Radius arm mounting point confusion

[Mucus72]

hi all,

last weekend I got round to doing most of the rear suspension in my build, and use the upper mounting holes for the radius arms. I did this because I am inherently lazy, and the lower mounting hole couldn't be used without me drilling out the inner skin first. 

I have listened to a few people and viewed a few old and one very recent) posts and I see inconsistencies. 

I would like to know which is the best option, and why you would use one setting over the other please. If there is a very tiny difference them leaving them in the upper setting where they are would be easier!

[k.russell]

It's a long time ago now but when I built mine in the early nineties I remember on the top mounting brackets on the de dion where the radius arm connects to the De dion were touching the chassis tubes when the car was loaded. When I enquired about this at the factory their answer was to "put it on the lower one then." so it's been there for 24 years.

[Jonathan Kay]

IF (Rigid member rather than Watt's linkage) AND (Road use) THEN (ASK: 'What are the inconsistencies?")

Jonathan

[Dartmoor7]

 

My understanding has always been that the top mount has a slight comfort bias, the lower a slight handling bias.

Forget it and proceed with your build.

[Jonathan Kay]

The Assembly Guide says (and I don't think it's changed since at least 1998, except for the provision about Watt's linkages WRONG, SEE BELOW):

15 Fit the radius arms to the upper chassis bush with the waisted section facing outboard, to provide maximum clearance to the brake calliper. The arms are fitted using M10 x 65 mm cap head bolts (fastener pack 30R011A Item 2) passed from the inside of the cockpit through the radius arm and secured with nyloc nut (8) and plain washer (14). The bolts should be tightened to 34 Nm with the arms parallel to the ground. This preloads the bushes in the correct position for when the suspension is properly loaded.

NOTE

On some series 3 chassis a lower fixing boss is supplied to give optimum suspension geometry when used only for race and track purposes. This setting is not recommended for road usage. 

...

It's a long time ago now but when I built mine in the early nineties I remember on the top mounting brackets on the de dion where the radius arm connects to the De dion were touching the chassis tubes when the car was loaded. When I enquired about this at the factory their answer was to "put it on the lower one then." so it's been there for 24 years.

Same questions, please: rigid or Watt's linkage and was that for road use?

But that "then" is intriguing.

Thanks

Jonathan

Edited: The recommendation did change between 2011 and 2014, see post #14.

[Dave Rayner]

I would go for the lower setting, it covers off all usage and I doubt if you would even notice the difference in ride.

[russjones]

Hi it's the upper setting you want as per build manual - we run our super sport r on track a lot and the softer setting covers off both - believe it or not the manual is right - they've been doing them for a while - if you don't like it change it laterr

[Richard Price]

Since I'm guessing the car is an SV, I'm also guessing that its watts linkage rather than standard trailing arms (aren't all SV's supplied a watts linkage?). Therefore the upper position is correct. 

[Mucus72]

Thanks all. I will stick with the upper setting as per the guide. It is a SV. It does have both options pre drilled. It is a radius arm set up, not a watts linkage one. This is now standard irrespective of chassis size.

But thanks for putting my mind at rest!

marcus

[Jim 123]

Not wishing to add to any confusion, but my well thumbed page 134 of the Feb 2015 Build Manual says:

"Fit the Radius Arm to the lower chassis bush with the waisted section facing outboard ......etc".

I went for the lower chassis bush mounting on my recent build.  My car is an S3 and had the holes for either option pre drilled in the inner skin.

I don't suppose that a compromise might be to fit one side on the lower bush and the other on the upper bush  :)

 

[Mucus72]

I've just checked with a mate and his factory build (recent, very last of the R400's before the re-brand to 420R) and his are in the upper holes. It's all rather inconclusive so for now I'm going with the uppers as the inner skin is only pre drilled here. I'm considering cutting out the rear wheel arches for both options to make it an easier change in future. But of course the seats would need to come out too, so it's always going to be a pita to change. 

[Shortshift]

In summary - the lower position will give less rear axle steer but at the expense of some ride comfort, whereas the upper position allows more comfort but at the expense of some control.

Difficult to explain without diagrams, and this is in any event something of a simplification with several factors being disregarded, but at a basic level:

With the link mounted in the upper mounting point, it is angled slightly downwards towards the axle mounting point with the vehicle at nominal ride height. When the wheel hits a bump, the axle on that side of the car is pushed upwards but in an arc (about the front mounting point) that, initially at least, allows the axle to move rearwards as it rises.  So the axle is not 'fighting' the bump - the geometry effectively provides some in-build compliance (the effect of the axle moving backwards as it moves upwards on encountering a bump).  But, unless both sides of the axle are affected equally by the bump, the rearward movement of the axle on one side of the car causes a steer effect - and the same effect happens as the car rolls, with the outer wheel going up (and slightly rearwards) whilst the inside wheel is likely to be 'falling' (and moving forwards slightly); as a result, the axle is no longer square to the direction of travel but is turned slightly - hence the steer effect.

When using the lower mounting position, the link is more or less horizontal in relation to its mounting point on the rear axle, such that when the axle rises (or falls) it does so in an arc that always 'pulls' the axle forward from its nominal position.  But 'pulling' the axle forwards as it encounters a bump is the reverse of providing a cushioning compliance effect and does, in effect, make the ride harsher; you're 'pulling' the axle into the obstacle that caused the disturbance.  But follow the argument through and you will see that the axle has a much reduced tendency to move away from being square to the direction of travel as it rises or falls - either in bump or in roll - hence the reduction in steer effect.

Two final points: I'm sure this could be explained with far fewer words but it's too late to do that now and, anyway, if you sketch it out (the links and arcs in side view) you'll see what I mean.  And finally, the extent to which this is noticeable for most of us in real-world driving conditions has to be open to debate...

James

 

[Jim 123]

A great explanation James!

[Jonathan Kay]

Not wishing to add to any confusion, but my well thumbed page 134 of the Feb 2015 Build Manual says:

"Fit the Radius Arm to the lower chassis bush with the waisted section facing outboard ......etc".

That doesn't add to the confusion, it's... pivotal.

For non Watt's linkage the Guides say:

• Up to 2011: Upper
• 2014: Lower
• 2015 (from Jim 123): Lower
• 2016 (current, excluding 160): Lower
• 2016 (current, 160): Middle!

The warning about the lower position being unsuitable for road use disappears with the recommendation to use it.

Which only leaves two questions: what's right and why did the recommendation change  (see post #2)?

Jonathan

PS:

I don't suppose that a compromise might be to fit one side on the lower bush and the other on the upper bush  :)

An Austin saloon was once built with one door on one side and two on the other. This is used as an exercise in thinking about quality control...

[nickh7]

I built my 97 with the arm in the lower bush and it was very tail happy in the wet and prone to snap lift off oversteer. After a very excitable moment on a wet roundabout I moved it to the upper. When I went Freestyle Garry said leave it up there its the best compromise, he new a bit more about handling then me and its been there ever since