The Triumph Dolomite Club - Discussion Forum

Hi All

I had an idea for for the rest trailing arms for Sprint and other RWD Dolomites as I remember some old post about them failing under high power/torque, something along the lines of they folded in on its self due to the torque.


Would anyone be interested in some uprated arms if I had them made? Probably would be “hard” mounted with rose joints.

I’m not an expert, but I believe the consensus is to have a soft rear end eg removing the rear ARB and not using a complete set of Polybushes.

Guess it depends on what you’re doing with the car eg daily driver, concours winner or track car.

Something like this...hope you don’t mind Dan !!

The arms I’ve seen failed have folded up at the ends where the top ‘roll’ doesn’t exist

RichardHyde wrote: ↑Tue Dec 31, 2019 9:45 am I’m not an expert, but I believe the consensus is to have a soft rear end eg removing the rear ARB and not using a complete set of Polybushes.

Guess it depends on what you’re doing with the car eg daily driver, concours winner or track car.

Something like this...hope you don’t mind Dan !!

I would be very careful about home welding and modifications of any suspension parts. They are under a great deal of stress and you could cause yourself all sorts of nasty problems.

Take the modification above, it is possible doing that could create a stress riser because the point at which you have welded those plates is stiffer than the rest of the arm. It will no longer flex in the way it was designed to. If you had an accident with a car modified in that way, I think you could have problems with your insurance or worse.

Boost All The Dollys wrote: ↑Tue Dec 31, 2019 1:07 am Hi All

I had an idea for for the rest trailing arms for Sprint and other RWD Dolomites as I remember some old post about them failing under high power/torque, something along the lines of they folded in on its self due to the torque.


Would anyone be interested in some uprated arms if I had them made? Probably would be “hard” mounted with rose joints.

Hi

Sounds a splendid idea, depending on cost, I would be interested in a pair!

Regards

Barry

I wrecked one when i spun the wheels through a pothole inadvertently which came to light as the nearside wheelbase being a few inches shorter.
looking at it I reckon a thick wall straight tubular one combined with a shorter coilover would be way better than trying to replicate the cheap to manufacture stamped steel thing, the floor and axle mounting are more than tough enough when in good condition to handle the harshness of rose joints which will allow free articulation movement , although if the arm is rigid it will overpower the tendency for the bush to resist axle articulation.

RichardHyde wrote: ↑Tue Dec 31, 2019 9:45 am I’m not an expert, but I believe the consensus is to have a soft rear end eg removing the rear ARB and not using a complete set of Polybushes.

Guess it depends on what you’re doing with the car eg daily driver, concours winner or track car.

Something like this...hope you don’t mind Dan !!

Yeah, but that’s a very compromised set up just to save one part. If they are failing because of the torsion induced by the bushes, poly or rubber, then having hard bushes and a stiffer arm would go a long way to increasing the strength of that arm.

jikovron wrote: ↑Tue Dec 31, 2019 8:51 pm I wrecked one when i spun the wheels through a pothole inadvertently which came to light as the nearside wheelbase being a few inches shorter.
looking at it I reckon a thick wall straight tubular one combined with a shorter coilover would be way better than trying to replicate the cheap to manufacture stamped steel thing, the floor and axle mounting are more than tough enough when in good condition to handle the harshness of rose joints which will allow free articulation movement , although if the arm is rigid it will overpower the tendency for the bush to resist axle articulation.

If it’s rose bushes/tie rod ended, they usually have a good amount of articulation, only issue might be the amount of bending force transmitted to the shock absorber.

Something like this is want I’ll be aiming for

Just one comment; product liability insurance.

I'm another one not in favour of messing too much with the original design!

In the first place, strengthening this part will only lead to to the stress going elsewhere and THAT part failing.

In the second place, failures are NOT that common and largely restricted to Hi-po cars that have been polybushed throughout.

In the third place, as Andy has said, you need to consider possible consequences if marketing a safety related item like this, it's one thing to mod your own car and see how it goes, quite another to sell the mods on the open market!

The normal point of failure is between the shock mount and the axle end bush, a point that has NOT been strengthened in the picture shown above!

My own experience seems to suggest that a stock arm in decent condition (they can and DO corrode and weaken) that is only polybushed at one end (to allow the arm to twist a little in service which is vital to the suspension geometry) will survive considerable abuse without failing.

The arms on the Carledo are the original ones from 1973, which were Ziebart protected from new and have very little corrosion evident. They are polybushed at the rear end only and have withstood my heavy right foot and 140+ bhp of Vauxhall power for more than 8 years to date, including innumerable "linelock burnouts" which must surely be the ultimate abuse for such a part!

My conclusion is, make sure your arms are in good order, don't go too mad with the poly and if it ain't broke, DON'T FIX IT!

Steve

Carledo wrote: ↑Wed Jan 01, 2020 11:55 am I'm another one not in favour of messing too much with the original design!

In the first place, strengthening this part will only lead to to the stress going elsewhere and THAT part failing.

In the second place, failures are NOT that common and largely restricted to Hi-po cars that have been polybushed throughout.

In the third place, as Andy has said, you need to consider possible consequences if marketing a safety related item like this, it's one thing to mod your own car and see how it goes, quite another to sell the mods on the open market!

