Ride Heights

[chris.richard]

I've dismantled the rear suspension, and I see there is a choice of two suspension mountings. The car was built using the lower mounting holes (giving a higher body height). Which setup does anybody else use and why? Now I.m using the Leda struts, presumably I can adjust my ride height ad nauseam.

Phots of bits will follow, they're arriving piecemeal, so i may as well post photos of complete kits.

[rutthenut]

Although you would initially think that the different suspension mountings themselves lead to a different ride height, I don't believe they have such a direct relationship as you might at first think.

With the strut rear suspension, it really is the spring length and platform height that dictates the ride height of the vehicle.

If you change the inner wishbone mounting point, it essentially changes the angle of the wishbone that goes to the lower part of the hub carrier. This will probably result in some small change in height, but not the one inch you might expect if using holes mounted one inch higher in the chassis, as the wishbone itself doesn't really take the weight of the car.

When playing with different spring lengths and spring platform heights, you will directly affect the ride height of the car. You will also affect the corner-weight distribution of the car, but that's a different topic I'm going to ignore for now.

The choice of wishbone mounting points will then probably allow the wishbones to be set in either a near-horizontal position, or else with the wishbone point up or down from that level. What you would want to avoid is a wishbone that points upwards (away from the chassis) as that will only increase as the suspension goes into bump.

As the wishbone goes through its arc of travel, it will push or pull the hub carrier and hence change the track width of the car. That in itself isn't a major concern, but this will also have the effect of changing the camber angle of the strut, which you do want to be aware of. Camber change will be reduced if the wishbone works around a horizontal plane, with more change being introduced as the angle increases.

The change of track width also affects the amount of driveshaft 'plunge', which has caused myself and others some problems if this either 'tops out' or 'bottoms out' under load, so check for free play of the driveshaft at extremes of suspension travel.

One of the views I have taken that is if the suspension travel is going to result in direct camber change when under a cornering load, I'd rather have that introduce some extra negative camber at the time, which may help to offset the major camber change brought about by body roll. To do this, the wishbone can be set to point downwards at its outer edge. As the spring compresses, the wishbone comes up to the horizontal, which cause a slightly wider track width, so puts in a bit more negative camber to the strut.

This does at least sound preferable to a horizontal wishbone going to an upward and 'shorter' position and removing some of the camber at the same time as the body is in roll, so giving positive camber in relation to the road surface, which overworks the outer edge of the tyre, has less grip, and so on.

But having seen photos of my car at work, the camber change from body roll is still the major problem to deal with so this is possibly of little real benefit.

There are other theories that you may want to consider as to the position of the roll centre of the car, which will be directly affected by the angle of the wishbone. I did work out some of this stuff some time ago (no, I don't have the records of that) but generally take the view that as all of this is a compromise for one effect over another, roll centre positioning was perhaps less relevant for this particular car design than that of controlling the major camber angle of the rear strut through reduced angular movement of the wishbone away from the horizontal.

Then you may also start to think about rear-wheel steering effects as the suspension moves up and down (well, I do, anyway). As if the track width changes at the hub carrier mounting, but not at the front tie bar mounting, then toe-in or toe-out may also be introduced by these track changes. It all gets rather messy trying to consider all of these implications after a while. Makes you think that having no spring travel is one of the easiest solutions to it all!

For more geometry considerations, on the Hawk chassis, you can think about using a higher or lower mounting for the front end of the rear tiebar, in conjunction with the chosen mounting for the inner end of the rear wishbone. This forms a large-based triangle and if this is higher or lower at it's front point it can lead to anti-dive or anti-squat characteristics - in theory, even if this isn't going to be noticeable in practice.

[mogul_x]

:confused:

John,

I've read and re-read that post. I understand the theory (I think), but I do have one question - which mounting holes are you using?

I've been told to use all of the lower mounting holes for attaching the suspension. This is probably fine for a road car like mine, although a little extra negative camber never hurts.

Are you using a combination of upper and lower holes, or have you revised the mountings entirely?

[rutthenut]

Originally posted by mogul_x
[BI've read and re-read that post. I understand the theory (I think), but I do have one question - which mounting holes are you using?[/B]

Hmm, yep - a short question, a long reply from me, but no direct answer. That sounds fairly normal!

As to the direct question, I am using the higher mounting points of the pair available for the inner wishbone location. Same too for the tie-bar, where that meets the chassis.

I've juggled around with these at different times and I would think that the lower points are only of use if setting up a higher ride height and increased ground clearance.

[mogul_x]

John,

On the front suspesnsion, I think the choice of mounting holes has a more pronounced effect on ride height.

I think the lower hole on the front suspension would be used for a lower ride height, and the upper would be used for increased ground clearance.

Of the two holes in the front lower control arm, which do you attach the spring / damper to - the upper, or the lower hole?

