The Mark Ortiz Automotive

CHASSIS NEWSLETTER

December 2016

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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.

 

 

A BIT MORE ON PROPORTIONING VALVES

 

I just reread your November newsletter on the proportioning valve.  My analysis leads to what may be a slightly different conclusion.  Let me lay out the logic and see what you think.

 

In straight line braking with a correctly set proportioning valve the rear brakes do not lock.  When the pedal is released the forward weight transfer stops (i.e. the weight transfer is to the back) giving the rear wheels more potential traction.  Consequently, even though the rear pressure does not fall until the front drops below the rear, the rear wheels are not at risk of locking.

 

What happens in trail braking?  Here one needs to set another parameter which is that the driver is following a constant radius.  Given this caveat the conclusion should be the same.  If the inside rear did not lock with the brakes on it should not lock with brake release as the inside rear tire has not lost traction capacity (it actually gains a little).  

 

Looking at another possibility: If the driver lifts off the brakes at the very same instant as he turns is it possible the rear brakes release slowly enough that the inside rear locks ever so briefly due to the "hysteresis"?  (i.e. the front brakes released while the car was going straight but the rears did not release until the car was turning)  Possible, I suppose, but one would need to look at real data to see if the lag time is sufficient to produce this effect.

 

I think it most likely the inside rear lockup often seen on front drive (and occasionally rear drive) cars is related either to 1. The driver is late releasing the brake pedal, or 2. The driver is doing what he wants/needs to do and having the inside rear lock briefly as it unloads is of no consequence to him.

 

I agree that having the inside rear lock momentarily isnít necessarily a problem, nor is having the inside front do so, as long as we donít flat spot any tires.  I donít think last monthís questioner meant to suggest that either.

 

 

If we imagine a car cornering at a constant radius with the brakes applied, and the brakes gradually being released, I agree that if the inside rear is not locked before brake release then it will not lock during brake release even if at first only the front brake force is diminishing.  This will also be true if we hold the car at a fixed lateral acceleration, which implies decreasing the turn radius in proportion to the square of the speed.

 

However, neither of these is what actually happens in trail braking, or at least neither of them is supposed to be what happens.  When trail braking is done correctly, the car is not on a constant radius with the brakes applied, and then held on that radius as the brakes are gradually released, nor is it held at constant lateral acceleration.  The objective is to transition from straight-line limit braking (maximum rearward acceleration) to pure cornering (maximum lateral acceleration) in such a manner that the car is kept at the edge of its traction circle/ellipse/perimeter throughout the entire process.  This means that the driver has to feed steering in as he releases the brakes, keeping the tires working to the limit of their capability the whole time.  Neither lateral acceleration nor rearward acceleration is constant.  Lateral acceleration is coming in as rearward acceleration is going out.  The vector sum of the two is close to constant.  This isnít easy to do perfectly.  The idea is to approximate the ideal.

 

We can feed the steering in more quickly, and the braking out more quickly, or we can have more braking and less cornering if we wish.  If weíre keeping the car at the limit, this choice will affect our line.

 

To brake harder, we must accept a larger turning radius.  If our turn radius is larger early, it will have to be tighter later in the turn.  This implies an earlier apex/tangent point/clipping point, and slower exit speed: an in fast/out slow line.

 

Conversely, we can have a tighter turn radius early, and a larger turn radius late.  This will give higher exit speed.  However, that implies less braking during turn-in.  That means we have to be at a lower speed when we begin feeding in steering.  That in turn means we have to begin our straight-line braking earlier, and accept a longer segment time for the last part of the straightaway and first part of the turn: an in slow/out fast line.

 

An in slow/out fast line is generally preferred any time the turn is followed by a straightaway of significant length.  This is sometimes called a type one turn.  Even if the turn has straightaways before and after, exit speed is worth more than entry speed, because speed we have at the start of a straight carries all the way to our cutoff point.

 

When the turn has a straightaway before it but another turn immediately after it, thatís called a type two turn.  For that, an in fast/out slow line makes sense, because we canít put better exit speed to use.

