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MG Midget and Sprite Technical - Check-strap advice

Hi all,

I'm in the final stages of designing and making a coil-over rear suspension system and I'm about to order some GAZ units. Having spoken to and e-mailed the GAZ technical department I'm somewhat unsure of some of their advice because a few of the dimensions they supplied on the phone did not make any sense when on the drawing board and disagreed with their written data. So while I have a query pending with them regarding rebound and bump loads I'd value any advice from this forum.

This type of conversion is not one I've designed before and my question is this: Are cheaper coil-over units generally designed to take the full bump and rebound (droop) loads or is it usual practice to include original (or similar)bump-stops and check-straps?

On race cars we did not use either but the coil-overs were expensive and the suspension had very limited movement. For this midget road car I'm looking at 4" of travel, soft-ish suspension and the possibility of damage if a unit fails on rebound. I'm inclined to 'belt-and-braces' to limit risk but there are huge geometric advantages (space is tight and I have a good notion where I'm going to mount the units) to not having to use extra bumps or checks. BTW, the coil-overs have built-in bump-stops but so far I have no elasticity or squish data for them.

All comments appreciated,

Nick

Nick Nakorn

Nick...

Try a website called mg experiance, there is a guy building a mg race car and he has coil over shocks

I dont know of anyone here working on coil over shock suspension

The problem with this is, the rear suspension is really well done from the factory minus a panhard rod

What most people complain about when modifying the rear suspension is itbecomes to hard and people break there teeth or bruise there brains with in a short drive

You will.want to make sure the shocks are adjustable, most that have gone to tube shocks turn them upside down and run them at there lightest settings

Take photos and keep us posted

Prop

Prop and the Blackhole Midget

my mate has used Gaz on a road car that also very occasionally goes on informal track use (not a Spridget and not racing) he thought they were good, he spoke with a director

as long as you give then the full information of your vehicle and stress the use of the car they should be able to work out their units

ETA: is Simon from Northamptonshire using coil-overs(?)
Nigel Atkins

Cheers Prop,

I think I've seen the project you're refering to some time ago. I seem to remember he had some expensive looking coil-overs that are probably a lot stronger than the one's I'll be using.

It'll be a while before I release any pics of the completed suspension because I want to make sure it all works as planned first - and that will be a while because I'm still redesigning the bodywork!

I did have some 'work-in-progress' pics but lost most of them when my lap-top went wrong. If I recover any from the back-up I'll post a few.

regards,

Nick
Nick Nakorn

I'd have to check but I don't think my road car has as much as 4" of travel in the rear suspension. It does retain bump stops but has no check-straps.

At one time Peter May retailed a kit to convert from leaf springs to coil overs and probably he still has a jig for it, or measurements.
Daniel Stapleton

Hey nick,

Here is a you tube vid. I think its the same guy on the mg experiance I was mentioning

Reconcider posting pics, this site is really geared for modification work, we have alot of members that are engineers by trade.

You can get alot of good insight, provided you have a drivers manual and dont mind a little ribbing

But yeah.. id love to see what your working on and I know others will be interested as well....not to many purist here

Prop

http://www.youtube.com/watch?v=_o5l3iP-zB4&feature=youtube_gdata_player

Prop and the Blackhole Midget

IIRC I have about 5" of travel on the rear of my frogeye, an increase from standard, and long ago ditched the lever arm dampers in favour of telescopics. The telescopics I used could function as check straps as they did in their original use and will have internal buffers to prevent shock loading, as they can function as check straps the original straps were removed, no problems experienced. The original frogeye bump stops were changed for 1/2 elliptic ones to provide a more progressive stop action. You could check with GAZ but I would expect any coil over shock with bump stop to also provide a rebound stop action unless specifically mentioned that they don't.
David Billington

I fitted Gaz Golds to my ZS180 racecar a few years ago, first time out under heavy braking the fronts bottomed out and soon smashed the internal end stops and valves. Gaz admitted they calculated the piston rod length wrong and fully rebuilt them, ISTR they took over an inch off.
David Smith

Hey guys,

many, many thanks for all the useful information. David, my fear is that a bumpy road could do the same for my units. Hopefully GAZ will get back to me soon. I think I'm probably going to keep the original bump stops and re-locate the check straps. It's more work with bracketry but well worth it. Hopefully the ride and handling will be much improved and the most of the bump-steer banished!

