suspension deficiencies.

[JohnD]

PS camber guage? DiY!

 

Thread hangs from centre of flange, read from scale on lower edge (see pic)

Scale must be calculated from Tan Alpha and the length of the flange.

Or measured in mms and then Alpha calculated.

John

[GT6Steve]

OK, I'm missing the difference between a parallelogram and a rhombus but we're all on the same page.  I think.

 

And as John says, illustrations are welcomed.

[DaveNotSoSideways]

God bless the digital protractor? Did my camber in a few moments to 0.1 deg by slapping it on the hub face? Also used to equalise castor....36.99UKP Also used to check camber gain through bump etc...

[GT6Steve]

As I was just laying beneath the car tightening things on the adjusted side a memory floated by...

 

Two decades ago (almost) I was hounding a multi championship wizard about how to setup my suspension.  {That from a position of greater ignorance than now if conceivable}

 

Anyway. Mr Rick Cline told me to learn to drive what I've got.  Sound advice.  I did.  I've learnt to drive the livin' shit out of what I've got.  Along the way Rick dropped clues for me to pursue that have improved things.

 

Still, his original advice may be the reason my competitors marvel at my car control.  Should I be doing all this fighting?  I can't wait to see what the effects of GTs castor observations have on the car in two weeks.

[GT]

Remember it'a not STATIC you're after, it's dynamic.

It doesn't matter what the gauges say with the car sitting on axle stands etc, it's what happens under roll and under load.

The majority of people forget this, as any increase in castor vastly changes camber angles under roll.

 

(Try jacking up one front corner of the car then watching your rear castor/camber gauge, the results are an eye opener!)

 

You have to think in terms of real angle of attack of the wheel to the road surface.....THINK "supermarket shopping trolley", then think of the effect of those wheels if you raise the back of the trolley onto 2 wheels  and then roll it deliberately through say 5 degrees.

 

What you get, is a compound angle so if you have ZERO castor on the back, you're going to get an inversion of that angle under full load with the power on, so this explains how the car goes from what appears stability to sudden power oversteer.

In reality there's nothing wrong, & your initial setup says all was fine.

 

Unfortunately as the car rolls, it changes the crazy "supermarket trolley" casters to an angle where they are no longer lined ahead and at a castor angle to the road......THIS is where things go awoll  ;D

 

Also remember,....

... the offset wishbones on the back inhibit dive, but it's absent on the front, so there's a fundamental inbalance between the heavy but diving front as you brake then turn in, then come on the power, and a rear which doesn't squat but alters camber and castor.

 

It was always this I found disturbing,- that "critical moment" whereas the initial turn-in on those Triumph cars is excellent, but as it goes in deeper and deeper, strange things go on.

You'll find a slight increase in rear track does marvels here (eg. 8").

 

Swung axle cars don't behave the same way at all btw, > they have totally different toe-castor-camber characteristics > the radius arm on those has a different length + different arc under load and roll + the rear track is constant and a fixed length, so in my view have no castor but alter toe.

Any errors in rear toe-in on a swing spring spitfire can rapidly degrade into violent slewing on power on-power off.

 

They also benefit from wider rear track than front tho...hence why they introduced it on post 1972 cars.

 

(not sure I made myself clear here, I hate trying to explain suspension, I'm no good at it!)

[charlieb]

Swung axle cars .........the rear track is constant and a fixed length....

It may not vary as much as a standard rotoflex car (I wouldn't know) but it certainly isn't constant, that's why when you've jacked it up at the rear it doesn't return to normal untill you've rolled forward or back a few feet.

[GT]

Do you refer to camber instead??

 

a swung axle car like a live axle car can't have changes in rear track because errr...hhmmm the axle shaft is always the same length.....(I hope so!) because if it had got larger the wheel would have parted company with the diff!  ;D

[charlieb]

The axle shafts are the same length but they move in an arc so the lower the car the wider the track (Track being the distance between the contact patches of the tyres)

[GT]

Track would be the distance between the 3 wheel bearings/hub.

 

The inner one taking almost all cornering loads is the diff stub axle (a 6205 ball race).

The 2nd one is a torrington and this just supports a small fraction of the weight of hub carrier and car geometry/aligning forces.

The outer one (the largest) gets the entire weight of the rear of the car/2.

Those distances never change, tho I do accept more negative camber instead of positive would alter the contact patch

 

On a Mk11 suspension they DO change, that's why they fitted a rotoflex/modern CV joint.

[GT6 Mike]

They also benefit from wider rear track than front

 

Hows that work then with regards to your previous comment..................... ;D

 

 

[GT]

If you widen the rear wheels by 1", you effectively also widen the track by 1" in total.

That's pretty obvious.

 

Post 72 Spitfires also had longer half shafts, so combining the 2 makes a fairly important difference, considering you can't do that to the front.

[GT6 Mike]

I understand exactly what your saying but perhaps you need to think about your descriptions a little better.

 

Rear track is measured between the rear wheels and is not constant on a swing spring set up.

 

Driveshaft length is fixed, dont understand why you need to mention the bearing's, they have ne relevance to the track !

[GT]

If driveshaft length is fixed, then is stands by definition the rear track cannot vary, simply because the distance "x" from the diff to the wheel centre is ALWAYS the same.

Rear contact patch,camber and rear track are not the same things, and a true zero track system is a live or dead axle as used on many Audi.

 

If the wheel centre is changed, as this is a fixed measurement from say 3 in 4 out to 31/2 in 41/2" out (8") then it stand to reason the track has been altered, as this has altered the "x" measurement.

