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September 2009

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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:  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.





My question relates  to the wheelbase on a 2700 lb. Outlaw bodied Latemodel.  We have no rules on the wheelbase difference from side to side.  My question is, any benefit to altering the wheelbase from side to side?  I was thinking of shortening the left side by toeing out the LR and toeing in the RR, then squaring the rear ties to the chassis to create lead in the lr tire.  In the front I was thinking of moving the LF suspension back.  This would create a shorter wheelbase on the left but have the tires still pointing straight ahead.  Is there any benefit to this approach and how much could you go?


It comes down to whether this helps you work whatever rules you do have.


What is the wording of your engine setback rule?


What rule(s) do you have regarding wheelbase?


What rule(s) do you have regarding rear axle housings?


Engine setback is 4in. measured from #1 plug to center of upper balljoint.

There are no rules stated for wheelbase.

There are no rules stated for rear axle housings.


This is pretty much an open competition class.  We run a 9 in. treaded tire so corner speed and momentum is the key to going fast and passing.  I am just trying to think outside the box a little.


No wheelbase rule at all?  Really?


Is there a minimum right side weight or a maximum left percentage?


How much ballast is in the car, and where is it?


What are your current static left and rear percentages?


Are you mostly running bowl-shaped momentum tracks, with large turns, short straights, and narrow speed range, or paperclip-shaped tracks, with tight turns, pronounced straights, and wide speed range?  How important is braking?  Do you use the brakes hard, or hardly use them?


No wheelbase rule at all.


No leftside weight rule - my car is about 62 % left and 49.5 rear.


Car weighs about 2100 lbs no ballast no driver.  Rules 2700 lbs.  We use about 400 lbs. of ballast mostly on left side.


The track in question  is a 3/8 high bank bowl shaped momentum track with narrow speed range most definitely.  We use very little brake if any on this track.


If the track is one where steady-state cornering is the main requirement, you’re probably about where you should be on rear percentage.  On a more paperclip-shaped track, I’d suggest more rear, to aid propulsion and braking.


It sounds like if you want to, you can move some ballast rearward and get more rear percentage, without needing to move anything else.  Is that correct?


62% left is a lot, but if you can get even more, I bet you’d go faster.  You want to start being careful doing sudden darts to the right, but turning left I bet you’d gain speed all the way up to 70% left, if you could get to that.


Have you tried more rear?  More left?


Can you get the ballast any lower than it is?


Returning to the original question, I don’t think leading the right front and left rear wheel, or leading both right wheels either, makes the car turn left more readily, if that was what you were wondering about.  The car is very sensitive to the aim of the rear wheels, but if you change their location slightly without changing the direction they point, that shouldn’t make much difference.


Relocating wheels does potentially give you a way to get a little more rear percentage, while meeting the engine setback rule, depending on how they measure.  If they lay a square along the side of the head and let its leg go out to the ball joint, perhaps extending that line using a string or another straightedge, moving the right front wheel forward doesn’t change the measurement, yet it gets you some rear percentage.  Ditto moving either rear wheel forward, or both of them.  Moving both rights forward makes sense when you have a wheelbase rule, and a wheelbase difference rule, and you’re after more rear percentage.  In your case, you don’t have those requirements, so I doubt that you’d



see an advantage.  I doubt that it would hurt, either, nor would moving the right rear back and the left rear forward.  It would mostly just be extra work for little or no effect once you dial the rest of the setup back in to suit the change.


One other question: if there’s no axle housing rule, does that mean you can run cambered rear axles?  If so, maybe a bit more rear percentage might even help you mid-turn.


There are reliability and efficiency issues as you try to angle the axle snouts with respect to the tubes.  Beyond about 1 degree, you need to run the axles with the barrel-shaped splines if you want decent life and reliability.  Even then, you are turning some horsepower into heat as the wheels turn and the splines rub.


Depending on the properties of your tires, you should see an improvement in cornering force up to at least a degree and a half of inclination into the turn (positive camber at the left rear, negative on the right rear).  My expectation would be that by the time you get the camber optimized, the splines will be rubbing quite a bit as the wheels turn, and you probably don’t want to make that worse by adding top-view angle to the snouts.


In other words, if you’re going to angle the snouts with respect to the tubes, go for camber rather than toe, unless you are thoroughly satisfied that you’ve already optimized camber.


You may find that as you increase tire inclination, your optimum pressure will be less.  That is, cambered tires will usually want a bit less air.  You want to find the camber that works best at its own optimum pressure, rather than at the pressure that worked best without camber.


It sometimes happens that the camber and pressure that are fastest will blister tires.  This definitely happens in the upper NASCAR divisions.  If you encounter that problem, you may have to be more conservative on your settings to make sure the tires survive.


If you run a spec, treaded tire, you may find it desirable to shave them.  This is hardly a secret, and you may be doing this already.  If you do shave the tires, and you can run cambered axles, I suggest shaving the tires roughly straight across, rather than camber cutting them.  Camber cutting makes sense when you can’t run cambered axles, but when you can adjust camber, it’s better to get the tread as shallow as possible all the way across.


Stagger will affect camber somewhat, so when optimizing camber you want to be working with an optimal amount of stagger.  There are ways to calculate that from a measurement of turn radius and banking, but you can also find it experimentally.  Drive into the turn on your racing line, at a bit below racing speed, disengage the clutch, and coast through the turn.  If you run a spool, see how much speed you lose.  If you have a locker, turn off the engine and see if you hear it click.  The idea is to find the amount of stagger that gets you through the turn under power with the least drag.




This will be worth some cornering speed, even if the car is not power-limited when cornering.  You can get the car to behave decently with some other amount of stagger, but then the tires are using up some of their grip budget overcoming the incorrect stagger: they’re fighting each other to some extent.  The grip they use doing that is grip they can’t use to make total lateral force.


You can also use a similar technique to optimize toe, both front and rear.  Again, you want the setting that drops your speed the least when coasting through the turn at slightly below racing speed.  You may find that at the best setting, the car feels a bit nervous or wanders a bit, although generally that is more of a problem on straights than in turns, with least-drag toe.  You do want to be comfortable with the car at the limit, but the idea is to run the least draggy setup that satisfies that requirement.


Note that this recommendation applies only to tire drag, not necessarily aerodynamic drag.  You don’t actually want aerodynamic drag, especially if you do little braking, but for almost any short-track application, you need maximum downforce from the body, even if it costs you some drag.