The Mark Ortiz Automotive

CHASSIS NEWSLETTER

March 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.

 

 

NEW LEANING TRIKE

 

In past issues we have considered design issues in three-wheeled vehicles, and considered the possibility of letting a trike lean when cornering, like a two-wheeler.  Tilting Motors has introduced a trike that leans.  It got featured on Jay Lenoís Garage just recently: https://www.youtube.com/watch?v=T3TumQ-ueMU.

 

Itís really just an ordinary motorcycle with two front wheels.  The front suspension has no roll resistance at all.  Its springing only acts in ride.  It has one coilover for each wheel, but these mount to a rocker that swivels freely about the x axis.  There are control arms for each wheel that appear to be equal length and parallel, so all three wheels lean with the frame.  When parked, the machine leans on the stock motorcycle kickstand.  To stop when riding, you have to put a foot down.  In motion, it steers and stabilizes gyroscopically just like a regular motorcycle.

 

So if it acts like a regular motorcycle, whatís the point?  The main advantage claimed is that it provides better safety in the situation where the front wheel hits sand or something else slippery while cornering and the bike tends to go down.  With two front wheels, they both have to hit slippery surface at once for the bike to go down, or at least thatís true if youíre far enough from the limit of adhesion so that one front tire, carrying half the front normal force, on clean pavement, will not slide out.  For most road riding, that will be so.  Therefore, there should be a genuine safety benefit.

 

It recently dawned on me that a leaning trike can be given a gentle self-righting tendency so that it will stay upright at rest by itself, without having any springing devices other than a single ride-only coilover bridging right and left rockers.  If those rockers provide rising-rate geometry, the vehicle will rise slightly when it rolls.  This will provide an induced gravitational self-centering effect in the suspension.  The effect would be similar to the induced gravitational self-centering effect in a steering system that results from front-view steering axis inclination combined with front-view steering offset (ISO) / scrub radius (SAE).

 

 

 

If an absence of ride compliance can be tolerated, and lightness is paramount, the system would do the same thing with a rigid link in place of the coilover.  That would probably be of interest primarily for human-powered trikes intended for pavement use.

 

 

EFFECTS OF WIDENING THE TRACK ON AN EXISTING VEHICLE

 

What effects should be expected from widening the track width on an existing vehicle, say by adding wider wheels with more offset? What changes to wheel alignment and other chassis settings are called for?

 

The effects will vary quite a bit from one vehicle to another.  In many cases the biggest factors will be fender clearance and changes in compliance effects.

 

In many cases a big change in wheel width or offset will be accompanied by changes to ride height.  Sometimes these are to increase ground clearance.  Sometimes they are to lower the car for competition or just for appearance.  Sometimes they are simply necessary to get the wheels and tires on the vehicle.  Often, an increase in track is accompanied by other changes in settings, but the changes are not directly caused or necessitated by the wheel and tire change; rather, both they and the wheel and tire change are related to a change in what the vehicle is to be used for.

 

If the objective is merely to get the look of the big wheels and still have adequate street operation, and we are up-sizing all four by the same amount, and if fender clearance is not an issue, most settings do not need to be changed.

 

If the vehicle has independent suspension with rubber bushings, we may see increased compliance toe-out in braking.  This will show up as directional instability when braking hard.  If this is encountered, it may be necessary to reduce bushing compliance, or maybe add toe-in to crutch the problem.

 

When we move the wheel planes outboard, there are effects on steering geometry.  These generally cannot easily be adjusted out or compensated for, but itís useful to know about them.

 

The front-view steering offset increases.  This increases caster jacking and SAI jacking.  Caster jacking rolls the car to the left when the wheels steer to the right and vice versa.  It de-wedges the car: adds load to the inside front and outside rear tires while reducing load on the other two.  SAI jacking creates an induced gravitational self-centering force in the steering.

 

Increasing the front-view steering offset also increases feedback through the steering from one-wheel bumps and brake pulsation.

 

Things get more complex when we add more wheel offset and tire size at just one end of the car.  Most often, we see this at the rear on rear-wheel-drive cars.  Typically, there is more room to

 

increase track and tire size at the rear, partly because the rear wheels donít have to steer and partly because manufacturers generally leave room for tire chains at the rear.  Also, cars often look good with larger tires at the rear.

 

If we stay with similar design and construction for the front and rear tires, but make the rears bigger, as a rule that will add understeer.  To counter this, we may want to add rear roll resistance and/or some negative camber in front.  Alternatively, we may want to just deflate the rear tires a bit.  This will amount to throwing away lateral grip to balance the car, but within limits it will improve longitudinal grip at the rear.  It will also make the car throttle steer more controllably, as rear breakaway will generally be gentler with lower tire pressure.

 

What if we have a beam axle at the rear and we increase just the rear track, not the tire size, and we donít change anything else?  The car will roll the same amount.  The rear suspension will have the same angular roll resistance.  However, there will be less rear load transfer, since we are reacting the same moment over a wider base.  This should increase understeer.  If we have limited slip diff, the understeer-inducing effect from that will be increased a bit.

 

Now suppose we have the same situation, and independent front suspension, and we increase the front track too, also without changing anything else?  We have already discussed the effects on steering geometry.  Front wheel rate in roll will not change a great deal, but with the increased track, front angular roll stiffness will increase.  Therefore, there should be less roll.  The effect on understeer gradient is harder to predict.  In most cases, understeer be somewhat reduced.