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Old 04-01-2012, 12:01 PM   #1 (permalink)
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Adjusting track to reduce understeer...?

I have been having a conversation with other driving instructors on a UK forum and one made the following comment which I found interesting....



Quote:
As a trained mechanic i am also aware of how the geometry of a car is set up to induce understeer when approaching the limits of grip...this causes tyre scrub which is very wastefull with learners doing mano's all the time....so a session with the calculator and tape measure revealed the fiesta required 5mm wheel spacers on the rear wheels to reduce this...makes a huge difference to how far it will keep rolling on full lock.


Could this really have any benefit in reducing tyre wear and FC?? I find it hard to see, personally. If my car runs freely with the steering in the straight-ahead position, surely the amount of time I will be free-wheeling with the steering turned by any amount will be quite small???

In the case of a learner doing a reverse manoeuver - we are usually doing only 2mph!

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Old 04-01-2012, 12:18 PM   #2 (permalink)
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Maybe if you own a Fiesta.
Every car is different. Even two identical cars would handle a bit differently depending on the tires, wheels and wheel offsets.
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Old 04-02-2012, 07:53 AM   #3 (permalink)
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I questioned the author of the original message I was quoting, and here is his reply: Comments??

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Hello kingsway

....so you will be well aware of something called ackermann steering....for others reading here is a link that explains it

Ackermann steering geometry - Wikipedia, the free encyclopedia

....although i should point out that the first diagram at the top is factually incorrect, as no car in the world goes round corners with both back wheels inline with the line from the centre of the corner radius....if a car did do this when going round a roundabout the rear wheels would be close to the roundabout and the front one's much further away....which we all know doesn't happen in real life....they balance out....both being a very similar distance from the roundabout.
So wilki helps a bit...but keeps it simple....for a very simple reason.
The calculations to work out the perfect Ackermann system would be a fruitless waste of time....as steering angle applied by the driver could be any angle....the speed the car is travelling at would apply a different amount of centrifugal force to the rubber bushes and thus the wheel location changes slightly....the amount the car rolls on the suspension alters the steering geometry by changing the angle the steering arms and track rod ends connect to the hub carriers....and so on for ever.....so forget that and keep it simple.

Assume the car manufacturer knew what they were doing when they designed the car....and then "introduced" some error (understeer) as understeer is deemed to be safer than oversteer when novices come across it....probably because it can be "felt" through the steering wheel. (i personally prefer steering control at all times....as most things can be steered through with the aid of good throttle control)
So thinking back to when i said both wheels balance out a similar distance from the roundabout....they balance at a point somewhere in between the wheelbase.
So i set about finding where the fiesta had it's balance point.

Handbook for wheelbase and front/rear track dimensions....and then turned the steering to full lock to see how much the front wheels move forward or backward when turned....measured the wheelbase on each side of the car with the front wheels turned....and averaged the readings for the inside wheels and again for the outside wheels....and then drew to scale as best i could onto paper....mark a line from the left front wheel to the right rear....and right front to left rear....and then measure where the 2 lines cross in relation to the wheelbase centre....if in front the front wheels want packing out....if behind the rears want packing...to make the 2 lines cross in the centre of the wheelbase.

The calculations i made to arrive at the nearest spacer dimension were obtained using the co-ordinate conversion function on a scientific calculator...(years of being a sheet metal worker helped here)....and that's about it.

If anybody decides to look at this on their own car....you will probably end up finding that the manufacturer REALLY DID know what was right in the first place....and then introduced the error deliberately....because the front and rear tracks of cars are nearly always different....and when made "right" with spacers the tracks will be almost equal... and all is sorted!
Further proof Ford knew what was what comes from the hub locator....specifically it's depth....the front hubs are just enough to locate the wheel securely....the rears are made much deeper....deep enough to take the spacers and still secure the wheel securely....without having to go to the vast expense of hub-centric spacers.
However i have said "probably" i would not EVER modify a car without doing the calculations for myself....and then giving it a proper road test checking for any wayward characteristics(for definition of proper) please ask Helen

The current shape fiesta when equipped like this is an absolute gem through corners....and i have owned a number of 200+ horsepower performance cars in my time....it "sets" itself into a corner with such ease and accuracy it must be one of the best cars handling cars in it's price bracket.
I then said 'Okay, so it improves handling - but does it have any real effect on rolling resistance and FC'....

