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MG Midget and Sprite Technical - Wider front more stable?

Hello all

After the pre-MOT checks I had to put the 5mm spacers on the front as the LH tyre was brushing the damper cover, not enough to worry about but enough perhaps to cause an raised MOT eyebrow.

The car seems much more stable in a straight line and into corners and resistant to body roll, with more pinpointy steering (that might just be the recent kingpin greasing).

Is this a 'real' effect or am I imagining it because I know the track is wider on the front? whatever it is, it feels better :)

Rob Armstrong

Not for nothing do I run 1" spacers on the front of my B class race car... ;o)
James Bilsland

I doubt if 5mm spacers would do much, but it's possible. Not for the wider stance most likely, but rather for moving the point of contact for the tyre in relation to the rotational axis of the king pin which will affect steering effort and directional stability as you describe. The same effect will be noticed with wheels with incorrect offset.
B Young

On a similar (but completely different) thought, I'm finding that on sprints I've got too much understeer while I very much like the way that the rear end breaks away and wouldn't like to change that. Are much stickier from tyres an over simplistic method of overcoming this or am I using a hammer to crack an egg (or whatever the expression is)?

Gary (who knows nothing about settung up a car) & Gaps
Gary & Gaps

Gary, are you running a large front sway bar? Those tend to increase understeer and I don't think that front tyres would make a lot of difference in that case. I'd suggest making an adjustable rear sway bar, pretty small diameter say 10 to 12 mm with a slider on the ends so that you can tailor the amount of roll stiffness and try that. I think you can reduce the understeer while keeping a bit of the tail happy feel you want when using the power.
Making your own rear bar isn't difficult if you have a spring shop in the area that can re temper the steel when you finish modifying the bar. I used a rear bar from a small Japanese car, about 12mm in diameter and just heated it with my torch and bent it to the new shape and then a local shop retempered it for about $20. My car wound up pretty neutral tending a bit towards understeer, but with oversteer coming on gradually with application of power. I think you'll be happy with the results.
B Young

Gary, Have you tried stiffening the rear dampers. also increasing the static negative camber and/or lowering the front ride height

Regards

James
James Bilsland

B Young,

I added one like you describe to the back of my frogeye, it came off the back of a Vauxhall Chevette. After heating the bend areas to a dull red to bend them the bar could still be bent more than it would ever see in use and return so didn't require any tempering in my case, I just used it as is and it has been fine. About 1/2" diameter and it nicely dealt with the excessive understeer in tight turns. I'd have to measure the arm length to really make any sense of the 1/2" diameter though.
David Billington

Rob,

Don't under estimate what any increase in track width can give in terms of grip on that axle.

On a FWD race car, a rear track width of 20mm less than the front is pretty significant on how the car behaves especially on turn in (corner entry)

So 5mm / side = 10mm total is not insignificant (especially on a car as narrow as a Spridget), it will reduce the lateral load transfer on the front axle and improve grip, sure there will be changes in your feeling at the steering wheel (mainly under braking and on initial turn in) due to changes in scrub radius (wider track caused by spacers = greater scrub radius) but wider track = more grip on any axle however it is created.

Gary / David,

I assume both of your cars are quarter elliptic being genuine Frogeye?

A lot of people overlook the differences bewteen quarter elliptic and semi elliptic in terms of roll centre height.

As far as I am concerned it is entirely logical that (everything else being equal) a quarter elliptic car will have more corner entry understeer than the same car with semi elliptic springs, therefore the prescence of a RARB (whilst pretty well unheard of on semi elliptic cars) would be desirable on a 1/4 elliptic car or a Panhard as a crude way of raising the rear roll centre.

But what I do not know is the rate of a 1/4 elliptic rear leaf spring, I would assume it is higher than a semi elliptic car to start off with a sensible chassis balance

Cheers
S Deakin

cool, it's not just me then!

I'd make it wider, but running out of stud length and space under the wings...

Think I'll keep it like this, though it seems more prone to locking up than before. Maybe it's cos it's faster cos it feels better.
Rob Armstrong

Spencer,

Good points and my frogeye is an original 1/4 elliptic. The front springs were always kept standard but with a 5/8" ARB IIRC and later fitted with wider spaced mounts to make it more effective, FG front so lighter. The rears springs were standard minus one long leaf to lower the rate and the ARB added, also 7/8" spacers to increase the rear track. Worked for me. Also telescopics all round and the front lever arm replaced with an upper link and 2 degrees negative camber, when I get it back on the road I think I'll drop that to around 1 to 1.25 degrees negative to benefit tyre wear.
David Billington

Is the track width thing a balance between front and rear?
So would narrowing the rear axle have the same effect as widening the front?

I am asking as I want to swop to a wirewheel rearaxle with modded steel wheel halfshafts to reduce rubbing in my swa body.
I have a panhard but don't like how it reacts under rebound.
And the slight increase in frontgrip would be a nicebonus :)
Onno Könemann

As far as I know narrowing the rear axle would only noticably affect the tendency to steer the car when one tire loses traction. That's one reason drag racers in the high horse power cars like the fuel cars run such narrow rear axles. If you don't like the panhard bar have you considered a watts linkage or similar?
B Young

Many thanks for the advice so far (and sorry to hyjack this thread). I also have telescopic front (frontline) front suspension which I believe has built in negative camber, 2 degrees comes to mind. In what whay does changing the settings of the front shock absorbers help/hinder the pointyness of the car. Currently I have them set at halfway.
Gary & Gaps

Doesn't count as hijack cos it's interesting :p

I found with my dampers if I had the back ones too hard relative to the front (or the front ones too soft) it had really bad rear wheel steering when going over bumps - tweaking up the stiffness on the front fixes it. If I set the front too hard it understeers and is uncomfy. mine are 4 clicks off the softest (12 point adjustable mini front shocks)
Rob Armstrong

Gary,

Dampers are such a minefield as far as peoples opinions go, some claim its a black art - usually brought about by a lack of data;

But I can give you my 'opinion' from my experience of 'tuning' mainly race / rally dampers

Really you need the force / velocity curves for your telescopic damper so you can see what part/s of the damping curve is altered when you twist the knob / rotate the adjuster.

Without knowing what your adjustment does:-

If, when you make an adjustment you JUST influence the low speed aspect of the bump curve (sometimes refered to as the 'nose') - digressive damping characteristics then this area seems to have most effect on what people term as 'steering feel / response' so a sort pointyness volume control, 'stiffer' up to a sensible point can help create this feeling.

If however the damper has 'linear' damping characteristics and adjusting them JUST increases the linear damping rate then this will probably not change the steering feel but just make the car feel 'harsher' for no other apparent gain.

If, (and is usually the case) the adjuster effects both bump and rebound, and also low and high speed then it is anybodys guess what the outcome will be.

So from the above you can see that without data you have to be careful about what is 'better' and why people get differing results / opinions.

Now whether a damper should be used to improve steering feel is whole different argument, but as a velocity sensitive device it is a quick, simple way of doing it.

I have offered in the past to get telescopic dampers dyno tested just to see what is being offered with these commercial kits, I have had my own telescopics made based purely on what I think they should be without seeing what the AVO etc... offereings are like, so the information would be of mutual interest.

Cheers

Spencer
S Deakin

This thread was discussed between 15/08/2011 and 16/08/2011

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