 

This gets really interesting when the turn has a straightaway after it, but not a very long one Ė or when a turn is followed by another in the opposite direction, with a long straight preceding the

 

combination and a short straight after, and thereís a tradeoff between entry speed to the first turn and exit speed from the second.

 

Perhaps Iím digressing.  But the point here that relates to inside rear lockup is that there isnít just one correct rate to feed the steering in and release the brakes.  It depends on the situation, and in some cases there can be rational arguments for different approaches.  Theoretically, thereís a correct lateral acceleration for any rearward acceleration, but there will be variances in what combination the driver actually uses.  Additionally, cars will differ in how much rear lateral load transfer they generate for a given lateral acceleration.

 

In any case, if the rear tires are close to the limit of adhesion in straight-line braking and then the driver begins releasing the brakes and feeding in steering, and at least initially rear brake torque does not diminish, what happens will depend on whether the increasing lateral load transfer unloads the inside rear more than the decreasing longitudinal load transfer loads it.  This can go either way.

 

 

IDEAL FRONT/REAR WEIGHT DISTRIBUTION?

 

Some friends and I have been debating a question:

 

"Ignoring any effects from aerodynamics, polar moment of inertia, or packaging (front engine, front/mid engine, mid engine etc), what would be the ideal front-to-rear weight distribution for a rear wheel drive asphalt-based road race car?Ē 

 

Some feel it would be the oft quoted 50/50% while I suspect it might be closer to 45/55% or so to better benefit acceleration and braking. 

 

Assume tire widths are free and can be adjusted front to rear as needed.

 

Any input would be appreciated.

 

We can definitely agree that weight distribution needs to be roughly appropriate for tire size distribution.

 

If we donít have rules constraining tire dimensions, then something else is bound to.  Assuming we can get whatever tire we want, generally packaging constraints set the limits.  If thatís the case, then generally the rear tires can be bigger than the fronts, simply because they donít have to steer.

 

If our only concern is to maximize lateral acceleration at constant speed, as on a skid pad, we theoretically want the weight distribution to be proportional to tire size, and we also want load transfer at each end to be proportional to weight distribution and tire size.  For example, if the car has 40% front, then 40% of the tire size should be there, and 40% of the total load transfer should

 

 

occur there.  We then have identical lateral inequality of tire loading front and rear in percentile terms, so we should be making equally good use of the front and rear tire pairs.

 

However, in most forms of racing we are not purely concerned with maximizing steady-state cornering speed.  We also need to maximize forward and rearward acceleration.  How important this is compared to steady-state lateral acceleration depends on the track design.  If itís a stop-and-go track Ė significant straights connected by tight, short-duration turns Ė then it becomes very important to be able to brake well and put power down well, and less important to have good steady-state cornering.  If itís a momentum track Ė few real straights, car almost always cornering, modest speed changes Ė then steady-state cornering takes precedence.

 

With rear drive, we want the car as tail-heavy as possible for best forward acceleration, at least up to the point where the car becomes wheelstand limited.  We also want it tail-heavy for best braking.  Up to a point, we can compensate by using more front brake, but there are packaging limits to front brake size, and endurance limits to how much stopping power we can get from front brakes of a given size.

 

Even for a momentum track, there is no penalty to having the car as tail-heavy as tire size constraints dictate, and having lateral load transfer proportional to weight distribution, as already noted.  But to improve longitudinal acceleration capability, there is a strong case for making the car even more tail-heavy, and increasing the percentage of load transfer occurring at the front to get the desired understeer gradient.  This will compromise steady-state cornering in favor of better braking and propulsion.

 

In most cases, there will be various constraints limiting how tail-heavy we can make the car.  The Chaparral 2E, for example, had all major components behind the driver, including the radiators.  The car reportedly had close to two thirds of its weight on the rear tires.  That could only have been increased by lengthening the wheelbase or by adopting a layout with the engine behind the rear axle.  At some point, penalties in vehicle size, transient handling characteristics, or something else will set a practical limit.