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MG Midget and Sprite Technical - Midget tracking

Hi all, sorry if this matter has been covered before, however after searching, I could not find the definitive answer.

Just fitted new wishbones (Barry Kings), kingpins, bushes & pins etc. to the front suspension I find that the steering alignment is a bit out. (due to the wear in damper splines, bushes and fulcrums etc. now corrected.

Tracking is quoted as 1/8" or 3mm toe-in, is this each wheel or the total of the two? Some say the 3mm is the total difference (front to back)across the two front wheels.

I only say this as some quote the measurement in degrees, i.e. -0.5 degree toe-in, for each wheel.

As a check, I used my old school maths (Sin 0.5 degree = 3mm/330mm) to find that half a degree was in fact the same as 3mm measured on the rim of a 13" wheel.

Others say that it does not matter where you measure the toe-in (tyre, rim, at the bottom or half way up!)

Can anyone clarify?

Thanks and regards, Tony


Tony G

First of all the tracking figure stated by the company is not necessarily the best figure to use so do not get too concerned with accuracy of measurement!

I tend to set mine at parallel as this gives me a far superior turn in. (That means that it is an overstearing car) For more conventional use simply get 3mm wider at the rear of the wheels (halfway up) than the front of the wheels (3mm toe in)
Robert (Bob) Midget Turbo

I'm off to get my tracking done and would like a figure to give the garage??
Be interested to know,
Jon White

1 degree toe in. Garage should be able to manage that. Make sure that they do it with the suspension loaded (i.e. standing on its wheels)
Guy

I use a long ally bar to set tracking to have a small toe-in. Most of the time, the users of the laser equips in tyre-centres don't know what they are doing (IME).

A
Anthony Cutler

So 1 degree toe-in = 3mm = 1/8 toe-in (on a 13 inch wheel)?

A couple of questions?

What does 1/16 degree toe-in = in degrees?

If degrees are calculated by the wheel size how does the difference in rolling radius accounted for in different tyre profiles, eg 70 series, 60 series, 50 series?
Daniel Thirteen-Twelve

What do you mean by 1/16 degree?
D O'Neill

Typically, a front toe figure relates to the 'combined toe' for a steered/axle pair of wheels. This makes sense because the position of the steering wheel, the direction of travel on the road of the car, the directional angle of the front wheels (ie each wheel's 'toe' figure) are all dynamically related - change any one and the directional position of any/all of the others will change in order to keep the car moving in the same direction before the first alteration was made. It is a common misunderstanding that one wheel's toe can be altered (by adjusting the tie-rod), say, to try and compensate for uneven wear on that one tyre. But, as I say, altering one wheel's toe will result in a change in the combined toe (ie the other wheel's effective toe will change) and the position of the steering wheel will have changed when travelling in a straight line. You can't alter one wheel's toe without it altering the other wheel's toe and/or steering wheel position - they all work interdependently of each other!

Assuming that a wheel rim clamp-on optical/laser tracking device is used on the front rims - ie the geometrical reference points are on the metal rims and not on the rubber sidewall of the tyres - then any tyre can be fitted to the rims, and the combined toe will stay the same as before the new tyres were fitted. The rims haven't changed, so the linear (ie mm or inches) toe will not have changed. The angular direction of the wheel rims haven't changed, so neither will the angular toe have changed.

According to my Dunlop optical toeing gauge, for 13" rims, a 1/16 inch linear toe (combined axle toe) equates to a 17 minute angular toe (there are 60 minutes in one degree - so 17 minutes is a gnat's whisker bigger than a a quarter of a degree.)

As always, I am open to be being proved wrong.




Andy Hock

Andy, you are a star!

As I see it:-

Toe-in for a midget is 1/8" total, measured between the rims of the wheel ( at centre of the hub height), one side difference (front to back) would be 1/16".

Your quoted degree measurement of 17 minutes (or a bit over .25 degrees) whould be between the central axis of the car and the angle of the wheel.

Great, this checks out with my school room maths too:-

Sin 0.28 degrees (17 minutes)= 0.005

1.5mm (1/16")/ 330mm (13")= 0.005

So setting your toe-in to 1 degree would be way too much!! four times in fact.

Mine were set to 1/8" each side! - correcting soonest.

Thanks again Andy.
Tony G

Tony, if your toe was 1/8" each side, then presumably, when the toe adjustment was physically made, the 'combined toe' (ie axle pair) was set to 1/4".

Yes, any one wheel's toe can be expressed (in either 'linear' or 'angular' terms) with respect to the centre axis of the car - but that figure is largely only academic because, as I said in my previous post, the front wheels (and steering wheel position and direction the car when travelling) are all interdependent on each other, they are not dependent on the centre axis of the car. Using the centre axis of the car as a reference line (datum) is only really relevant during car design and body/chassis damage/repair. That is why in the 'tracking industry' any toe figure nearly always relates to a combined axle pair of wheels.

Andy Hock

IIRC the manual says something like 'toe-in parallel to 1/8". It's nice to take measurements if you can, but using a long ally pole, I position the pole at wheel mid-centre (also horizontal) and like to see it parallel to the rear body on one side, and with slight divergance on the other.

I used the same method for Honda ATR on front and rears (front toe-out of course; rears slight toe-in) and for the first time since the car was new, the tyres wore evenly and lasted another 30%; at £150 a corner, that was welcome.

I've never had any success with the 'drive-over' pads you can buy, BTW.

A
Anthony Cutler

Anthony, I, too, am sceptical about the 'drive-over' pads that you mention (eg gunsons Trakrite). Yes, I'm sure that it will show the toe of the one wheel that it is measuring (assuming that the steering wheel is facing 'true north', so to speak), but, as I say, it's the relationship between the two front wheels that goes towards the axle 'toe' figure.

In my mind, the only way to seriously use the 'drive-over' pads would be to have two, each one used simultaneously under each wheel. By summing the two readings, the axle toe will be found. Of course, as using the 'drive-over' pads require some movement of the wheels, using two would really require the steering wheel to be locked in the straight ahead position (thereby eliminating movement of the steering gear during the test).

Having said all of that, I really concur with Bob's (Midget Turbo) words of wisdom - trial and error will achieve the best 'feel', steering and tyre wear. (However, it's always nice to know just how awry the 'official' recommended toe figure is, compared to the optimal setting found by trial and error).
Andy Hock

This thread was discussed between 13/11/2011 and 14/11/2011

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