instability

[roger001]

it seems to me that a often found fault in stratos handling is straight line instability, which vanishes when under load, my basic understanding of geometry leads me to think that this is due to varying rear wheel toe in angle when under load or not, presumably contributed to by the rubber bushes in the lower wihbone/tie bar assembly, one answer is to upgrade to the later type wishbones with better bushes(just done), the other is to replace the forward tie bar bush (rubber) with a sperical joint, has any one tried this and if so does anyone know the dimensions of the original rubber bushes, also used at the inner end of the front wishbone.

[rutthenut]

Roger,

I remember John Whalley claiming issues with stability at high speed on his Strat some years ago - allegedly it would need about two or three car-widths of track (i.e. all of it) when travelling at 120mph and above.

I've not found this at all on my car and I think he really just wanted to point out that he has travelled at that speed (haven't we all).

Anyway, I take the few that the Stratos is inherently unstable due to its design - short wheelbase and mid-engined weight bias - which is what was wanted for rallying. The short wheelbase does mean that maneuvres at high speed can be a bit more 'touchy'.

I hadn't really thought of the problem as being related to power on and power off situations though, but there certainly will be differences in that area.

Another area for consideration is just the airflow under/over the car, which can lead to the front lifting at speed and directional control being reduced perhaps due to geometry changes as well as the lesser weight on the front wheels. At low speeds, the weight transfer fore-and-aft is appreciable, another corollary of a short wheelbase and a relatively high centre of gravity. This weight transfer may be some of the cause for stability changes when on and off the throttle.

I feel that my car improved a small amount in respect of airflow management simply by putting an 'undertray' in the front 'under-bonnet' area, so most radiator air now exits up through the louvre panels rather than going under the car or out of the wheel wells.

Back to suspension mountings, my car has always been fitted with rose joints at both ends of the front and rear tie-bars, with the rear wishbone having a rose joint inner and nylatron type outer bushes. The front upper arms have nylatron inner bushes too, with the lower arm having a rose joint mounting at the inner end.

The front tie bar mountings can contribute to instability under braking if they aren't too solid, as they get put under a lot of stress when knocking off a lot of speed.

If you have a look at many standard road-going boxes (and Strat's with softer bushes) you can often see evidence of bush compliance by putting your foot on the brake pedal and rocking the car - the wheels can often be seen to change toe angles. If this happens at high speed, it will certainly cause undesirable geometry changes. Again, this has more effect on a short wheelbase car such as the Strat.

I guess the same is true of the rear suspension, but the front gets extra loading under brakes, whereas the rear is not going to get quite the same amount of extra force under acceleration (unless you've got a real monster of an engine in there!).

One area I have found to lead to major instability in both straight line and high-load cornering is if the bushes on the rear hub carrier have any play at all. Whenever there is any 'give' in these bushes, the toe angle of the wheel is affected dramatically, which has much more effect than any play in the tie bar, due to the triangulation of the lower suspension arms.

The lower rear bushes are a form of nylatron, but I did have a set that went soft, possibly due to grease contamination from the bearing/CV joint above. I wouldn't have expected the bushes to suffer from this, but it certainly made the car a handful to keep on track until I replaced them with newer bushes. Think of the loading on these bushes when cornering hard with a slick-shod car, as the weight of the engine is right there too. The power and torque transmitted through the driveshaft are miniscule in comparison to the loads that can arise with high speed cornering on a high-grip surface.

The design of the rear hub carrier on the Hawk mimics that of the Stratos, with the (reversed-A) wishbone outer points being bolted through from the outer edge into the hub carrier. This does mean that each of these two bolts are effectively in a single shear type of mounting, rather than the preferred double-shear style, which would require an additional outer brace - for which there is no room inside the wheel. If it's all torqued up nice and tight, this isn't so bad, but you do have to keep an eye on things in this area. It does however put more stress on the bushes and I feel that even the nylatron units have more play than I would like in this area.

Stick a jack handle [carefully] through the spokes of a wheel and lever on it in a horizontal direction to view the toe angle changes possible at the wheel. Then look closer to see where the play is evident - most likely to be the rear hub carrier bushes, but the tie bar mount may also contribute to geometry changes here. As I've got r/j mounts in that bar, I don't have issues with that. Maybe that has more effect than I thought, hence your question about upgraded bushes!

I've considered changing the hub carrier mountings, or at least the bush construction, to generate more effective control. Using bronze or other metal bushes is an option, but would no doubt wear terribly - making a worse problem - and may generate too much friction for good suspension action, so that idea is probably out. Using a larger metal sleeve in the nylatron bush should be easier, so having a smaller plastic area in the mounting point, so having less scope for flexible travel. A rose joint outer would be better still, which would require wishbone modifications at the outer end, so has to be considered for the future. At the moment, I think that keeping a check on the state of the bushes and the security/tightness of the mounting bolts will continue to work ok.

I don't have any measurements that would help you in changing your suspension bushes, but consider that the rose joint fittings may now be classed as standard on the HF3000, so you could be allowed to fit them for use in the production class.

Think that's enough woffle on that topic from me!!!

Cheers,

[chris.richard]

my brain hurts now!

[rutthenut]

Originally posted by chris.richard
my brain hurts now!

Hmm, it did come out to be quite a long post didn't it :rolleyes:

Hopefully it isn't all complete cr@p, as it resembles the thoughts that go around my head :eek:

[Matt P]

well you said it all appart from one thing i have found when driving customers cars. that is the stearing rack they fit are normaly to quick. a normal road rack would do but when a 2.5 trun lock to lock item is fitted it makes the cars feal very twitchy indeed. the cars dont need a great deal of lock because of the short wheel base.

i hope this adds a little
matt

[GMC]

Well done John, I remember you also wrote an article recently on "Spring rate for competition". Did you ever think of writing a book ? A best seller within the Stratos Club ?

[rutthenut]

Originally posted by GMC
Did you ever think of writing a book ? A best seller within the Stratos Club ?

A best seller?
It'd cost me a fortune in paying everyone to take a copy!
:eek:

[David May]

On the subject of those nylon lower wishbone bushes - does anybody know what is the best maintenance for them. I've heard that some oils and greases effect them. Should they be just left to die dry? How long do they last?

Dave M