The Mark Ortiz Automotive

CHASSIS NEWSLETTER

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to the Motorsports Community

September 2012

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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.

 

 

TERM ďSTATIC DEFLECTIONĒ AS USED IN RIDE FREQUENCY CALCULATION

 

In ride frequency calculations what is your definition of 'static deflection' when used in the form f = 188/sqrt(x) in cpm?

Static deflection of the chassis (or spring with a motion ratio of 1) as the car sits on the scales at setup? i.e. Taking just the spring: spring free height - spring height at static load = x

Or static deflection at some estimated 'g' loading?

I have found many opinions on this but no clear cut answer. 

The equation shown is for English units (inches and pounds), and gives the undamped natural frequency for the sprung mass in cycles per minute.

 

Static deflection as used here means the amount that the corner of the car being analyzed deflects under static load (car sitting still in the shop), from a real or theoretical condition where the suspension is extended to the point where the spring completely unloads.  In some cases, we can measure this directly, and be fairly accurate.  However, in many cars, the suspension cannot actually extend that far; something stops the motion before the spring completely unloads.  Also, in many cases the wheel rate in ride is not a constant.

 

Therefore, it is often best to use a calculated value for static deflection: weight of the sprung mass at the corner of the car being analyzed, in pounds, divided by the wheel rate in ride for that wheel in pounds per inch, at the ride height we are examining.

 

We should note that it is really the mass of the sprung structure that we are concerned with, rather than weight or load.  The equation takes the mass as weight in Earthís gravity because that is easy to measure.  However, it should not be supposed that the frequency changes when we adjust wedge or when the car sees cornering loads.  It might, a bit, if the wheel rate is not constant, but if the system

 

is linear it doesnít.  It also does not change due to aerodynamic loadings, if the system is linear.  The frequency does change with fuel burn-off, or if we move ballast.

 

If we have an existing car, and we want to know its undamped frequencies, we can get a good approximation of the sprung weights by taking the springs out and measuring the weight of the unsprung components with our wheel scales, and then putting the springs back in and noting readings, then subtracting to find the sprung weight.  When we read the scales with the springs in, we need to have the car in an unwedged condition, meaning equal left percentage front and rear, and equal rear percentage left and right.

 

When there is a lot of unsprung mass, as with a 300 pound rear axle, and the unsprung mass is roughly centered left to right, but the sprung mass is very left-heavy, there may be a noticeable difference between equal total left percentage front and rear, and equal sprung left percentages.  In such a case, we should try for equal sprung left percentage front and rear: get the left percentages equal, after deduction of the unsprung weights.

 

Fortunately, especially in race cars, we donít need a high degree of precision in these measurements or calculations.  Even if we do get really good precision, the result is only an approximation of actual car behavior, because we have damping, tire compliance, and various other things that cause the running natural frequencies to vary from calculated undamped frequencies.

 

Nevertheless, it is a good idea to try to take a look at the calculated natural frequency relationships, particularly for a lightly damped dirt car.

 

 

THEORY OF TRACTION ON DIRT

 

I would like to know your thoughts on a tires behavior on dirt.  You have said in many previous newsletters that chassis dynamics on dirt behave pretty much the same as they do on pavement for a given level of grip and I agree.  However there is still much confusion among racers, a language problem really, about terms such as side bite vs. lateral acceleration capability and the like. 

In the case of side bite on dirt, when is elastic load transfer more beneficial than geometric load transfer?  Is there really a difference and if so, why? 

So my request/question is: would you run through the different phases of traction available as a dirt track goes from a soft tacky surface to a dry hard surface and what chassis dynamic parameters are most pronounced during the phases of that progression for both lateral and longitudinal grip.

I was reading your old newsletter on traction in the snow and got to thinking how early on in the night on a wet dirt surface the compaction/shearing mechanism must be the dominant player in traction, very quickly followed by sticky adhesion between tacky dirt and a cold tire and then finishing up with a dirt surface harder than the tire itself, in which the interaction is much more like

 

pavement.

Just looking back through the index of old newsletters and was surprised that this topic wasn't brought up.

Actually, Iím pretty sure I have addressed this in the past, although it was probably in the course of discussing something else Ė possibly why wide tires work.  Consequently, it may not be apparent from the back issues list that I have taken the subject up, and certainly itís been a while.

