring/bump stop setups there.
There are a number of sanctioning bodies running dirt Late Model races in the US, and they all have their own rules. Somewhat similar cars run in Australia and New Zealand, also under local rules. Here are the rules for three prominent US sanctioning bodies:
World of Outlaws (formerly STARS series)
On all of these, new rules for 2017 are underlined. IMCA has new rules this year prohibiting any kind of suspension stops whatsoever on the front, and also requiring only one constant-rate spring per wheel. They even prohibit spring rubbers (between the coils). There is one exception allowing a “take up” spring on the left front. In dirt Late Models such springs have been used with a droop stop on the shock body that makes the rate increase toward the droop end of the travel, as opposed to the “tender springs” seen on lowered production cars in road racing, which provide a very soft spring that is coil bound in normal operation and is just there to keep the main spring in place when the car is jacked up.
IMCA also has a new rule requiring “standard weight aluminum or steel axle tubes only”. I don’t see those provisions in the WoO or WISSOTA rules. WoO and WISSOTA do have “catch all” rules saying anything “non-standard” or innovative in any way is to be submitted for approval in advance. I don’t know if they actually check for heavy axle tubes. WoO does prohibit any separate insert inside an axle tube.
Well, okay – why would you want to run a very soft RF spring? Wouldn’t it do the same thing if you just set the RF ride height lower? It would, pretty much – but you couldn’t get the car through tech. All the linked sets of rules have very stringent requirements about the height of the left rear corner of the body, the amount of tilt in all the horizontal surfaces, the radius of the edges, the
amount of dish in the quarter panels – anything that could affect the car’s ability to generate downforce and lateral force when running in aerodynamic yaw.
From a normal suspension design standpoint, a lot of what’s done on dirt Late Models is nonsensical. That is, it doesn’t help the wheels follow the road surface or minimize camber change or bump steer. Rather, it’s about aero. It’s about having the car level and at legal heights when it goes through inspection, and at the same time getting it to run as nose-down, tail-up, sideways, and tilted as possible at speed, to get lateral force and downforce out of that big slab-sided body.
So how does a soft RF spring play into this? It makes the right front corner drop more, and the right rear and left front rise less, when the left rear hikes to full droop or close to that. The bump rubber is then necessary just to provide at least a little cushioning and prevent destruction of parts when the suspension goes to full compression. The soft RF spring also makes the car gain less wedge when the LR hikes. The car corners with the left front lower, so less air gets under the car there. That helps overall downforce. The RF has more tendency to drag the nose on the ground, but that is easily dealt with by trimming the bottom edge.
The strategy could probably work without a hold-down shock. The car would just come up a lot at the RF on the straights.
The heavy axle tube makes less sense. It probably doesn’t hurt much if the suspension is topped out and the entire left rear corner is effectively unsprung anyway, but I can’t see it helping. I don’t see any rules that restrict ballast location other than requiring it to be within the frame and not on the bumper, so a heavy axle tube doesn’t offer any advantage in terms of c.g. location. I could maybe see some point in hiding ballast by using heavy tubes, and then adding phony ballast weights made of aluminum rather than lead in wacky locations, just to fake out the competition. But I think I’d rather have heavy frame tubes instead of axle tubes. Of course, adding or removing a heavy axle tube would be easier.