The Triumph Dolomite Club - Discussion Forum

I seem to remember that lowering the rear suspension actually raises the centre of gravity and it affects the axle nose angle. I think Steve Grace wrote an excellent article on this in the old DSR days.

I know that SprintSpeed sell adjustable radius rods. Do they need to be adjusted longer or shorter to correct the geometry, lower the CofG and correct the axle nose angle….

My memory says lowering raises the "roll centre" (whatever that is) on a car that already has a relatively high roll centre (even worse on TR7s apparently) and that therefore, on a Sprint, you shouldn't drop it more than an inch or you'll make it worse rather than better. Other models like Toledos, which start higher, can successfully be dropped further, i've proved that empirically with the Carledo which is/was almost 2" below stock. It doesn't seem like a big deal to me as dropping the front a similar amount leaves you very close to the practical limit as regards subframe, sump and exhaust clearances.

I actually swapped the original Carlton 3/4" nut head sump plug for a later, thinner, recessed Torx head plug to lessen the risk of ripping the plug and sump apart on sleeping policemen and the like, it's THAT close! And actually happened to a guy I know, who ripped the plug from a Carlton sumped Redtop in a Sprint. With predictably disastrous results.

I've not bothered to adjust the nose angle as it seems to work as is, lowered on stock tie bars. Though I HAVE put extra hard red Superflex bushes at the axle end of the tie bars to help with "axle tramp" under extreme duress. With length of the trailing arms being the pivot, an inch or two of drop can't put many degrees on the nose angle, it'd still be within normal suspension travel limits. And the difference between the (fixed) prop angle and the (moving) trailing arm has to be subtracted from the effective nose angle so you're not affecting the "real" nose angle as much as you think. Probably an extra washer at the front end of the axle fitting and a corresponding thinning of the rearmost bush would be enough to adjust it up a bit.

My understanding of the physics says that lowering the car would lower the diff nose, so you'd need the tie bars to be a tad longer to reinstate the original geometry. But, since a lower nose angle will give more room for axle torque reaction before the diff hits the pinion snubber under heavy acceleration or during "burnouts" (my favourite showoff manoever) i've though that's all to the good and left well alone, though I guess limiting axle tramp that way is actually a good idea too! Even if hitting the snubber repeatedly on every hard start is gonna be tough on the bump stop/pinion snubber and the body above it!

My two pennorth, Steve

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'73 2 door Toledo with Vauxhall Carlton 2.0 8v engine (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine (The Dolomega)
'72 Triumph 1500FWD in Slate Grey, Now with RWD and Carledo powertrain!

Maverick Triumph, Servicing, Repairs, Electrical, Recomissioning, MOT prep, Trackerjack brake fitting service.
Apprentice served Triumph Specialist for 50 years. PM for more info or quotes.

Richard,

I was told by Rob that the adjustable tie bars also give you the option to to ensure that the rear axle is central in the body. Unsurprisingly, apparently BL tolerances weren't great with regard to rear axle alignment and these give you the option to correct that.

He also told me that the other way to drastically lower the rear of a Dolomite, without radically affecting the geometry of the trailing arms, is to modify them, so that the bush hole for the rear axle mounting is higher up. Therefore the arms retain their normal static position on the car. A lot of work and probably only necessary, if you are racing.

I remember, 30 odd years ago, lowering my Sprint too much. 2" IIRC. I don't recall any bad handling issues but it was constantly grounding out at the front, and sometimes at the rear. This happened a lot on a camping trip to Scotland with 4 people in the car. On one occasion, in a country lane near where I leave , I experienced a loss of power and was struggling to get up a hill. I stopped, popped the bonnet release, and went to the front of the car to have a look in the engine bay. Before I could lift the bonnet, I noticed that there was a huge pile of leaves in front of the car, right up to the top of the front panel. I had literally snow-ploughed them up the lane. I went back to 1" lowering springs after that.

Ah yes I think you’re right…lowering the rear raises the Roll Centre. I’ve found other old post that confirm this. Wish I could find the diagram that shows the standard vs lowered car…

Would lowering the rear of the car mean that the top of the axle is pushed backwards and that having a slightly shorter tie bar would bring it back into the correct position ?

Richard,

Best to speak to Rob about it, he is very knowledgeable.

Glen.

I think it's the other way round! You have to look a the rear axle like a pair of unequal wishbones with the axle as a hub/wheel. So the trailing arms are the lower wishbone and the tie bars are the upper. Because the tie bars are shorter than the trailing arms, as the axle goes up into the body (or you lower it) the angle of the diff moves downwards as the tie bars pull the top of the axle forward. So you'd need to LENGTHEN the tie bars to compensate.

This would, however, only compensate for the difference in nose angle. Roll centre is another kettle of fish. Whilst I don't really understand what it does, the limiting factor appears to be the angle of the lower trailing arm to the road in the "rest" position. On a standard car they are angled slightly down at the axle end. The limit of lowering seems to be when the trailing arm is dead parallel with the road. Which is why Rob recommends raising the axle mounts on the trailing arms, this keeps the trailing arm angle where it needs to be for roll centre conrol whilst lowering the body over the axle. But, as he says, it's a pretty extreme and radical mod to acheive something you don't really need on anything less than an out-and-out race car.

Many years of trackdaying the Carledo makes me confident you won't need it, the stock geometry lowered to the most practical level and with judicial polybushing and some Gaz asps and good brakes, wheels and tyres, had me looking for more horsepower from the 140+ I had cos I was confident the chassis could handle it comfortably. The car's next iteration will hopefully be around 220bhp which I think should be more than enough for a 3/4 (760kg kerb weight) tonne 2 door! That's close to 300bhp/tonne power to weight ratio!

Steve

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'73 2 door Toledo with Vauxhall Carlton 2.0 8v engine (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine (The Dolomega)
'72 Triumph 1500FWD in Slate Grey, Now with RWD and Carledo powertrain!

Maverick Triumph, Servicing, Repairs, Electrical, Recomissioning, MOT prep, Trackerjack brake fitting service.
Apprentice served Triumph Specialist for 50 years. PM for more info or quotes.

Lowering the rear suspension of a Sprint does raise the ‘roll centre’, which increases the propensity of the car driven on/near the limit to suddenly break away and likelihood of the car changing ends, rather than a progressive slide. One inch seems to be the recommending max lowering of the axle. Of course, tyre size can change ride height without changing the suspension height. The only way I have seen to lower suspension height without the detrimental effect on ride height involves mounting the rear tie bar front mounts INSIDE the car, just above the floor where they currently sit. I have seen it done in one very fast Sprint and also in a Lotus Cortina. However, mounting the links inside the car requires more work to seal the cabin from the outside (fuel ingress and fire in case of an accident).
I used adjustable rear tie bars when racing, but simply to locate 195.60x14 race tyres centrally within the existing rear arches (flaring the arches wasn’t allowed). Even then the edges of the outer bodywork were flattened and the inner arch panel beaten a little.
Having the front suspension set lower than the rear increases understeer, so front was lowered same as the rear. Bilstein shocks, revalved to suit, race springs, 500lb front and 200lb variable rear. Uprated adjustable front sway bar. No rear sway bar. Adjustable front links (as well as rears).