GT6 Roto. Rear Shocks. Comment invited.

[chrishawley]

The worst aspect of my GT6 has been sorting out suspension and handling problems. Most things are now acceptable except an extremely unpleasant ride from the rear. Using chassis brackets I'd tried various shocks, but with no good result. 'Short' SPAX were too hard even on the lowest setting. Standard 'Spitfire' shocks gave overall acceptable ride but with periodic 'spinebuster' jolts over rough conditions. Adapted Mini front shocks were the worst - rough and with an uncontrolled rear end and spinebusters now and then.

Following no particular logic I took a punt and removed the chassis extension brackets and installed 'full length' SPAX i.e. mounted to the inner wheel arch (£££sx). Result: Pretty satisfactory. Tadge vague at the rear but handles bumps and irregularities comfortably.

So what's a reality check on this? What makes the SPAXs better? The chassis extension brackets force the shocks to run off perpendicular. Whereas the SPAXs are square to the axle movement. Relevant? I also noticed that even with the SPAXs on full extension the hub needs to be jacked up by a good half inch when fitting. i.e. the bottom stop of the shock is holding the half shaft well clear of the chassis. So is it really the clearance betwen the half shaft and the chassis as a critical factor?

Reality check on that?

In practice I have an acceptable situation. But the SPAX has 24 possible settings. The blurb says says the user should adjust up by 4 and down by 1 untill the best ride is achieved. But doesn't give any criterion for what 'best' might be. Any opinion of what 'best' looks like on a GT6?

 

[Clive]

When the shock is angled onto the original (spitfire etc) mounting point, it effectively reduces the effectiveness. Using the extension brackets are halfway. Vertical the shocks are most effectice (stiffer?)

I have a rotoflex setup on my spitfire, but CV shafts, means I can use the original chassis points. But mine is not vague.

I would be wary of not having full droop available, that could cause an issue if it runs out of travel. It won't happen under normal circumstances, it may happen when other things start to go wrong, making it all worse.

You could make some new extension brackets to suit the shocks, that may work. Otherwise there is s beetle shock which is just about identical to OE rotoflex shocks.

As to setting shocks up, start soft (1/4?), and gradually harden until you go too far. Then back a bit. It is rather all trial and error, which is why adjustable shocks are often pointless. You will know when they are too hard, the car will get skittish on cornering. 

[NonMember]

The 'spinebuster' effect is caused when the shock doesn't have enough compression movement available. Spitfire shocks are designed for a slightly longer fitment than you get with the chassis brackets. There's not a lot of movement available on the GT6 rear suspension so you can't afford to lose any of it - including the droop, as Clive says.

[chrishawley]

Thanks both.

I'd hypothesised initially that the spinebusters were the leaf spring hitting the rubber stops on upward deflection. But on inspection there's no witness of any contact between the buffer and the spring. So perhaps that suggests the 'short' shocks are hitting bottom on upward deflection.

But I've never had to think about this in detail  before, but what controls (maximum extent of) downward deflection of the axle? If I jack the rear end, with shocks removed,  then both half shafts are sitting on the chassis. So are my spinebusters as much due to downward axle deflection with the half shafts making contact with the chassis?  But with the 'long' SPAX I'm not getting spinebusters (at least on first test drives). So as opposed to my previous mickey mouse arrangements is it simply the case that the downward deflection is restrained by the dynamic resistance of a correct shock that it actually up to the job?

More experimentation today!

 

[Mark B]

The sudden jolt sound like the shocks are bottoming out, fully compressed, before full droop. In the past I have marked the shock whilst off the car, fully compressed and fully extended with a  marker. When fitted you can then get a good idea of how much travel you have. Need to also test with a passenger and luggage, a little extra weight can make a big difference to the amount of shock travel available.

