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caster
#1
  This topic is about my 1960 Triumph TR3A
red72b bill duran
TX, USA   USA
1960 Triumph TR3A "Trevor"
1972 MG MGB "Ms B"
I haven't yet taken the measurements and done the math to calculate the exact caster angle on my 1960 TR3A, but just by placing a square on the floor against the front tires, it appears to have a positive caster. The car seems to be not inclined to drive straight ahead, but needs lots of steering input to stay straight. Just completed a many year restoration on it and all steering parts and suspension parts are new. Steering box was rebuilt by TRF and tires are new Firestones of the right size. Toe-in may be slightly off (too much toe-in), but the appearance of excessive caster has me concerned. Car is sitting unladen and I think the caster is supposed to be measured with 150 lbs in both seats according to one of my restoration books.
Am I trying to make a car that's almost 60 years old drive like a new one or is this caster thing a possible problem? If it is, how can it be corrected?

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malbaby Avatar
malbaby malcolm baker
kyabram, Australia   AUS
Not sure on the TR3 upper control arms, but are they in 2 pieces, and asymmetrical, and have been assembled incorrectly...shorter arm to the front instead of the back ?

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
"Caster" refers to the fore/aft tilt of the steering axis (the line around which the front suspension pivots when turning side to side). I don't see how you could be measuring that by putting a square against the front tires. Caster is also relatively unaffected by suspension movement, so it shouldn't matter how much weight is in the car.

Could you perhaps be referring to camber instead? Camber is how far the wheels are tilted away from vertical, which is easily measured with a carpenter's square against the tires (although it's more accurate to measure against the wheels). And the TR3 front suspension is supposed to have positive camber. (2 degrees static laden, 1/2 degree full bump, 1 degree full rebound)

There is yet another relevant angle, known variously as "steering axis inclination" or "king pin inclination", which is the tilt of the steering axis again, but side to side rather than fore/aft. It is supposed to be fairly large, 7 degrees ("static laden" ).

On a flat, level road, it should track straight. But most roads are not level from side to side, they are deliberately tilted so that rainwater will run off faster. It's normal for the car to want to follow the tilt to some extent, which means that usually on a 2 lane road it will pull just a little towards the side of the road. But it should be a fairly steady pull, easy to compensate by putting a little force on the wheel. If the road is not flat (eg ruts in the pavement), the steering does get somewhat erratic compared to modern cars; as it will want to either climb the rut, or turn down it (or both).

On a stock TR3 suspension, the upper A-arms are very nearly exactly the same length. There is a very slight difference, but it's only the thickness of the sheet metal. Because of that, the arm on the right (with the ball joint towards you) should be on the outside of the arm on the left, which is kind of counter-intuitive. But doing that wrong will make only a tiny change in caster.

The later TR4s were modified to have 3 degrees of positive caster, which is supposed to improve steering stability (at the price of increased steering effort). Some people like to fit those components to the TR3. If so, that might explain your car having around 3 degrees positive caster.

But again, positive caster is supposed to improve steering stability, not make it worse. So I think you might have something else going on. The very first thing I would do is look at the toe-in adjustment, as it's fairly sensitive and just a tiny bit of toe-out will make the car want to alternately turn left or right. The book allows up to 1/8" toe-in; it's probably best to shoot for closer to that than the other limit (0).



Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild



Edited 2 time(s). Last edit at 2019-02-09 09:48 PM by TR3driver.

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CJD john durant
Southlake, TX, USA   USA
Which way is the car pulling? Also, how old are your tires?



John
Southlake, TX

'55 TR2

auprichard Andrew Uprichard
Jackson, MI, USA   USA
Wow, Randall - what a very thorough and complete summary on caster/camber/toe-in. It is great to have you around to address these issues.

Regarding the initial question, you can put caster on a TR3, but you would have to have the later upper A-arms and trunnions with caster, so if you used stock components, you will have zero caster. Like Randall, I am wondering if you mean camber - and one thing you may want to look at is whether you have the ball joints fully tightened and drawn into the upper A-arms. Some time ago I came across aftermarket ball joints which, when fully drawn in, impinged on the suspension tower and required an eighth or so to be machined off.

Other than that, as Randall says, driving will be affected by excessive toe-in (or out).

CJD john durant
Southlake, TX, USA   USA
In reply to # 1595262 by auprichard

Other than that, as Randall says, driving will be affected by excessive toe-in (or out).

Or bent frame/suspension. Hopefully Bill will answer the tire/pulling questions to figure it out.



John
Southlake, TX

'55 TR2

red72b bill duran
TX, USA   USA
1960 Triumph TR3A "Trevor"
1972 MG MGB "Ms B"
My apologies for using the wrong term for my problem. I fully intended to say "camber" as that is what I suspect is the problem. The tires are new Firestone F560 165/15 radials purchased from Coker last year when the car was going together. They are not NOS, but new ones made in Mexico, but not sold by Firestone here. I have found a seemingly fool proof way to measure the toe-in and plan on doing that in the AM to see if it is where it should be .A very interesting comment on the ball joints. Another item to check while looking at the toe-in. I checked the frame several ways to be sure it was OK before all the rest of the resto proceeded so I don't think that is the issue. Thanks for all the info from everyone, what a gold mine of expertise this is!

CJD john durant
Southlake, TX, USA   USA
That makes sense. The positive camber helps reduce the steering load, and with the thin tires the TR's were designed for, does not hurt handling. If you add wider tires, then the positive camber could become a problem.

The reason these cars tend " to be not inclined to drive straight ahead" as you said, is that there is zero caster. Caster is the steering angled measured for and aft. Positive caster makes a car track straight...but that also adds driver steering load in a turn. So the positive camber and zero caster are both designed in to the TR2/3 to reduce the steering load on the driver. Unfortunately, there is no adjustment for either!! They are either spot on or something is bent. I am convinced that most alignment technicians that find a problem resort to bending the vertical link to fix an issue with cater or camber. That just compounds the problem by placing a bind on the trunions, in addition to not fixing the real problem, i.e. that something else is damaged in the suspension.



John
Southlake, TX

'55 TR2

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