Quote:
Yes definitely....reduced error in the tracking improves tyre wear and fuel consumption...although i do not have figures to prove that....i can easily tell the increased willingness to roll on full lock....which simply must mean reduced effort required to move the car....have you ever heard cars in car parks going very slowly....but squeeling the tyres?...this is what causes it.
I'd have thought we spend so little time with wheels turned, that any differerence would be un-noticeable?
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Old 04-02-2012, 08:27 AM   #4 (permalink)
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With MacPherson struts, doesn't the wheel camber change dramatically on full steering lock? Does that have an effect?
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Old 04-02-2012, 09:06 AM   #5 (permalink)
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the guy kind of gets it, but makes it waaaaaaaaaaay too complicated at the end.

Ackerman means the inside tire turns more then the outside tire because the outside tire has further to go. To get "correct" Ackerman you do indeed know the wheelbase of the car.

However, lets look at the simple case. When you are going straight changing the distance between the rear tires (rear track) does not change how hard the car rolls down the road. I suppose there are wind resistance issues but that isn't what we are talking about.

Now another extreme. Imagine a kids wagon, and we turn the front 90 degrees and pull sideways. Again, it doesn't matter how far apart the rear tires are, the wagon turns quite easily.

If the front and rear tracks are different, the car will indeed wander down a rutted road. But it will wander down the road quite easily.

Changing rear track does not change rolling resistance.

For the mac strut guy, Caster is the angle you are talking about. Caster does a few things including changing the camber as you turn the wheel. but it doesn't really change the "direction" the wheel is turning when you turn the steering wheel.

FWIW, all cars have caster - between that and mechanical trail it is what makes the car go straight down the road instead of wandering randomly all over the place.
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Old 04-02-2012, 11:38 AM   #6 (permalink)
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Going back to the original question of if a wider rear track can reduce understeer. Recalling my autocross days, I don't think so. 5mm is also so little, it is really a fine tuning tool. Keep in mind, this is only when approaching very near the tractional limits at moderate to high speeds though. This is not going 10mph in a parking garage.

The same can also be tuned by increasing pressure in the front tires, removing the front anti-sway bar among a host of other things, see below. Cut and pasted from EXECstudio.com - Tech: How to modify Oversteer and Understeer

Understeer, Oversteer and how to fix it:

Understeer is common on 90% of the road cars because it tends to be a safer setup for daily driving conditions. Oversteer happens when the car gets loose. In this situation the rear tires slide out first

Oversteer can be dangerous especially at high speeds as it requires good driving skills to keep control of the car.

Understeer Corrections

Raise front tire pressure.

Lower rear tire pressure.

Soften front shocks. Stiffen Bump.

Stiffen rear shocks.

Lower front end.

Raise rear end.

Widen front track.

Install shorter front tires.

Install taller rear tires.

Install wider front tires.

Install narrower rear tires.

Soften front sway bar.

Stiffen rear sway bar.

More front toe out.

Reduce rear toe in slightly.

Increase front negative camber.

Increase positive caster.

Soften front springs.

Stiffen rear springs.

May need more front suspension travel.

Install wider front wheels.

Use softer front compound if possible.

Use harder rear compound if possible.

Remove weight from front of vehicle.

Add weight to rear of vehicle.


Note: This is a sample of the methods used to correct various handling problems. Books have been written on this subject. Not every correction will always work as expected. Stiffer front sway bars will in many cases will decrease understeer because of decreased body roll and better camber control. The best rule is to change only one thing at a time and keep notes.Shock adjustments have the greatest effect on corner entry and exit.
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Old 04-02-2012, 11:41 AM   #7 (permalink)
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Now if the question really is how to reduce tire scrub while cornering by adjusting the effect of ackerman, that should be a different thread.
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Old 04-02-2012, 01:31 PM   #8 (permalink)
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A few random points:

- There are a number of cars where the rear wheels are closer to the inside of the corner than the front wheels are in steady-state cornering. I've clipped curbs in my parents' 2nd-gen Rx-7 because of that, for instance.

- Adding FRONT track can help reduce understeer, except when the track increase gives you a scrub radius that is too large. (In this context, I mean the distance between the center of the contact patch and where the steering axis intersects the ground.) When you go too far out, the extra scrub radius adds to understeer and makes the car drive like a truck. You can correct that by changing the suspension geometry so that the steering axis point moves outward. Usually a 10mm increase in track (and 5mm increase in scrub radius) won't be enough to feel, though.

- It is possible that other mechanisms than the above result in a feeling of less understeer in a Fiesta when you add 5mm spacers to the rear.

-soD

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