 

Dirt is a lot like pavement, in that itís basically just grit and some gravel, with a binder.  The big difference is that the binder is clay, and its behavior is highly sensitive to water.  It gets soft when it gets wet.  If itís really wet, it gets greasy.  If itís somewhat wet or just damp, it gets tacky or sticky.  As it dries further, it gets hard and brittle, and not sticky at all, with the result that the grit itís binding can be torn loose fairly easily, and some of the clay itself can be loosened to create a fine dust.

 

We occasionally end up racing on greasy-wet or sloppy dirt, but we and the race promoters try to avoid that.  Thatís why we mud-pack before qualifying.  To get traction in outright mud requires a very toothy tread Ė very low rubber-to-void ratio Ė that is also self-cleaning.  Off-road tires are often this way.  Dirt racing tires are not really optimized for this condition, because we donít spend much time on such surfaces.

 

Itís a bit hard to determine whether lateral load transfer has the usual effect in serious mud, partly because when there is so little grip there is very little lateral load transfer, and the vehicle is going every which way, making it hard to discern whether itís loose, tight, or what.  However, we can say that mud does not reward a narrow tire the way snow does.  It generally rewards a tire that floats rather than penetrates, but has a tread that penetrates.

 

Moving to less wet conditions, dirt becomes firmer, yet still soft enough for a tread to indent.  A dirt track may be like this for a fair period of time, particularly for the heats.  The tires can grab dirt and throw it.  If a cushion builds, this is when it will happen.  One might think that concentrated loading of the tires might benefit grip on this sort of surface, but experience suggests that there is no reversal of the effects of wedge.

 

As the dirt dries further, it reaches a state where it is still sticky, but too hard for a tread to leave an impression.  This is generally the condition in which the track is fastest, and also the condition in which it most resembles pavement.  The surface rewards contact area.  Edges and grooves cease to have much effect.  Slicks will actually be very fast.

 

With further drying, we will see the track either go to a blue-groove condition or an outright dry-slick state.  In the blue-groove condition, the surface absorbs enough rubber to visibly blacken.  The rubber serves as a binder, and also may chemically bond with the tires.  The surface will often be shiny.  Slicks will work on this sort of surface.

 

Alternatively, the surface may be loose and sandy enough so that it gets hard, but with a lot of sand and dust on top.  The surface then has properties a bit like asphalt or concrete that is either very dusty or wet: there is a hard, gritty surface with a lubricating layer on top.  Sipes in the tread blocks become useful for creating edges to penetrate the lubricating layer.  Some people believe that in this situation tire load sensitivity reverses, due to the importance of penetration.  My own observations contradict this: wedge still seems to tighten the car.  The surface seems to reward slightly lower tire pressures than a firm but tacky surface.  Wide tires still seem to beat narrow ones.  Generally all cars in a class will use the same size, but the faster classes are allowed bigger sizes, and nobody runs narrower tires than the legal maximum.

 

On dry pavement, tires work by a combination of chemical bonding and mechanical interlock.  On wet pavement, chemical bonding is largely prevented and the tire relies on mechanical interlock.  I think the only time chemical bonding can occur on dirt is when the surface is either tacky or blue-grooved.  But whether chemical bonding is involved or not, more contact area helps, and lateral load transfer hurts.

 

I donít know what ďside biteĒ means, if not lateral acceleration capability.  I have not encountered problems from using or interpreting the term that way.

 

Is there a difference between geometric and elastic load transfer?  No.  The tires canít tell where their loadings come from.  However, it is quite possible for people to be confused, and think that either geometric or elastic load transfer is affected one way by a setup change, when in fact it is affected the opposite way.  This sort of confusion is particularly likely among people who think they can tell by body attitude how tire loads are distributed.

 

For example, suppose we reduce the geometric roll resistance at the rear (lower the rear roll center).  The car will then roll more, and there will be more elastic load transfer at the rear, although the overall load transfer at the rear will be less.  The rear will stick better.  The outside rear corner of the car will ride lower, creating the appearance of increased lateral load transfer at the rear.  An uninformed person can then easily conclude that load transfer is helping side bite, or that elastic load transfer is helping side bite.

 

I still have yet to encounter a case where lateral load transfer helped side bite on dirt, although I have encountered plenty of people who have concluded that it does, due to misunderstanding of when lateral load transfer is being increased or decreased.