[Pete Lewis]

this is the must have problem thats is forever raising its head on supposed upgrades

we just dont have the chassis dynomometers used by manufactures development so you pay out a fortune but there is no plan other than trial and error 

in setting the thing up, as they cost a lot of ££s very few will admit to its worse than the orrible black things  if they are blue you stand a chance 

but how you can tell the bump and rebound forces are working correctly is not for the faint hearted and you are forever getting under to tweak the settings

which change with the loading of the car 

not putting a downer on these but so many fit the nicely boxed and  bright units and 200yds down the road wonder what have i done 

one trick if its bottoming out is retrive the bump 0 rings from inside the top can and cut and remove one   there can be up to three fitted 

pete

 

 

 

[Gary Flinn]

Chris

I've noticed you have queried the Mini damper conversion previously, what make of Mini damper did use and are you sure they where the correct open/closed lengths?

The Koni 80-1717 worked well on my Mk2 Vitesse and are the correct Open/Closed lengths 

Gary 

 

[chrishawley]

3 hours ago, Gary Flinn said:

I've noticed you have queried the Mini damper conversion previously, what make of Mini damper did use and are you sure they where the correct open/closed lengths?

The Koni 80-1717 worked well on my Mk2 Vitesse and are the correct Open/Closed lengths 

I don't think I'm sure of anything!

As I understand this particular point: for Koni 80-1717s open 292mm/closed 216mm. What I had were NOS Uniparts(GSA 130 I think) with a range 325/215mm. I note that the TSSC option for roto-with-brackets (Koni gt42270) has a very short operating range of 309/261mm

I suspect that I've entered a area of naïve whopperhood in basing my considerations on static dimensions with (as Pete points out above) little account of the dynamic nature of suspension behaviour.

I was kind of assuming that since the Konis worked for others then the Unipart equivalent would have the same general characteristics - at least to give me some clues. What I did not expect was an uncontrollable rear end knocking and hopping around like nobody's business. 

Only just occured to me there may be another factor. The chassis brackets I've been using are those that came with the car when I bought it. But these are straight and fairly short unlike the TSSC ones with the longer extension canted upward. I can picture that that may easily make an inch worth's of difference to the relative movements.

I can see I'm going to end up settling on the full length SPAX as being the best option. Perhaps these are proving better simply because they're similar enough to the OE setup and not having a muggins (me!) in the middle thinking he knows better!

 

[Ian Foster]

Chris

I use the Koni 80-1717s on my Mk2 with a bespoke bracket, which also corrects the fore/aft alignment (as posted previously). We we rebuilt the car in 2010/11, we went away from the arch mount due to significant corrosion and fatigue issues.

As stated the Konis have and open/ closed length of 292/216 which works well. The units are adjustable but must be removed from the car make adjustments.

I use the dampers with a spring that we have uprated by adding an additional leaf in the centre (now 7 leaves) and the ride is actually really good now.

Ian

[chrishawley]

Another round of swaps and substitutions: End result is that 'long' spax (i.e. mounted to inner wheel arch) produce the only effective result. At a '1' setting the back is wallowy; at '8' it's stiff and hoppy. So I've settled on '2' as the working compromise.

So that works but I still don't see why the chassis bracket shocks were so unsucessful for me. is the something about shock operating at an angle to line of force that enters the considerations?

[Gary Flinn]

On 03/08/2022 at 18:35, chrishawley said:

Another round of swaps and substitutions: End result is that 'long' spax (i.e. mounted to inner wheel arch) produce the only effective result. At a '1' setting the back is wallowy; at '8' it's stiff and hoppy. So I've settled on '2' as the working compromise.

So that works but I still don't see why the chassis bracket shocks were so unsucessful for me. is the something about shock operating at an angle to line of force that enters the considerations?

Chris

Maybe you had some faulty Mini dampers with the Chassis extension brackets?

As I've said, the Koni Classic 80-1717 gave an excellent ride on my MK2 Vitesse

It sounds like you've got your car riding Ok now so tick with the SPAX Dampers using the Wheel arch top mounts

Gary