I've checked my back-up drive and sadly I've lost nearly all the pics of my suspension build but, though I have lost the 'in construction' shots I'll make an effort to re-shoot all the components in their finished forms.

best,

Nick
Nick Nakorn

Universal tapered bump stops made from closed cell polyurethane fitting over the damper shaft would seem to be a neat solution. You could slim them down in order to reduce their rate if needed.
Really looking forward to see how you get on.
Stuart
S BREEZE

Stuart, a good idea. It'll be a while till the car is on the road because of the extensive mods the car is undergoing, but once up and running quite a bit will become clear as various road-tests get underway.
Nick Nakorn

Nick, I did away with the check straps on mine with no probs. Its just a conversion to vertical telescopic shocks but the same principal. The shocks are some cheap no make Mini rears.
john payne

John, I think my set up might be more prone to breakage. If the suspension has limited rebound travel, as it would if the axle is bolted to a leaf spring, there's not too much risk if a damper breaks because the spring has limited travel. But if a coil-over unit fails on rebound the whole unit might disassemble and then the only physical item stopping the axle from drooping is the unit on the other side and the prop-shaft in the tunnel! True, it's unlikely that the car wouild ever be in a situation where the coil-over breaks and the car is high off the ground. But it could happen over a fast hump-backed bridge so I'm being cautious.

I remember many years ago in the 1970s I was driving my Triumph Herald in a spirited manner when the rear independent suspension 'tucked under' over the brow of a crested left-hand corner and the result was a bad spin and bent suspension; luckily I didn't hit anything and all was well - nevertheless it shows how if the geometry allows it it might very well happen..
Nick Nakorn

Nick, I'm no expert but surely cars like caterhams have a similar setup and I don't think they use any for of backup in case of breakage. I can see what you mean about the leaf spring setup though, in fact I did have a bracket break on my old shocks and the only reason I knew about it was the knocking noise!
john payne

John, good point - many run multi-link coil-sprung live axles.
Nick Nakorn

Here's a general shot of the links in situ - I've had the axle all connected to it previously (articulates very well) but lost the photos. As soon as I get some written stats from GAZ I'll buy the units and make all the mounts and brackets. It's frustrating having these delays as I want to get the underside painted once all the bracketry is finalised.

Nick Nakorn

One of the semi-trailing arms and its bracket.

Nick Nakorn

Looks good. I'd always thought a setup like that would require parallel links but I can see how you've done it without the need for the top link to protrude though the bulkhead.

Are you putting turrets in for the top shock mounts?

Make sure you keep all the measurements in case you want to market it as a kit!!
john payne

Hi John,

thanks for the encouraging comments. Until the car is able to be tested (a long way off) I'm only able to assume it will work as designed. No, I won't be turreting the upper coilover mount but will be reinforcing the structure from inside the car; it would most likely be okay without such modification because the 1500 shell is pretty beefy already but it won't add much weight. As for selling kits; I might be tempted if it all turns out as planned!
Nick Nakorn

It seems my belt and braces approach has paid off. I've just received this from the Gaz Technical Department:

"I have spoken to David regarding this & he said that the piston back plates on a normal 1.9” or 2 ¼” coilover is not designed to take too much load, the exact figures we don’t have but ours are basicly the same design as most of the other damper manufactures out on the market, A strut is different as this will have a check stop on the piston rod to protect the piston as these normally take the full droop lad but a small coilover is usually in a wishbone so not a lot of loading on them."

So not only are their coil-overs not designed for full rebound loads but they have not been designed for any particular rebound load compared to the designs of their struts. It seems that is normal for the industry. Incorporating a check strap in a new location will not be too difficult but I'm glad I made the enquiry.
Nick Nakorn

Nick,

That does seem an odd reply as plenty of cars use them and they are the only rebound stop provision. IIRC my mate recently fitted a full set of GAZ dampers to his Caterham and it has no additional check straps, maybe I should warn him his front and rear axle is in danger of falling off if he gets air born.
David Billington

David, if the front of Caterham is traditional double wishbones then it is likely that the full droop of the suspension is less than the full extension of the coil-over so no problem. Having looked at loads of pics on the net I note that, at the rear, live axle Caterams seem to have their trailing arm axle-mounts behind the centre-line of the axle so, again, it is likely that the full droop of the suspension is limited by its own geometry. In contrast, my set up has the lower trailing arm mount ahead of the centre line while the upper is behind and above the centr line: there is thus no built-in stop other than the prop-shaft hitting the bottom of the tunnel. I will have straps! :-)
Nick Nakorn

