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Z Cars rear suspension 
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Joined: Sun Oct 19, 2014 10:10 pm
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Post Re: Z Cars rear suspension
Really interesting analysis Spider, some which I even understood! I've been following this thread and others on the rear suspension for a while and have read many of the earlier threads from Team GTM too. It seems to me that in theory the GTM design is a great idea and should work really well. Unfortunately in practice it fails to consistently deliver on the promise. There seem to be a few reasons for this that include variability on the bulkhead strength (possibly more of a problem on Libras than Spyders?) leading to flex, a degree of give in the trailing arms themselves but in my opinion the biggest problem is that there are too many links in the chain - just count how many ball joints there are, and whilst they might be okay when they are all brand new even a small amount of wear in each joint adds up to the potential for quite a bit of movement. Given the length of the trailing arms and the direction that the forces are applied to them in cornering small amounts of slack in the joints compound up to noticeable changes in the geometry. The idea to give dynamic camber in bump to compensate for roll is great until the taking up of all the slackness in the many components pushes it back the other way!
I've seen pictures of my car on track where it is clear that the negative camber at rest has become visible positive camber under load and the same movement will also be tending towards the toe coming out too which both add up to a spin! The worst aspect of these changes due to the "slop" is the unpredictability as the changes build up with load and weird thing happen.

I've been improving mine by replacing worn joints and tweaking the geometry at rest to compensate for what's left. My bulkhead seems pretty stiff so I'm definitely not getting the flex others have described and I don't see any significant movement even when using a big bar on the wheel so I think mine is getting towards as good as it can be on the original set-up but I can definitely see why people would want the Z-Cars, if nothing else for consistency. I think the benefit you'll see depends on your starting point - could be a massive difference or maybe not that much. I hope I'm in the latter bracket and as such I can't justify spending the money but maybe one day! Meanwhile I'll go down the ARB route next as I think that will give a bigger return on investment!


Mon Jun 12, 2017 1:54 pm
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Post Re: Z Cars rear suspension
Thanks Sanzomat. I agree with everything you say about the standard suspension. I am very encouraged by your bulkhead stiffness especially since yours is Spyder 015 and mine is 030. (Hopefully that stiffness has carried through) Likewise there is no visible camber change when I heave on a wheel. On my to-do list is to apply some serious "cornering" load while monitoring camber with a digital level. The easy way would be a winch or ratchet strap between the bottom of the two rear wheels. I even have a 100 kg spring scale to measure the force. The other thing I would like to measure is the deflection at various points so I can see exactly where most of the compliance is. If the bulkhead is the main culprit, a brace along the lines of the Z Cars cross-tube would help a lot. It would also be nice to find an alternative for the four bearings/bushings. I suspect rod ends would be no better wrt wear-rate and play.

The nice thing about the Z Cars suspension is that bulkhead or rod-end deflection in the plane of the bulkhead is needed to allow camber compliance whereas the GTM system changes camber by flexing the bulkhead perpendicular to itself - a much weaker mode. Add to that the amplification of forces and deflections inherent in the geometry. I did a rough calculation and compared to a double wishbone system with equally soft attachment points, the GTM system has about 10 times the camber compliance.


Tue Jun 13, 2017 3:58 am
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Post Re: Z Cars rear suspension
Be really interested to hear how you get on with your experiments Spyder. I'd try and see if you can measure the toe change whilst you're putting load on the suspension as this was always highlighted (rightly or wrongly) as one of the major problems.

Have you got access to the TeamGTM archives as they did a lot of research into the rear suspension on there and it might be useful for you?

Also, rose joints have been fitted to the original suspension but its really difficult and the arms ended up having to be remade. The problem is the unusual thread size (11/16" I think) - hardly any rose joints are made with this thread. I looked into it and the only ones I could find were in America and not really suitable for heavily loaded suspension. They would be a much better bet than the TRE's if you could find a way to make it work I think.


Tue Jun 13, 2017 11:50 am
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Joined: Sat Oct 22, 2016 12:41 am
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Post Re: Z Cars rear suspension
Jaykart1227 wrote:
Be really interested to hear how you get on with your experiments Spyder. I'd try and see if you can measure the toe change whilst you're putting load on the suspension as this was always highlighted (rightly or wrongly) as one of the major problems.
I wouldn't expect much toe change. Just looking at the geometry of the outside wheel during cornering, the lower ball joint should move directly inboard, the upper BJ outboard (not as much) and the effect on the toe link would be minimal. I will definitely try to measure it though.


