The Mark Ortiz Automotive

CHASSIS NEWSLETTER

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to the Motorsports Community

November 2010

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

 

 

HIGH AND LOW SPEED BALANCE, AND TORQUE ARM LENGTH

 

I am currently autocrossing a 3rd gen Camaro.   I am using a Spohn torque arm.   The weight balance on the car is 51% front.  The torque arm is the same length as the factory.  With a wheelbase of 101 inches, I can't help but feel the length of the arm should be more around 50 inches or half the wheelbase.  Is there a way to determined the length?  Also, is there a way to figure braking into the equation?
 
My experience is the stiffer the front, the better the car turns in the more sweeping turns, but in the tighter turns, the car will push.   I feel that the long torque upsets the balance of the car.   My experience is with Late Model dirt track racing.  We used torque arm with a length of about 36-38 inches however, no sway bars and braking is quite different.

 

Short answer first: the torque arm isn't the problem, nor is anything else in the suspension, in all likelihood.  Most cars, especially rear-drive ones, tend to be too tight (inclined to understeer) in low-speed turns when they're well-balanced in high-speed turns, and be too loose (inclined to oversteer) in high-speed turns when they're well-balanced in low-speed turns.

 

The tight turn/tight car effect results from the off-tracking of the rear tires inside the fronts in tight turns and outside them in sweepers; the increased power transmitted through the rear tires to maintain steady speed as turn radii and speeds increase; and the the increase in locked axle push effect in tight turns, when any kind of limited slip diff is employed.  The effect isn't desirable, but it's normal.

 

There is really no way to fix this using just suspension and tires.  However, rules permitting, it can be fixed with aerodynamics.  If the aerodynamic center of pressure can be moved far enough rearward, the car can be given good balance in both high-speed and low-speed turns.  In fact, it can even be made to push in the sweepers and be loose in the hairpins, chicanes, and slaloms.  All it takes is a fair amount of rear spoiler or wing.  If the rules allow that, everything is easy.  If the rules preclude that, we are stuck with tuning the suspension around an aero balance problem.

 

To some degree, we can crutch the problem with relatively stiff low-speed rear damping, but that really just makes the car loose in/tight off, perhaps more so in low-speed turns where transients are more abrupt.  That isn't really the same thing as making the car looser slow/tighter fast, steady-state, but it can help the car turn in on the slower segments.

 

So the immediate problem at hand doesn't relate to the torque arm, but what about the torque arm?  How exactly do these devices work, and what does it take to optimize them?

 

A torque arm is a beam that extends forward from the rear axle, usually alongside the driveshaft, used to react drive torque (My) acting about the axle centerline (in side view to the car).  The front end of the arm is attached to the sprung structure in a manner that locates it vertically but not horizontally.  The arm therefore reacts only axle torque, and locates the axle only rotationally.

 

The arm will also react braking torque, if the brake calipers are mounted to the axle tubes.

 

Other members must be provided to locate the axle longitudinally and laterally.  The Camaro layout is the simplest possible: two trailing links and a Panhard bar.  The trailing links are a bit below the axle, and roughly horizontal, for minimal bump steer.  The layout is very compact.

 

The most common method of locating the front end of the torque arm is a drop link.  It is also fairly common to use a rubber or plastic bushing, and simply let the end of the torque arm slide in that.  In dirt Late Models, it is customary to create a compliant drop link by using a coilover for the link.

 

The stock Camaro layout takes simplicity to the extreme by not even having a slider or a drop link just a really big, soft single bushing.  The torque arm itself is a stamped channel-section piece.  This works fairly well, but there is some undamped vertical compliance in the bushing, which can potentially lead to wheel hop.  Some aftermarket torque arms, such as those from Spohn used by the questioner, substitute a nicer-looking triangulated tubular weldment for the stamped arm, with either a slider (lower-priced systems) or a drop link (higher-priced versions).  These attach either to the transmission tailhousing, as the stock arm does, or to the nearby crossmember.  There is little or no change in the geometric properties of the system.  There is a reduction in vertical compliance at the front end of the arm, with freer motion longitudinally at that point.  This reduces compliance axle wrap, which can in some cases become oscillatory and cause wheel hop.

 

The torque arms in dirt Late Models are deliberately made highly compliant, but the compliance is damped.

 

The height of a torque arm does not affect its properties, nor does its angle, but its length affects the amount of anti-squat under power and anti-lift under braking.  If the longitudinal locating linkage provides near-zero bump steer, the long arm in the Camaro provides something close to 100% anti-squat: the rear suspension does not compress or extend very much under power.  The shorter arms used in dirt Late Models provide considerably more than 100% anti-squat: the rear actually lifts noticeably under power.  In this form of racing, torque arms are often called lift bars.

 

It is also possible to get comparable amounts of anti-squat using tension-compression links.  One advantage of a torque arm over such options is that it is at least potentially possible to get an anti-squat support force in the suspension that is not sensitive to ride height.  This potentially allows somewhat greater amounts of anti-squat to be used before wheel hop is encountered.

 

Dirt Late Model suspensions often have large amounts of roll oversteer.  The rear wheels move forward when the suspension extends and rearward when it compresses.  This causes the inside (left) rear wheel to move forward in rightward roll and the outside (right) rear to move rearward, aiming both rear wheels out of the turn.  This geometry adds an additional anti-squat/anti-lift effect, sometimes called thrust anti-squat/anti-lift.  This component generally does increase as the car lifts, although it is possible to make it not do this.

 

The Camaro layout also reacts braking torque through the arm.  Since the rear brakes only do about 30% of the braking in limit straight-line braking, anti-lift in braking is in the range of thirty to forty percent in that condition.  In gentler braking, the rears do a somewhat greater percentage, and the anti-lift is correspondingly greater.

 

More anti-lift might lead to wheel hop in braking, if the rear wheels come close to the point of lockup.  Surprisingly large amounts of anti-lift can be used if rear brake percentage is limited so that the front brakes always lock well before the rears.

 

In dirt Late Models, it is common to use a lot of rear brake to help the car turn in.  To allow this without wheel hop, the brake torque is commonly reacted in a manner that creates pro-lift, roughly countering the thrust anti-lift that comes with the roll oversteer.  In the most popular layout two trailing links and a birdcage left and right, with the calipers mounted to the birdcages the upper trailing linkon each side is inclined steeply upward at the front, so that the side view instant center is behind the wheel and well below the wheel center.

 

The torque arm length does not affect cornering balance unless there are asymmetries in the car spring or wheel rate split, a markedly off-center c.g., or the torque arm itself off-center in the car.  Such asymmetries can cause the car to gain (or lose) left-turn wedge under power.  They can thus be used to tune entry and exit balance in an oval track car.  However, in general we cannot use these tricks in a car that has to turn both right and left, because the wheel load changes we create will affect car behavior oppositely in right and left turns.

 

My advice with the autocross Camaro would be to: leave the torque arm as is; use plenty of overall roll resistance, since the strut front end has poor camber recovery, especially when lowered; have enough of that roll resistance at the rear (probably use a rear anti-roll bar) so that the car has good balance in the tighter turns; add rear spoiler to stick the rear in the sweepers.