[rutthenut]

Originally posted by mogul_x
On the front suspesnsion, I think the choice of mounting holes has a more pronounced effect on ride height.

I think the lower hole on the front suspension would be used for a lower ride height, and the upper would be used for increased ground clearance.

Of the two holes in the front lower control arm, which do you attach the spring / damper to - the upper, or the lower hole?

Yep, the front mounting points have a direct relationship on ride height and much less on the geometry.

I'm using the lower mounting points at the front, which does give a lower ride height. But that really is a combination of spring length, platform height and the chosen mounting. So it would be possible to use the upper mounting with a shorter spring and still get a lower ride height.

The only change relating to geometry will be the angle of the spring/damper unit when compared to the lower wishbone (and the upper mounting point). Using the upper mounting point would decrease the angle and so make the 'decreasing rate' of the suspension that bit worse. But the change isn't going to make all that much difference, this another case of theoretical math and perhaps less in the measurable world of driving the car. This has already been discussed with possible differences in the top mount position of the spring/damper unit anyway.

As usual, this leads me on to yet another area of related suspension activity that you may want to consider. That is the possibility of having pre-loaded spring settings, with the spring seat being used to compress the spring by a certain amount in the stationary position.

One initial effect of this would be to extend the damper rod until it reaches it's fully extended position as well, which is then counteracted by the vehicle weight when the car is on the ground. I've not played around with any of the maths on this as it seems even more involved than the other areas I have looked at. This technique is probably more useful in race cars (single seaters, especially) and can improve the turn-in aspects of the vehicle by changing the dynamic change of weight transfer on entry to a corner.

Anyway, I have ignored that since it requires more in the way of specific damper rod lengths, spring lengths, predetermined ride height settings and so on, as well as the decision on static spring rate against the preloaded rate. I just thought I'd throw it in to see if anyone else has comments on this setup technique.

[rutthenut]

A couple of other points I recall about the lower front suspension arms, which may appear as a 'gotcha' when you set yours up.

This is from memory when I first installed my suspension with Spax shocks, so may be different these days with alternative (better) dampers and I also believe that the suspension arm construction has been modified slightly since then.

First problem was when using the lower mounting point for the shocker, the damper body could clash with the edge of the suspension arm when on full droop. Solution, if this is still a problem, is to file/grind away a slightly larger radius on the upper edge of the arm, where it may meet the shock body.

I've a feeling this may not occur any more, so let me know if it does still crop up with the latest shocks and arm fabrications.

Second issue was more to do with the Spax adjustables, as the lower adjuster was a slotted screw that was to the side of the unit when mounted. This meant that it was inaccessible once fitted into the suspension arm. Solution for that was to drill a small access hole so that a screwdriver could be inserted through it to turn the adjuster.

This is not a problem if you are using the AVO or later shocks that Gerry supplies, or any others that put the adjuster at 90 degrees to the mounting hole at the base of the shock.

Besides those possible glitches, if you have the arms that use rose-type joints at their inner edge, this gives you two areas of adjustment for camber at the front wheel, with the upper and lower wishbone lengths being adjustable. Just make sure that there is enough thread engaged in the end of each arm for safety purposes.

When I wound the camber to a maximum of 4-1/2 degrees, I had to thin down the locknut on the top wishbone, to allow the rose joint to be screwed that bit further into the wishbone. That shouldn't be an issue for most people though as more normal camber angles are easily obtained with the available adjustments.

That was an excessive amount of camber that I went to for a while to get the front slicks to work across their full width - which was a problem made worse when using soft (i.e. road spec) spring rates that gave me plenty of body roll and ruined any camber angles set up on both front and rear wheels. Stiffer springs improved the body control noticeably and allowed the camber to be set up to more sensible levels. With road tyres there is less grip and lower speeds, hence less body roll due to cornering forces.

[chris.richard]

Wow, I'm going to have to read that lot several times! That'll teach me to ask questions!!

[roger001]

Decisions, Decisions!

However if you have an early Transformer chassis there is only one mounting point, which makes for one less variable to worry about!

[mogul_x]

Originally posted by rutthenut
First problem was when using the lower mounting point for the shocker, the damper body could clash with the edge of the suspension arm when on full droop. Solution, if this is still a problem, is to file/grind away a slightly larger radius on the upper edge of the arm, where it may meet the shock body.

I've a feeling this may not occur any more, so let me know if it does still crop up with the latest shocks and arm fabrications.



Tried installing mine over the last couple of nights, and the problem still exists. It seems to have improved, given the fact that only one of the control arms clashes with the Shock.:confused:

On my car at least, the left hand assembly goes together with no problems, but the right hand clashes at full droop. I estimate that I need to remove 1-2mm of material from the upper edge of the arm to correct the interference.

By the way, my front shocks are the Gaz units.