Single Shear Rear Coilover Mounting
Turbo Mini Forums

Has anyone ever had any problems with the usual/standard way of mounting rear coilovers on the shock stud? I know 99.9% of minis with rear coilovers are just bolted straight on like this, but wondered if anyone has actually had problems i.e. sheared the pin, as it's not an ideal set up.

Cheers,
TC

www.topcatcustom.co.uk

Well my standard ones never failed even after a pasting on the track. Now running sc rear arms with uprated stud. Even so the pins do look weedy.

If you consider the loading on that pin it really isn't very high. Cars with coil overs are usually lightened/stripped out.

Next to no mass over the rear wheels means the pins are never really that stressed. A simple calc would settle your mind when you consider how lightly loaded they are.

True, and the shock is help upright so it never has to resist bending. Cheers

www.topcatcustom.co.uk

I had no issues with mine mounted to fairly solid shell.

I hit a proper nasty bump in the road at 70 that made me almost crash it was that bad. No damage at all to the rear. The fronts the lower bracket bits hit the subframe but again no damage.

The main issue is caused by having to run a compliant bush in there. If you could be sure there was no articulation outside the plane of the arm, you could run a solid bush in there! That would reduce the risk of stress fracturing the pin!

In practice, the shock is mounted right up against the arm casting, so it's as good as it get's really.

I've always had concerns about the other end of the coilover shocks though. The arch top was never designed to take spring loads, only miniscule loads from the dampers. I assume nobody has ever encountered a fracturing problem here, otherwise it would be widely known by now?

Steve
DSN Classics Ltd
www.dsnclassics.co.uk

Steve, firstly thanks for the circlips, great help.

I have fabricated new rear turrets from 4" ERW tube, so big enough to house coilovers without offset tops, they have 3mm tops with stiffners as well.

I never used to think dampers created much force unlike springs- but in reality I think it is more- the spring mounts suffer little shock loading as the spring cushions any loads, the dampers (if reasonably stiff) thrash the hell out of their mountings as it's all direct feedback. I'm doing some work on a completely original RS200 at the moment, which has 8 dampers on it- to cope with the work and stop overheating, albeit technology is better now and can use 4 if modern spec.

www.topcatcustom.co.uk

With coil overs the entire spring load is passing through the top mount. The loads the turret see's is the same regardless of spring rate. Although, there might be some transient differences in the way the load is transmitted. This may increase the risk of fatigue stressing.

If you apply a particular upward force at the wheel (hitting a bump!), the mass of the car provides the opposing force. The spring compresses until it reaches equal forces in both directions, beyond this the car will start to lift. If you fit stiffer springs, the car will simply start to lift sooner given the same upward input and the same body mass bearing downwards.

Basically, a bump will always provide a specific force upwards and the body is always the same to provide a downwards force.

In a conventional setup the spring loads are all passed through the subframe via the rubber cone, of course.

Damper loads are minimal. Yes, the stiffer they are, the greater to forces transferred to the turret in a transient way. But, compared to the loads from a coil over setup, they are miniscule.

Steve
DSN Classics Ltd
www.dsnclassics.co.uk

Never cared about bottom mounting pin, as for thetop mounting i was going to do the cage to it, dont like the idea of the std arch and coilovers with heavy road use. I have recently damaged a spring on my clio due to a pothole also dented the steelie too.

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Steve, do you not think that if you had a ultra soft dashpot type spring that could just take the weight of the car on the back, and a damper set to "rock solid", then when you hit a speedbump at 60mph, there will be a lot more force through the shock mounting than the spring seat? After all the shock absorber is absorbing the shock... and the spring is effectively just taking the weight of the vehicle.

I agree there is more force acting on on the spring seat due to mass, but the forces shock mounts see are very high spikes which would cause fatigue more so than the slower bigger forces on spring seats.

Think I waffled on there but you may see my point..?!

www.topcatcustom.co.uk

The forces are simply a function of the weight down and the input up.

What I'm trying to say (badly!) is that the weight of the car is always the same, so that gives the same 'resistive' force all the time. Any given input from the road is applied against that. What joins them together cannot change the load seen by the turret. The load is simply a result of the force applied by the bump. A 3" speed bump driven at 30mph will always give a specific upward force. That force is fixed by the height of the bump, ramp angle, etc. Nothing to do with the car.

In the extreme, as you suggest, if you were to put a solid bar in place of the strut, the shock loads could cause stress fractures where a spring applies the load more progressively. The final load applied will still be the same though. It's the 'vibration' that causes the stress fractures, not additional load.

Steve
DSN Classics Ltd
www.dsnclassics.co.uk

One thing I'm missing here is the dynamics of it. I may be talking taurus poo as a result!

In a static state what I have said is true. i.e. push up on the wheel with whatever fixed between and the load seen at the turret will always be the same.

However, the dynamics of the damper compared to a spring mean the forces are transmitted through the damper far more than the spring. The greater the acceleration of a damper, the stiffer it becomes. Whereas, a spring is the same rate regardless of acceleration applied to it.

Interesting discussion though!

Steve
DSN Classics Ltd
www.dsnclassics.co.uk

The thing you are assuming is that a bump will apply a certain force, whereas I believe if would give a certain displacement. If you imagine the car body as constant height, never changing even over bumps then a bump will displace the wheel by its height regardless of spring rate. Therefore the stiffer the spring the higher the force, as force = stiffness x displacement

We agree that it is fatigue that kills the mounts, thats the main part! I'm a bit tired but think I agree with your points Steve!

At least the general consensus is that single shear seems to work ok for us :)

www.topcatcustom.co.uk