GT6. Correct toe-in. Can it be tweaked?

[chrishawley]

After innumerable suspension and steering problems, to cut a long story short, I settled on a laser wheel alignment. All well and good and the car came back with everything 'on spec'.

Except that on the road it's not quite right: On the straight it just feels a bit vague and doesn't self center that well. Would it make sense to experiment with a tadge more toe-in at the front? Maybe an extra 16th inch? What about the rear? Best left alone?

Overall the handling's pretty reasonable so it's more about tweaking rather than fixing.

As ever, advice would be much appreciated.

C

[Clive]

Has the castor angle and camber been set too?

Setting toe angles is a doddle, I use a tape measure, but castor requires a tad more thought. till do-able at home, guides available on youtube etc.

My "camber level" is a bit of wood, with a length of string and a small weight on the end, calibrated in degrees. I ought to try the ebay special I bought (about a tenner)

[Pete Lewis]

we have a post on basic DIY geometry settings  you dont need lasers and most modern places have no idea what the triumph specification is 

most specs require a static load adding  at 150 Lbs on each seat  some later do give unladen kerb weight settings but following the basics is easy  just follow what spec is noted by model year in the WSM  books 

realy it doesnt need a tweak it needs to be right

have a read of out DIY post 

 

[Clive]

10 hours ago, chrishawley said:

After innumerable suspension and steering problems, to cut a long story short, I settled on a laser wheel alignment. All well and good and the car came back with everything 'on spec'.

Except that on the road it's not quite right: On the straight it just feels a bit vague and doesn't self center that well. Would it make sense to experiment with a tadge more toe-in at the front? Maybe an extra 16th inch? What about the rear? Best left alone?

Overall the handling's pretty reasonable so it's more about tweaking rather than fixing.

As ever, advice would be much appreciated.

C

Another issue, did they do  4 wheel alignment? if the rears are out, the car can be horrendous.

[Pete Lewis]

again a bit of straight 6ft timber against the rear tyre and sight it along the sill line each side should look the same 

a reasonably parallel gap showing the rears are fairly square to the cars centre line not |/  or \/  or /\  more | |

its not rocket science 

and  a  trakrite will save you £££s   and takes about 30seconds to do a check  costs less than  £60

[JohnD]

The trick is, to find a way of putting a line, straight and parallel to the midline, down each side of the car.    I've made up two beams (PVC tubing) that clip onto prominent parts at each end, and marked to position the string.     

But instead, I use a laser leveller, that sends dead straight light rays across the marks at each end.   Simples!

 

[JohnD]

 Chris Hawley,

Just reread your OP, and you say, "On the straight it just feels a bit vague and doesn't self center that well"   This can be a symptom of incorrect REAR toe.

If your GT6 Mk3 is after KC20,000 then it has a swing-spring, and the toe is adjusted by radius arms.     The mount bracket for these is on a flange at the front of the wheel well, where corrosion can often weaken the metal.     

Check the toe, which should be 0.8-2.4mm of toe-in, and that the mounys are secure.

JOhn

[ahebron]

Do you need 'fat bastards' to do this on a GT6 or is that only for Herald/Vitesse

[Pete Lewis]

all depends on what spec you read and what model year most apart from late GT6 all give a spec with 150 LBs on each seat  

so in general rent a crowd or fat  both work 

not using the correct set up and static load ends up with a waste of time and ...money if youve paid for the treatment 

once you set it loaded you can then measure the unladen result and note it in the little black book  

Pete 

[chrishawley]

Coming back on this one.

Firstly, thanks for all the advice above, duly observed

I've been all over it again. Everything measures up 'by the book' But the basic problem doesn't get any better. Which is to say: satisfactory steering and handling up to 55- 60mph but vague and poor centering (not 'planted') at higher speeds than that.

So maybe I'm barking up the wrong tree? Should I be looking at steering rack bushes, lower steering coupling and suchlike. Would that fit in with the symptoms? The only part not replaced in the rebuild was the lower column coupling - so perhaps that's a prime target.

Opinions?

[JohnD]

Steering that gets poor at speed says aerodynamic lift to me.   Spitfires with their lighter engine were fitted with a 'chin spoiler'.   Fit on of those to your GT6, or else a full airdam, spoiler.   I can send you patterns and photos of my old Vitesse with one if you like.

