The Mark Ortiz Automotive

CHASSIS NEWSLETTER

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February 2007

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WELCOME

 

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: mortiz49@earthlink.net.  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.

 

 

MODERN DRIVESHAFT JOINTS VS. RUBBER DOUGHNUTS

 

I am running a Historic Formula 2 March 712 in Europe.  I am working hard to improve the handling.  To make the car lighter, first I removed the heavy doughnut with heavy strong driveshafts and I saved a total of  7 Kg after the modification to modern CV joints.  Can I expect a improvement on handling?  I think the rubber doughnuts are likely working as a spring and are not so nice to drive on corner entry and exit and I am losing power on acceleration (and the polar moment is much higher; that means the acceleration is different.)  Can you give me more information about that?

 

Handling should be helped a little.  The difference may be too small to feel or even measure, but anything that reduces weight should help handling, once the car is optimized for the new weight and weight distribution.  In this case, roughly 50% of the mass in question is unsprung, and that improves the payoff, at least when there are any bumps.

 

Rubber doughnut joints are compliant in torsion, and this can create surging: an oscillatory longitudinal acceleration of the car, caused by the joints wrapping up and unwrapping.  Any jerky application, release, or reversal of torque can provoke this.  This is not strictly a handling issue more of a driveability issue.  However, we do partly control the behavior of the chassis with the application of torque to the wheels, and anything that reduces our control of torque to the wheels inevitably hurts our ability to control the car.

 

As for the addition to the wheel rate from the doughnuts, no doubt there is some.  However, we need wheel rate; we aren't necessarily seeking to minimize it.  We can compensate for any contribution from the joints by running slightly softer springs.

 

We do want to know wheel rate as accurately as we can, and the addition from the joints complicates this a bit.  If we are really fastidious, we can measure the joints' contribution fairly accurately by putting the car on the scales, removing the springs, jacking the sprung mass up and down, and noting changes in the scale readings.  Probably the doughnuts have a somewhat different rate when

 

 

everything is in motion, but measuring their rate statically should be at least as valid as measuring a tire's rate statically.  Of course, all this is moot if the rubber doughnuts aren't there anymore.

 

Regarding improved acceleration due to reduction of moment of inertia, that's also good, but I expect the effect here will also be small.  It will be nowhere near as big as a similar moment reduction at the flywheel or clutch, because the drive shafts turn much slower.  If the engine turns, say, four times as fast as the shafts, then any given saving at the flywheel is worth sixteen times as much as the same reduction at the shafts.  The engine has to accelerate the flywheel four times as fast as the shafts, and any given inertia torque at the shafts looks only one-fourth as great to the engine because it is reduced through the gears.  Still, even if the effect is small, it is surely helpful.

 

 

ZERO-DROOP SETUPS

 

[Continued from previous question]

The next question I have is about suspension droop travel limit fwd and aft.  I have seen that modern single seaters have no droop at all.  Now I am not sure about a historic racecar like my March F2 (year 1971).

  

The shock stroke (fwd and aft) during hard driving is about 20mm.  I know aft shocks should have more travel than fwd shocks but how much?

 

Corner exit on acceleration the car is pitching fwd up and with no droop can help to get more weight on the car fwd because the unsprung weight will help to hold the nose down.  Until now I made the shock adjustment with more rebound but I think that's the wrong way to fix the problem.

 

The only reason it makes any sense to not let the suspension move freely in droop is to control ground-clearance-sensitive aerodynamic elements.  From every other perspective, making the suspension top out prematurely is a bad thing.

 

For best mechanical grip, we want the suspension to extend freely until the springs reach zero load, and then stop.  If the suspension extends further, so that the springs hang loose, that doesn't hurt grip but it can cause the springs to beat up the shocks, or the spring retainers or adjusting collars, or other pieces, if it happens very often.

 

If the suspension tops out before the springs unload, that abruptly unloads the tires, but it also keeps the ground clearance from growing, at least as long as we don't pull the wheels off the ground.  That's bad for mechanical grip, but it's good for aerodynamics if we've got a floor, a valance, a splitter, or a wing that has to be near the ground to work well.

 

From what I can tell by pictures on-line, the March 712 has a fairly broad chisel-shaped nose, with two small nose wings on the sides of the chisel.  The radiator is in the nose, fed by an intake below the leading edge of the chisel, and exhausting through an outlet on the top of the chisel. The little

 

wings are up fairly high compared to later cars.  It doesn't look like the car would be highly sensitive to ground clearance at the front.  The car pre-dates tunnels and diffusers; the floor isn't designed to make downforce.  It has a rear wing, but this would not be highly sensitive to ride height either.  So I don't think the car would realize the same advantages as a more modern car from zero-droop suspension.

 

There is no hard and fast rule for the relationship between front and rear shock travel, as shown by travel indicators on the shock shafts.  In tail-heavy, rear-engined cars, it is common for the front suspension to have a higher natural frequency and smaller static deflection than the rear.  That is, the front suspension is stiffer in ride than the rear, relative to its sprung mass.  This normally results in more suspension travel at the rear than at the front.  Also, it is normal to have more aerodynamic downforce at the rear than at the front.  This will result in the shock travel indicators showing more travel at the rear than at the front, if the motion ratios are similar.  Looking at photos, it appears that the motion ratios at the front and rear of the March 712 are fairly similar.  So if the travels are similar, it may be that the rear springs are a bit stiff relative to the front but not necessarily.

 

Keeping the front end from lifting under power will only add a little bit of load to the front tires and concomitantly reduce rear tire loading a little.  The amount of rearward load transfer, for a given forward acceleration, depends entirely on the height of the center of gravity and the wheelbase.  How much the nose lifts wouldn't matter at all, except that more nose lift does result in a slightly higher c.g.  If the front of the car seems to lift excessively under power, stiffer front springs will reduce that.  You may want to combine the stiffer springs with more rear anti-roll bar or less front anti-roll bar.

 

Some readers may find it surprising that the front end lifts less with stiffer springs, but that is indeed the case.  A stiffer spring doesn't mean more upward force.  It means less travel for a given load change hence less extension travel for a given load decrease.

 

Ordinarily, we don't want to keep load on the front tires under power; we want load to transfer to the rear so we can put power to the ground.  This is true unless the car understeers excessively on corner exit.  This is not uncommon in rear-engined cars, but a more common complaint in racing generally is that the car spins the wheels and/or oversteers on exit.  A car that is too tight (understeers too much) power-on is a problem that racers in many classes would kill to have.  If the back tires stick too well, just feed them more power, goes the reasoning.

 

This doesn't always work, however.  There may be no more power available; the driver may have the throttle wide open already.  This is often encountered in Formula Fords.  Or the understeer may simply increase until power breaks the rear tires loose completely, whereupon the rear end suddenly snaps out.

 

The March appears to have very short control arms, especially the uppers.  That means that either the camber recovery goes away markedly as the suspension extends, or the roll center rises as the

 

 

suspension extends, or both.  Having that happen at the front would worsen a power push, and stiffer front springs would reduce the lift.  Restricting droop travel would keep the nose down too, but the

effect would be abrupt, and the roll resistance would abruptly increase, worsening the push.  More spring and less bar keeps the front end down more, without that problem.