Nick,

I don't buy that as there is no physical stop on any of the suspension components front or rear other than the dampers. When you say "limited by its own geometry" are you indicating you think the trailing links may come into physical contact with the axle and act as a stop, I don't think that is the case as the loads would be tremendous to do so and would bend the tubing as it isn't that great in size and wouldn't take the bending loads to act as a stop. I can check with my mate but I suspect he will just indicate the dampers are fully extended and no other component is acting as a rebound stop.
David Billington

David, Not sure what you mean exactly but you may be right. I know for a fact that my geometry has no physical stop other than the prop-shaft and tunnel. I'm not concerned about front suspension (yet). If Gaz can not say what their load parameters are then I'm going to assume they might not be sufficient. Anyway, I've decided to go ahead with relocated straps.

Nick Nakorn

Nick,

Have you asked GAZ if their coil over dampers have an internal rebound buffer. Dampers that I have dismantled that can function as check straps have had one although much smaller than the external bump buffer but then the loads are much smaller as stopping the unsprung mass rather than the sprung mass.
David Billington

David, no I haven't but I've had conflicting info from them from the start concering other aspects of their units. At least I now have a 'don't know' in writing from then while the other conflicting info was part verbal and part written. The only other maker in my price range is AVO and they didn't reply at all.
Nick Nakorn

Now the coilovers have arrived I've made the two top brackets and am in the process of making the bottom brackets for the axle. Here are pics of the shiny new coilovers and one of the top brackets in situ prior to top coat of paint and drilling of floorpan.

Nick Nakorn

odd - uploaded two pics to above comment and only 2nd one is showing.. here's the first:

Nick Nakorn

I think you can only upload one pic at a time.

Nice shiny coilovers!

I think I would have welded in some beefy plates where the brackets are mounted but I do tend to go a bit OTT with such things!

Keep the progress reports coming!
john payne

John, I agree about the beefy plates. The reinforcing will happen on the other side; inside the car and will include a brace between each side. You'll also note that as well as the 1/2" diameter bolt being in double shear, the bolt is very long and continues through the 'chassis rail' via the 3/8" check-strap mounting hole. The long bolt being turned down to that size for about half its length and re-threaded. (I used two bolts welded together as a temp to make sure it all fitted but it'll be a proper turned item.
Nick Nakorn

Yes, the other side would be the best place, easier to weld for a start! I like the way you can remove the brackets in case of wear, repositioning etc.

I noticed the long bolt, similar to my initial tele shock conversion which broke, due to rust and a lack of decent bracket!

One thing I was going to ask was about lateral location. Do the semi trailing arms do it all or will you have to use a panhard rod?



john payne

John, the idea is that the semi-trailing arms take care of all the lateral location. If, in practice, it turn out that they are not sufficient then I'll think again but I'm pretty sure they'll work.
Nick Nakorn

One of the lower mounting brackets (axle) finished. the other one I'll weld up tomorrow. I'll mock the whole thing up on the car before welding them to the axle; even though all my drawings say it'll be fine I sometimes make mistakes!

Nick Nakorn

Hi Nick, Any updates on your rear suspension?
The lower damper bracket, could it be bolt on similar to the leaf spring fixing instead of welding it on?
Would be very interested in it if it became available.

Cheers Phil
Phil Lawson

Hi Phil,

the suspension is finished and fitted along with a 5-speed Type 9. The brackets are all bespoke and welded to the axle-case. Since then, I've been working on the body design and have started building it so I haven't had a chance to test and evaluate the suspension. But hopefully that will happen next year; I work slowly as I have health problems so I'm not rushing it.

By the way everyone; I fitted some straps in the end and made new mounting points for them on both axle-case and body. Well, belt and braces!

Cheers,

Nick
Nick Nakorn

Nick,
if you've used the usual modern made rubbish rubber straps then you might be well glad of your custom system
Nigel Atkins

Thanks Nigel. I hope the new straps are good, I bought them from Moss. They look like traditional straps but the extent of their reinforcement is unknown unless I cut them up!
Nick Nakorn

The Moss ones are reinforced
Paul Walbran

Paul, indeed - it was the extent of their reinforcement I was questioning.
Nick Nakorn

Nick,
there was a thread on check straps not so long back and IIRC(?) the different variety of materials they are available in
Nigel Atkins

Cheers Nigel. Well, the Moss ones are currently installed and I expect they'll be fine.
Nick Nakorn

This thread was discussed between 20/02/2014 and 19/08/2014

MG Midget and Sprite Technical index

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