[Pete Lewis]

 a worn steering coupling will show as free movement in the hand wheel 

steering racks are damped by adding/removing shims from the preload cap on the rack body

what tyre  pressures do you use ????

Pete

[Ian Foster]

Chris

Self centering of the steering comes from castor, which is adjusted on the small chassis cars by adding and subtracting shims from the lower wishbone attachment points. Adding at the rear and subtracting at the front moves the bottom of the vertical link forward increasing castor. Having the rear end out of alignment can also create stability issues which might be detectable by varying (and sometimes scary) behavior coming on/off the power.

NB most Red Bull Soapbox steering issues come from lack of castor (and dubious engineering generally).

Ian

 

[Nick Jones]

15 hours ago, chrishawley said:

satisfactory steering and handling up to 55- 60mph but vague and poor centering (not 'planted') at higher speeds than that.

I echo Ian’s comments on castor. I have yet to find an alignment place that knows what it is, let alone how to measure it. This includes one that claimed their machine had measured it without the steering being turned (I was watching them closely), which is impossible.

It is diy-able, though a bit tricky, with some simple tools, which can be homemade, and a bit of simple calculation. This might not get you an accurate absolute measurement but but you can get it the same both sides and measure more and less. At the moment it sounds like you want more. You’ll know if you’ve gone too far as it makes the steering heavy (heavier).

Comment regarding rear tracking are also relevant. The swing axle set up can be particularly fidgety if even slightly toe-out, especially if it is low with negative camber.

Nick

[Pete Lewis]

worth a look at the offset  wishbone pivot brackets  it not the first time they get mixed and fitted  upside down 

not looked it up but from memory !!  front is up and rear is down 

whatever they are not both the same way of offset 

Pete

[Steve P]

Sounds very much like my Vitesse after i converted from swing axle to roto and CV shafts,speeds above 70 saw the car trying to drive itself from the rear,it was toe in on one side and out on the other.

Steve

[Pete Lewis]

thats  easy to see just stick 6ft of straight timber against the rear  tyre wall and sight it down the sill line 

just compare n/s to o/s  for where it is all pointing   so you get   l - l   not /--/  or \--/ 

together with getting the toe correct 

Pete

[Clive]

There is a simple camber/castor gauge about

https://www.merlinmotorsport.co.uk/p/ada-castor-camber-gauge-adaccg

Me being a tightwad, used that picture, enlarged to A4 and printed it off. Stuck it to a bit of rigid plastic, bit of strong thread with a weight. Works a treat, really simple to use. 

[Pete Lewis]

for castor the old school was   hold foot brake firmly on ,   turn wheels 20deg in   set caster gauge     turn 20deg out and measure the castor 

leave the brakes off gets some silly results 20deg fixed by simple chalk lines on  the floor if no turntables are available 

I remember years ago some of those who knew better dropped a van in the pit when it just rolled off the turntables  on a 20deg turn out 

dont you just love the red face brigade   Ha 

Pete

 

[Gully]

Just a single shim can make a noticeable difference to castor and wheel centring. When I adjusted the camber on mine, I added 3 shims to each of the brackets and left out the small shims that had previously been on each of the rear mounting points. After correcting the toe, the steering felt woolly and didn't self-centre (although it did when reversing). I added back the additional shims to the rear mounts, corrected the toe (to the same value as before) and the steering centred once more in normal driving.

Gully 

[chrishawley]

I’ve still not cracked it (sob) so I remain open to all suggestions. I spent time today looking for factors which might contribute the issues. Found a couple of not-really tight bolts on front susp and a rather frail steering coupling. Tightened and replaced respectively. But no change to the problem so are ‘incidentals’.

 

Tyre pressure? I wish I knew, as all my gauges give different opinions!!! But my estimate was 24f/26r psi which would be ‘book’ for a roto GT6. I tried whacking the pressure up by 6psi (ref previous thread on this subject) – major deterioration in handling, skittish and all over the place.

 

Although the susp is standard roto all over I did spot that the antiroll bar seems to be the ‘fat’ one from the non roto GT6. Any opinions about whether the ‘wrong’ ARB might be a problem? But since my issue is lack of straight line stability at speed I don’t immediately see how the ARB would be implicated in this much.

 

Checked the front fulcrum brackets on front suspension and they are in the right position and right way up.

 

I’m pretty sure the rear end is ok. I had the setting the laser guys did and remeasured that myself. I then ‘unadjusted’ it adding or subtracting toe and either way the handling deteriorated so that would seem confirmatory.