 

I understand what you are saying in terms of suspension arcs and so forth but in terms of what appears to induce stability, having wider rear track than front substantially improves it, very much like Citroen used substantially wider front track on their DS>CX>>Xantia FWD active suspension cars than on the rear to exactly the same effect.

 

The Xantia TDI-E is an example of a quite hi tech solution to many problems which failed commercially due to poor reliability. It's interesting because Citroen have for years thought outside the classical "envelope" and devised ultra ingenious solutions themselves.

The Triumph GT6 esp with EFI & with it's equally heavy front is quite similar in many ways, and it's probably why the initial turn-in is so good, and why widening the rear further makes it even better.

[charlieb]

If driveshaft length is fixed, then is stands by definition the rear track cannot vary, simply because the distance "x" from the diff to the wheel centre is ALWAYS the same.

GT, this is nonsense, "track" as I said is the distance between the contact patches of the tyres & of course it varies. You can't start using a commonly understood word to meen something different!

 

and a true zero track system is a live or dead axle as used on many Audi.

Yes on these axles track does not vary.

[DaveNotSoSideways]

You get stacks of change in contact patch widths between zero camber and fully loaded or totally unsprung on a swing axle.

 

If you take into account that the contact patch on a swing axle can vary between the outside and inside edge of the tyre, scrub or effective tyre width track could vary by as much as roughly 3" or so...per side.

 

One also has to factor in that the shaft effectively shortens the track between contact patches as you go either side of a dead level shaft...however going to negative increases contact patch distance and postive camber reduces it, yeah...We run flat tyres don't we?

 

Obviously the effect is lessened as usually one wheel goes up to negative camber and one drops to zero camber or postive, if they both bumped and drooped to exactly the same level then you get no track change (contact patch differential?)

 

However what you get is loads of cross axle change as the effective centre of the vehicle changes compared to the effective loaction of the track width between contact patches...? The actual axle length from the diff to the hub is totally irrelevant as the wheel is stuck on the end at 90degrees so incurs VAST changes in relation to the road. Hense large changes in distance between contact patches.

 

Tempted to get out a protractor....

 

So the effective change in track width between the two tyre contact patches is not relative to one wheel but a cumulative effect either added to or negated by both axles.

[charlieb]

Good point, the contact patch is not in the centre of the tyre (except when the camber is 0°) Strictly track is the distance between the centre of the bottom of the tyres so while the track gets wider as you lower the car from 0° the contact patch distance first gets narrower as the tyres roll towards their inside edges then gets wider. In cornering it must vary all over the place!

 

Still none of this helps Steve go any faster  ;D

[GT6Steve]

Still none of this helps Steve go any faster  ;D

 

 

Well, I must applaud "helping Steve go faster" as a laudable goal but this thread has developed a lot of other good information.

 

We'll be testing the new setup in the first weekend of April.  I'm a bit suspicious of the whole thing as the right side looked way out whilst the left side looked fine?  Don't yet know how that got dialed in.

 

We'll test and adjust at Willow Springs.  A fast desert track that suits my car.  Really a perfect test track.  Since I couldn't afford new tyres for the next race I'll have a good comparison.  Willow is much rougher than cal Speedway so if the shockers are the issue it'll show early...

 

Time will tell ;D

 

[spitNL]

God bless the digital protractor? Did my camber in a few moments to 0.1 deg by slapping it on the hub face? Also used to equalise castor....36.99UKP Also used to check camber gain through bump etc...

 

Dave, I know how to measure camber with my protractor, but how do you manage to measure castor with it?

I have been thinkin about this but I can't figue it out. :B

 

Frederick

[DaveNotSoSideways]

You can equalise castor with it, but you cannot measure the castor! (well I hear you can calculate it from some readings but I never got a realistic number from the calcs)

 

You need to attach the gauge to the hub, horizontally in my case, attached to the wheel studs wth the wheel nuts, so bolted on.

 

I then set it up so the gauge reads zero with the wheels dead ahead/straight and at equal ride height on both sides.

 

I then rotate the hub 20 degrees each way and make a note of the reading at +20 and -20deg.

 

When both sides to EXACTLY the same thing you have dead even castor!

 

I wasn't that worried by what the castor was but it worked out to be around 3.8 I seem to remember. I had one side with less castor than the other, so the idea was to simply equalise both sides. I know the protractor allowed me to make precise adjustments to the suspension to 0.1deg. Which by the feel of the car was well worth the 4 evenings it took me to work on camber, castor, bumpsteer and general things.

 

 

[logicaluk]

dont suppose you a wright up on how you did all the rest (camber & bumpsteer ect) suspention set up has always been a mistery to me.

 

Dan

[spitNL]

Thanks for explaining Dave.

Now I can sleep again. ;D

 

I thought you were going to measure the castor by somehow attachting the protractor to the vertical link.

 

 

Frederick

[GT6Steve]

An interesting contrast emerged at this weekends race.

 

Firstly, the rear castor adjustment made a major improvement in the cars handling.  It was stable and predictable.  I didn't have to do a lot of corrections during corners.  

 

Secondly, the track surface contrast between Cal Speedway and Willow Springs identifies a weakness.  Cal Speedway is generally smooth as glass whilst Willow is rougher than a cob.  The car bounces and jumps around at Willow but is still stable with the bumpsteers corrected.  I still want to run them all again and will probably buy new shockers for the rear but for now, All is good... ;D