Wed Jun 14, 2017 12:09 am
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Joined: Sat Oct 22, 2016 12:41 am
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Post Re: Z Cars rear suspension
Jaykart1227 wrote:
Have you got access to the TeamGTM archives as they did a lot of research into the rear suspension on there and it might be useful for you?
I have subscribed to that forum but haven't been able to dig up any old threads with the search function. Can't find an archive area either. Do you need to be a paid-up member?


Wed Jun 14, 2017 12:48 am
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Post Re: Z Cars rear suspension
As far as I know you don't have to pay on that forum (or perhaps donations are welcome as the person running it is doing it at his own cost I believe) however to access the archive you either had to be a paid member of the old version of that forum (where the archive came from) or own a car that belonged to a previous paid member.

I guess to explain a little, it used to be a different forum run by others and you had to be a paid member to access it. When the forum collapsed for one reason or another it was agreed the huge archive of information shouldn't be lost and someone set up a new forum to host it (at great personal effort i believe). it was generally agreed that to be fair the archive shouldn't be made public, as it wasn't originally, hence the reason it can be accessed by previous paid members or future owners of their cars.

However a lot of fiberglass has passed over tarmac since then so I guess you should be able to get access if you ask nicely! :D
Your car isn't Kels old car is it? He was on the old forum i believe... :wink:


Wed Jun 14, 2017 9:33 am
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Post Re: Z Cars rear suspension
Spider wrote:
Jaykart1227 wrote:
Be really interested to hear how you get on with your experiments Spyder. I'd try and see if you can measure the toe change whilst you're putting load on the suspension as this was always highlighted (rightly or wrongly) as one of the major problems.
I wouldn't expect much toe change. Just looking at the geometry of the outside wheel during cornering, the lower ball joint should move directly inboard, the upper BJ outboard (not as much) and the effect on the toe link would be minimal. I will definitely try to measure it though.


I don't think the toe out issue was related to the geometry on bump or rebound, more the compliance of all the various joints and components added up to cause the effect, I think the toe control bracket was identified as a possible source of flexing but its been a while since I read through it all on TeamGTM. It would be worth trying to get access to the archive as per Libras post above, some useful info on there including some very in-depth studies completed by a Jaguar suspension engineer.


Wed Jun 14, 2017 10:12 am
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Joined: Sat Oct 22, 2016 12:41 am
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Location: Brisbane Australia
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Post Re: Z Cars rear suspension
Thanks guys. I will ask for access to the archive.

Libra 134. I bought my car unfinished (all but finished) from a guy here in Australia. He imported several Libra kits and one Spyder to build and sell on. The demise of GTM put a stop to that.


Wed Jun 14, 2017 1:02 pm
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Post Re: Z Cars rear suspension
wrt getting to the Team GTM archives, Libra 123 is spot on. I asked nicely and was told it had to be car that had previously belonged to a paid up member of the old site. I found out that the builder of my car had been a member (although it was on a different registration so had to use the chassis number!) Having found out the name of the builder it was interesting to read the posts from when the car was being built and tested and find out a bit about the history of the car. Still worth asking though. Craig seems like a nice guy.


Wed Jun 14, 2017 1:19 pm
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Post Re: Z Cars rear suspension
So, I finally had some time to work on the car yesterday. Been itching to get at it since picking the car up from z cars last weekend.

Firstly, I'm really pleased with the kit and it looks great - frees up a load of room under the rear clam as well and looks 'right'. As you know z cars had my car so they could test fit the kit and check it all went together ok and the suspension had been re-fitted when I picked it up but only loosely to get the car rolling etc. I spent some time yesterday checking things over and looking into a couple of minor issues that z cars made me aware of. For those of you who are buying a kit I'll go through them below + some things you need to be aware of:

1. Engine steadies - The engine steadies (2 No) have been made so they are now adjustable with a 16mm threaded section providing the adjustment. Having fitted it all together its apparent that the bars are fractionally too long so that, when in the approximate position, you can't get a half nut on the thread to lock them. Its an easy fix and the bars need to be fabricated a tad shorter to provide a better range of adjustment. I've spoken to z cars about this so future kits will incorporate this change (mine will go back ti be altered). Mine is a k series car of course, I don't know what the arrangement is for other engines?

2. Bearings - The new kit uses MGF / Metro 100 / Lotus Elise sized wheel bearings instead of the previous ford offering. This obviously makes things a lot easier as existing drive flanges etc will fit without needing a machined sleeve etc. Be aware that the kit doesn't include new bearings and they require a press to fit the bearing to the arms and the drive flanges to the bearings. I supplied new items and got z cars to fit them for me - much easier.