 

So caster. Guess it’s time to buy a proper gauge. In the meantime what if I simply experiment with an extra 1/16th shim on each of the rearmost brackets on the front lower arm? Are there potential disasterous consequences of such a straightforward bit of experimentation? At least to gain some insight into how making changes affects road behaviour.

 

 

 

 

[PeteH]

9 hours ago, chrishawley said:

I did spot that the antiroll bar seems to be the ‘fat’ one from the non roto GT6

Am I right in thinking the "fat" anti roll bars are only recommended for Swing Spring equipped cars?. Or does that only apply to Non Roto cars?.

And could Tyre Age be having some effect?. I ask because some tyre`s have issues towards the end of their recommended life despite having sufficient tread? Especially if not having been used for a long period?. eg; during a rebuild?.

Pete

[Clive]

10 hours ago, chrishawley said:

Tyre pressure? I wish I knew, as all my gauges give different opinions!!! But my estimate was 24f/26r psi which would be ‘book’ for a roto GT6. I tried whacking the pressure up by 6psi (ref previous thread on this subject) – major deterioration in handling, skittish and all over the place.

Although the susp is standard roto all over I did spot that the antiroll bar seems to be the ‘fat’ one from the non roto GT6. Any opinions about whether the ‘wrong’ ARB might be a problem? But since my issue is lack of straight line stability at speed I don’t immediately see how the ARB would be implicated in this much.

So caster. Guess it’s time to buy a proper gauge. In the meantime what if I simply experiment with an extra 1/16th shim on each of the rearmost brackets on the front lower arm? Are there potential disasterous consequences of such a straightforward bit of experimentation? At least to gain some insight into how making changes affects road behaviour.

I use 25psi (on my SP gauge!) all round on my Roto spitfire. As you have found, higher pressures make the care less stable, all from experimentation.

You are correct, fat ARB is for swing spring cars only. Fitting one to an non swing spring will give a little more understeer, already an issue with a GT6! But as you say, not related to straight line stability.

You mention the steering joint being replaced (I hope ypu haven't fitted a new std type, if so expect to replace it again in a few months) but worth checking the rack, mounts and also the inner ball joints. Also, is te car lowered on the front? If so you may be getting excessive bump-steer, though it would not happen on a smooth/flat road

So yes, castor. Try experimenting. If you have shims front and back on the lower wishbones, swap one from the front to the rear (keeps toe correctish) 

This is a decent explanation  https://suspensionsecrets.co.uk/caster/

And yes, you can buy a caster gauge, but the simple gauge I posted about above is very easy to make. Just ensure it touches the wheel rim top and bottom, not the tyre wall. Or fork put a tenner and get a digital box gauge, then cut a length of steel bar to fit your wheel. (I bought one whne I have to investigate my prop angles, really handy.) https://www.ebay.co.uk/itm/303995486885?hash=item46c78b02a5:g:~0cAAOSwPZpgnRT6

[Ian Foster]

11 hours ago, chrishawley said:

24f/26r psi which would be ‘book’ for a roto GT6

Chris

I set to 28f/25r on my Mk2, with 175/70/13 tyres. This is on the basis that the car is front heavy and is in similar proportion to what Nick Jones uses on his Vitesse Mk 1.5 conv.

I used to run the same pressure all round and in the low 20s, but I think the current arrangement works better.

My car was set up on a very good modern 4 wheel alignment machine as has a bit of neg front camber. Counting shims I have 4r/0f on the o/s and 5r/2f on the n/s. Car tracks and centres well and tyre wear is even.

I ran with the thicker 7/8" ARB for many years and prior to rebuild also used a home built rear ARB, but have since uprated the rear spring by adding another leaf (in the centre), which has reduced body roll, which was may original intention when changing/adding ARBs. Turn in (ie less understeer) is defiantly better with the standard 9/16" front ARB. (nb standard ARBs all seem to have a set (twist) built in)

Ian

[Mark B]

I recently acquired a Trackrite. I had set the front and read tracking with tape measure and string on my mk2 Vitesse,  and the car drives and handles well. 

I gave the Trackrite a try on the rears, backing the car over the gauge gives different readings from rolling the car forward. So assume there is a lot of twist and flex in the rotoflex doughnuts and trailing arm bushes. I can imagine the rear wheels being at all sorts of strange angles on the road.