Bump Stops in rear arches

[Jeremy]

I have to say the response to my post really surprises me given you all seem right thinking and experienced owners

Speaking as a wholly inexperienced owner I am happy to defer to Jag's engineers...they worked out bump stops were needed, and that's good enough for me:

[Mich7920]

[Heuer]

I have had first hand experience of driving my E-Type with and without bump stops. Consider driving at speed and approaching a tight left hand bend:

1. Standard Bump Stops

As the car enters the corner it begins to lean and as it does so the hub carrier begins to make contact with the bump stop. The design of the stop means it progressively reduces the rate of body roll and once the hole in the donut is compressed there is little further movement. The car is now stable, leaning in the corner and can be steered on the throttle. It has maximum grip and is handling the way it was engineered.

2. Bump Stops Removed

As the car enters the corner it begins to lean and continues to lean further until the shock absorbers bottom out. This is an extra 1" to 1.5" of movement so the lean angle is greatly increased. When the shocks bottom out they do so abruptly which causes the car to jerk and you feel the steering wheel shimmy in your hands. The extra lean takes weight off the nearside tyres reducing their grip. The off side tyres are forced sideways rolling towards the side walls. The road surface will have imperfections and they will cause the suspension to move up and down repeatedly bottoming on the shocks which causes more shimmy in the steering. Once the car straightens up any undulations in the road will cause the suspension to flex resulting in the exhaust hitting the road or the shocks sharply bottoming out causing discomfort to driver and passenger. You have 1"+ less ground clearance on the move.

So we have compromised handling, cornering, grip, ground clearance, ride quality and safety simply because those two bits of seemingly superfluous rubber have been removed. Every car on the road (apart from some E-Type's!) has bump stops fitted and 99.9% of those are internal on the shock absorber.


Jaguar had their unique IRS though and decided to forgo internal bump stops (which are service item and need replacement) and opted for an external stop which can be easily seen and replaced if it starts to deteriorate.

For those who have no bump stops there are two ways forward and neither are particularly palatable. First would be to reinstall the stock items but this will involve welding and may not be acceptable to those with a freshly restored car or are unwilling to give up their wide wheels. The other is to install bump stops in the shocks which involves removing them from the car, dismounting the springs and fitting the stops. Unfortunately the Boge shocks (others may be the same) do not allow you to remove the top mount to get the stop pushed on, which means the stop has to be cut and slipped on. This is no problem as the standard dust cover will hold it in place and allow it to function.

The big problem is selecting bump stops that will replicate the action of the stock items in terms of compress-ability and range of movement. Also the stock items are in a different place and angle. It will be a case of experimenting with the myriad styles and compositions available until the car behaves as Jaguar intended and that is going to take a lot of time and mileage. Jaguar probably went through many iterations of bump stop size and composition before settling on the specified part so all we can do is make a guess and hope it works or at least reduces the problem. It will never be as good as what Norman Dewis was happy with though. Best option is to use what Eric (64etype) used - he did sterling work thinking it through.

Interestingly people removed the bump stops to fit wider wheels/modern tyres to improve road holding when in fact they have achieved the opposite

[timhum]

David,
Not doubting your observations but don't the springs come into play when the car is cornering in that they counteract roll?
thanks
Tim

[64etype]

This old racing photo comes to mind when the subject of E-type bump stops arises....

[Heuer]

David,
Not doubting your observations but don't the springs come into play when the car is cornering in that they counteract roll?
thanks
Tim

Anti-roll bars counteract roll, springs store energy and shock absorbers dissipate the stored energy from the car body movement. They work together obviously but in a roll the springs only store the generated energy, not the extent; the shocks determine the rate of roll not the extent (until they bottom out); the bump stops control the extent of the roll; the anti-roll bar counteracts the roll by attempting to lift the opposite wheel. Stiff suspensions do reduce body roll but compromise NVH - on the E-Type you would need to fit stiffer torsion bars, springs and shocks but you would still need bump stops. The tyres are also a suspension component but un-sprung weight as against the body which is sprung weight.

The shock absorbers are valved to match the coil spring so the stored energy is released in a controlled fashion. This is why Koni shock absorbers are such a poor match to the E-Type suspension - the user can mess with those valves and there will be only one sweet spot for the springs and good luck with finding it!

Jaguar worked hard to find the correct spring stiffness and the Girling shocks valved to match the weight and dynamics they wanted. The Factory put dabs of red, blue, yellow or green paint on the springs to make sure they were not mixed on the individual IRS. The springs were also date stamped.

[kgb123]

OK, well I went away for a couple of days and when I come back find I've a lot of reading to do

Also I'll get a picture of my current shocks and springs arrangement. there seems to be a similar fitting on the top of the springs to that shown in a picture on this thread. THE IRS has clearly been worked on before, perhaps the previous person has gone down an avenue already.

Pics tomorrow.

Kevin

[MarkRado]

Perfect and detailed explanation from David - should be in the knowledge base

[timhum]

David,
Thank you for your explanation. I think I've got my head around it but then had another thought; why doesn't the front suspension have similar bump stops?
Tim

[cactusman]

Same thought occurred to me

[Heuer]

Torsion bar suspension has many advantages like being to easily adjust ride height to compensate for engine weight, small profile along the length of the car and unlike coil springs does not need heavy reinforcement of the mounting points. One of the disadvantages is, unlike springs, torsion bars cannot provide a progressive rate of travel. Jaguar used that to their advantage as the torsion bar itself would not allow full travel getting stiffer and stiffer as it was twisted. It almost acted like its own bump stop and they also used a front anti-roll bar. The effect of the IRS should not be underestimated as that also contributed to the dynamics of the front suspension.

Having said that practically all other cars using torsion bars do have bump stops e.g Citroen Traction Avant, Morris Oxford, Porsche 356/911 etc. This is because over rotating the torsion bar will put the shock piston outside its intended travel and we are back to where we started. Got to hand it to Jaguar, they had the experience and of course Norman Dewis to develop the car over hundreds of thousands of test miles and with careful selection of components proved they could do better!

[Heuer]

I should a.dd that there is little opportunity to fit bump stops on the E-type front suspension because the front bodywork, aka bonnet, is moveable. Normally the bump stops are fixed to the inner wings but that is not an option.