The Mark Ortiz Automotive

CHASSIS NEWSLETTER

July 2014

Reproduction for free use permitted and encouraged.

Reproduction for sale subject to restrictions.  Please inquire for details.

 

 

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.

 

 

PIN LEAD OR TRAIL IN STEERING GEOMETRY

 

Whatís the thinking of the Porsche 919 at Le Mans?  The front wheel bearing is ahead of the ball joints.  Indy cars have been doing this for years.  Chapman did it on Jim Clark's Lotus 35 Indy car.  Don't make no sense to me.

 

I call that pin lead: the spindle pin is ahead of the steering axis in side view.  It is also possible to have pin trail: spindle pin behind steering axis.

 

When the amount of pin lead or trail is small, it may be difficult to see whether itís present.  Therefore, more cars may have some pin lead or trail than is commonly supposed.  When dealing with production cars, in most cases we donít even think to try to measure it.  It only gets attention when we are designing, making, or modifying uprights or spindles, or possibly if we are checking them for damage.

 

However, some common devices that weíre all familiar with have obvious pin lead or trail.  Bicycles and motorcycles almost always have the steering axis behind the axle: pin lead.  Swiveling casters on shopping carts and furniture have really dramatic pin trail.

 

We most often see pin lead in independent front suspensions with conventional ball joints (or sphericals used as conventional ball joints) but it is also possible to design a spindle for a beam axle that has a little pin lead or trail.  We just need to make sure we still have adequate steering movement.  The same applies to an independent design that uses a kingpin as in a beam axle.  MacPherson struts can easily have pin lead or trail.

 

With a dual ball joint design (meaning two uppers and/or two lowers), we can adjust the effective pin lead or trail without any need to change the spindles or uprights, although some change in wheelbase may occur.  Different uprights may still be required to obtain desired wheelbase when changing pin lead.

 

 

So there is no law of nature that says the steering axis has to intersect the wheel axis.  We can lead or trail the wheel axis with almost any kind of suspension.  The question is: what are the effects of doing that, and when might we want to use pin lead or trail, or avoid it?

 

The main reason for using pin lead is to get a lot of caster without a lot of trail at the ground plane.  Caster is the side-view inclination of the steering axis.  Caster makes the front wheels lean into the turns when we steer.  Depending on how much scrub radius (or front-view steering offset) we have, it also de-wedges the car when we steer: it jacks the inside front corner up and the outside front corner down, adding load to the inside front and outside rear and reducing load on the other two wheels.  That makes the front wheels more equally loaded when cornering, and the rears less so, which reduces understeer.

 

This effect is particularly useful on street circuits and slow, tight road courses.  Itís generally good for autocross.  It is less desirable on a high-speed oval, where we can use asymmetrical static camber and are more concerned with making the car steady and stable and less concerned with making it turn in quickly.

 

Unless otherwise indicated, trail is the side-view distance from the point where the steering axis meets the ground plane, back to the contact patch center.  This is approximately the moment arm on which lateral forces at the contact patch act about the steering axis.  That determines how forcefully the steering tries to center itself when cornering, and also how much force the driver has to exert to keep the car running straight on a laterally sloped surface.  If trail is insufficient, the steering will feel numb.  If trail is excessive, the steering will wear the driver out, especially with unassisted steering.  Driver preferences with regard to these effects can vary quite dramatically.

 

For a given tire diameter and pin lead, caster and trail are inextricably related.  More caster implies more trail.  If we want the benefits of added caster, without making unassisted steering intolerably heavy, the only way to get what weíre after is to use pin lead.

 

Are there downsides to pin lead?  There can be.  Not all forces acting on the upright can be considered to act at the ground plane.  Some can be considered to act at the pin.  Any time there is thrust but no torque on the upright, we have a force that can be considered to act at the pin.  A rearward force acting this way will create a de-centering force in the steering if there is pin lead.  This can lead to instability, and in some cases oscillation, in the steering.

 

One common source of that kind of force is the drag the tire makes just by rolling down the road, as opposed to the induced drag it makes when running at a slip angle and generating lateral force with respect to itself, which is partly rearward with respect to the carís direction of travel.  The drag that comes purely from the tireís own internal resistance to rolling straight ahead does not produce a torque at the front spindle.  It only produces a rearward thrust at the pin.

 

 

 

 

Another source of thrust at the pin without torque on the upright is braking or drive force exerted through a jointed shaft: propulsion force for a driven front wheel, or retardation force for a front wheel with an inboard brake.  Propulsion force with pin lead can be expected to produce an increasing self-centering force with power application.  Retardation force from an inboard brake can be expected to produce major steering instability if combined with pin lead. 

 

Therefore, pin lead with driven front wheels and outboard brakes is not necessarily awful, but pin lead with inboard front brakes is probably a combination to be avoided.

 

Pin trail makes sense for cars that need to run straight, particularly if they have very small front wheels.  A dragster with small front wheels would be an example.

 

 

FRIC SUSPENSION BAN

 

Your recent article on hydraulic interconnection of suspension systems was certainly timely.  I see that the FIA is now going to impose a mid-season ban on interconnection of front and rear suspensions in Formula 1.  What do you think of that?

 

From what Iíve been able to read, this is supposed to be a cost reduction measure.  Detractors say itís the result of lobbying by smaller teams that are hoping to suppress innovations mainly used by the bigger teams.

 

My take is that in a series with mandatory turbochargers and complex energy recovery systems, the suggestion that a mid-season ban on passive suspensions involving front/rear interconnection is needed for cost reduction rings pretty hollow.

 

With or without front/rear interconnection, suspension systems can be made complex and expensive.  Is front/rear interconnection of any kind intrinsically expensive?  I donít think so.  How expensive is a Citroen 2CV?  That has interconnected front and rear suspension.  How expensive is a Hydrolastic Mini or MG 1100?

 

For future seasons, I would suggest considering limitations on number and type of suspension components, rather than a ban on front/rear interconnection of any kind, if it is decided that such measures are really needed for cost containment Ė which, as Iíve said, is in itself a bit of a reach when applied to passive suspension of any kind at the F1 level.

 

If the objective is to equalize competition, and perhaps justify racing as a means to ďimprove the breedĒ, how about requiring the teams to tell the public what theyíre doing with interconnected suspension?  That would be refreshing, and certainly more informative than the confused blather Iíve been reading on the subject from my fellow journalists. 

 

 

 

I understand that the reason the FIA is giving for a mid-season ruling is that because FRIC systems can influence the carís pitch attitude and therefore its aerodynamics, they constitute a violation of the ban on movable aerodynamic devices.  If thatís true, thatís truly ridiculous.  Any suspension system influences the carís aerodynamics.  If a passive suspension that offers better control of the carís aerodynamics is a movable aerodynamic device, then all the third springs people have been using all these years have been illegal.  Indeed, suspension systems in general have been illegal since 1968 under this logic.