3. Driveshafts - Once z cars had fitted the new wheel bearings and bolted the kit on, we had a slight issue with the driveshafts binding up when we tried to torque up the hub nuts. After checking things out, we concluded that the outer CV joint body was just making contact with the back of the housing where the bearing is pressed in. It is only just making contact and needs a small spacer on the back of the driveshaft before assembling to sort out. We didn't have anything suitable to space it with when I was at z cars but in the week I ordered some 30mm type A washers which do the trick with a bit of fettling. I'll take some specific photos of this and post them later on so you can see what is required. Its an easy fix, fit a spacer to the driveshaft and away you go. I fitted the spacers yesterday and tried the suspension in its full range of movement and there are no issues with them binding or bottoming out at all.

4. Dampers - I had a better look at the space for the damper yesterday to see what sort of clearance there is to the engine frame with the new set up as it was marginal with the old suspension. Bare in mind that I have not set the track width properly yet ( it could be slightly out either way) and have set the negative camber to the middle position (which looks far too much), the damper is too close for my liking on the engine mounting side. I don't think this is a symptom of the z cars set up as it was the same before but I'm going to grind away and slightly modify the engine frame to give decent clearance. Again, I'll post some pictures of the frame when I've done it. This isn't a problem for me as I'm taking the frame off to be powder-coated anyway but something to bare in mind for those of you just bolting the kit on to running cars. I'm using protech shocks and other types may not be a problem. I suspect that with proper suspension settings the clearance may be greater but I'm going to take the opportunity to make sure whilst I can.
The protechs are quite thick at the bottom end compared to Gaz shocks so the damper body comes quite close to the tyre but with the 5mm wheel spacers I run there is adequate clearance but if you are running protechs with a wheel and tyre bigger than mine (see below) you might need some thicker spacers. This is dependant on final suspension set up of course so may vary when the suspension is properly adjusted.

5. Wheel / tyre clearance - I'm running 7 x 16" wheels with 215/40/16's so a standard set up. Without spacers the tyre comes within 5mm of touching the arms so wider tyres would be an issue. However my car runs 5mm spacers all round anyway and with these fitted theres plenty of clearance. Keep in mind you will probably need spacers if running a wider rim or tyre than my set up but thats no problem.

Some pictures below, hopefully you can see what I'm getting at above. I'll post some details of the driveshaft spacer later. Very pleased with the kit in general - can't wait to see how it performs.

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Sun Jun 18, 2017 8:48 am
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Post Re: Z Cars rear suspension
I think Protech use bigger diameter springs (2.25"ID) than the original Gaz/Avo ones (1.9"ID), so that'll be why the engine frame clearance is less.

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Sun Jun 18, 2017 10:27 am
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Post Re: Z Cars rear suspension
sidewinder wrote:
I think Protech use bigger diameter springs (2.25"ID) than the original Gaz/Avo ones (1.9"ID), so that'll be why the engine frame clearance is less.


Yes, I think the early ones did but my protechs replaced a set of Gaz that were on the car when I got it and they have 2.25" as well. A bit of judicious fettling will sort it out :D


Sun Jun 18, 2017 1:53 pm
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Post Re: Z Cars rear suspension
Looking good. Do you know the offset of your wheels - a smaller ET number might not need the spacers?

Setting the track wider (presumably with different spacers either side of the rose joints?) might improve the clearance from both tyre to shock and shock to engine cradle as the shock would then move out at the bottom but fixed at the top so lean over in its plane a bit?

My Gaz have 2.25" springs and at the top I have to rotate the spring so the stiffener on the cradle fits into a gap between coils or it rubs slightly (pulley end) which is far from ideal but doesn't seem to be an issue.

Bet you can't wait to get it on the road but if you still have to get the engine frame off and coated I'm guessing its a while yet - hope the good weather hangs around long enough!!


Sun Jun 18, 2017 3:52 pm
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Post Re: Z Cars rear suspension
My wheels are et25 so they are marginally further out (wider track) than standard but not a lot in it. The spacers will have to stay as without them I think the tyre is a bit too close to the arm for my liking but as you say if the arms are spaced a bit wider on the brackets it would help a bit, I've not set the rear track properly yet so that may be an option - I've got the car on my flat patch in the garage now so I'll be able to take some accurate measurements and get the track right and see how it is. It has just enough clearance as it is though so any more will be a brucey bonus.

I've conceded that this summer has gone for me now but its got to be all together again this year for sure!


Thu Jun 22, 2017 8:39 pm
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Post Re: Z Cars rear suspension
anyone have a update on how the kits are going ?


Thu Jun 29, 2017 1:41 pm
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