The normal point of failure is between the shock mount and the axle end bush, a point that has NOT been strengthened in the picture shown above!

My own experience seems to suggest that a stock arm in decent condition (they can and DO corrode and weaken) that is only polybushed at one end (to allow the arm to twist a little in service which is vital to the suspension geometry) will survive considerable abuse without failing.

My conclusion is, make sure your arms are in good order, don't go too mad with the poly and if it ain't broke, DON'T FIX IT!

Steve

I understand the if it ain’t broke, don’t fix it, but if rose bushes/tie rod ends are used, they provide the articulation rather than the steel arm.

And yeah, selling them would be difficult without proper testing and insurance, maybe I’ll develop this and run it for a while before providing them.

And I’m not suggesting that everyone who’s making more than standard power should immediately buy this when it’s available, I’m more suggesting that if people want the best handling car, compromises shouldn’t be made because the rear arm might not be up to the task.

If I’m planning on running 250/300/350 hp and want to have a good, solid chassis set up, the last thing I’d want is the suspension folding up on its self and there be no real solution.

I see this thread as more of an experience and knowledge gathering for what I’m likely to face if I start running higher hp.

Boost All The Dollys wrote: ↑Thu Jan 02, 2020 7:13 pm

Carledo wrote: ↑Wed Jan 01, 2020 11:55 am I'm another one not in favour of messing too much with the original design!

In the first place, strengthening this part will only lead to to the stress going elsewhere and THAT part failing.

In the second place, failures are NOT that common and largely restricted to Hi-po cars that have been polybushed throughout.

In the third place, as Andy has said, you need to consider possible consequences if marketing a safety related item like this, it's one thing to mod your own car and see how it goes, quite another to sell the mods on the open market!

The normal point of failure is between the shock mount and the axle end bush, a point that has NOT been strengthened in the picture shown above!

My own experience seems to suggest that a stock arm in decent condition (they can and DO corrode and weaken) that is only polybushed at one end (to allow the arm to twist a little in service which is vital to the suspension geometry) will survive considerable abuse without failing.

My conclusion is, make sure your arms are in good order, don't go too mad with the poly and if it ain't broke, DON'T FIX IT!

Steve

I understand the if it ain’t broke, don’t fix it, but if rose bushes/tie rod ends are used, they provide the articulation rather than the steel arm.

And yeah, selling them would be difficult without proper testing and insurance, maybe I’ll develop this and run it for a while before providing them.

And I’m not suggesting that everyone who’s making more than standard power should immediately buy this when it’s available, I’m more suggesting that if people want the best handling car, compromises shouldn’t be made because the rear arm might not be up to the task.

If I’m planning on running 250/300/350 hp and want to have a good, solid chassis set up, the last thing I’d want is the suspension folding up on its self and there be no real solution.

I see this thread as more of an experience and knowledge gathering for what I’m likely to face if I start running higher hp.

Anything over 250bhp and you'll be looking for a different axle too as that's about the limit of what a Sprint unit can handle.

Im not sure that rose joints won't provide a little too MUCH articulation and force you to adopt a Panhard rod to control axle sideways location adequately.

If I was to be designing such an item (with KISS as my guiding principal) i'd keep the tubular bushes at each end and merely replace the pressed arm with a nice strong bit of tube, a U bracket welded to it to support the shock mount and some sort of drop plate for the ARB if you keep it. The main thing to get rid of, if practical, is the "kick up" at the axle end which introduces the fatal stress point.

Whilst most failures of the originals are probably overload induced, it's by no means exclusive. A couple of years ago, I changed one out that had failed on a 1500 auto, you'd be hard pushed to find one LESS stressed than that! Old age and corrosion were the culprits in that case. The failed trailing arm was only the thin end of the wedge on that car, I don't think i've EVER seen a Dolomite quite so rotten that was still in regular use, nor one with SO many patch repairs! It was scrapped shortly after, as it's chances of another legit MOT were effectively zero.

Steve

How would it need a panhard rod? , it's effectively more controlled than a bush with radial squish given a rose joint has none at all if preserved with cover boots lol

Power output wont be the issue 140,,,210,,250 that's only an issue for excessive heat build up in the bearings and hypoid sliding tooth mesh, 1st gear torque @ the axle is more relevant for arm collapse, and also compressive shock loads fed into the trailing arm when a low powered car cant overcome tramp, my 1500 would violently tramp under harsh acceleration, in contrast the 3.9 engine would easily smoke the offside rear with 4 passengers without tramp but setting off hard under grip did bugger one over a few weeks that was in good order but creeped into a bent condition and the wheel spin though a pothole instantly killed another decent condition one with squishy old bushes in.

Imo dont sell them though unless you can prove the design has nothing that could cause cyclic failure etc.

If I was going to unrated the trailing arms.
I would use a heavier gauge metal (U Shape )
And simply Refabricate what’s already there but stronger.
And look at the top arms as well.

My Sprint has a Panard rod set up
Copied from Aluns QuickSprint.
It just tightens and supports the rear better.

I imagine the 'give' in the bushes and limited travel allows for a panhard rod without the geometry of the 4 link fighting the arc of a panhard rod aslong as its roughly parallel with the axle at rest position.