4CV rear suspension foibles

JohnW

JohnW

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This discussion split from thread linked below on 12/1/22 by admin as requested:
https://www.aussiefrogs.com/forum/index.php?threads/gordini-double-shaft-djet-engine.144933/
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4cvgordini said:
Perhaps it's an I.P. thing but I have always wondered why people don't craft their own Gregoire rear suspension kit.
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I've asked about this exactly in France. To replicate them properly you need some dimensions and spring specifications, a fair bit of work having been done by Gregoire to get them right, and that needs an example to measure up. They are very rare! I asked with a view to maybe getting a batch made professionally - I reckon you could sell them perhaps in the dozens (they won't be cheap). The answer I've had is that the the key component - the top spring housing piece - is a real practical problem being forged aluminium. I'd have thought you could do it in steel but it's out of my league. It would need a design exercise to do it safely.

I'm still a bit surprised though, as you are. There must be a good 1,000 4CVs in France I'd have thought. I reckon you'd sell several in Australia and we have perhaps a tenth of that number of cars.
 
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Very ingenious progressive rate suspension system. Greatly improves behaviour of the 4CV, which had/has pretty firm rear springs. It was designed and I guess marketed by the very interesting engineer, Gregoire. There's a Renault Prairie traytop in Victoria with the variant actually fitted standard to some of those vehicles. I'm not aware of a single 4CV in Oz that has it and there are very few in France. I was told that it cost about half the price of a new 4CV.

It's clever and complicated and works well - I've ridden in a car with it fitted in France.
 

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Looks interesting but a bit sketchy on details.

What locates the rear half shaft tubes in the factory setup? Is the tube rigidly attached to the g'box or does it have a setup similar to the R8/10 that doesn't show in those pictures? I don't see any trailing arms or such in the article.
 
schlitzaugen said:
Looks interesting but a bit sketchy on details.

What locates the rear half shaft tubes in the factory setup? Is the tube rigidly attached to the g'box or does it have a setup similar to the R8/10 that doesn't show in those pictures? I don't see any trailing arms or such in the article.
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All the rear-engined Renaults have the same trunnion pin pivot system to locate the swing axles. The pins increased in diameter over time, that's all.

The trailing arms don't locate the swing axles with respect to their mount on the transaxle, but rather keep the whole power pack straight in the car's chassis and therfore allow the softer mounts that make the cars so much smoother than the 4CVs.
 
a little bit of wear on those trunnions represents a lot of movement at the wheels ,a meter or so out ,i recall having my head out the drivers side window reversing my 4cv and being more than a bit surprised at just how far the wheel moved when i let out the clutch ,i have seen those reverse tramp rods. in the day that went front the axle tube to the rear onto the chassis rail ,two bits of flat bar .attached to an eye ,at the chassis end like a spring hanger and a strap around the axle tube with an eye attached .like a trailing arm on a conventional car rear suspension ,but to the rear, were the chassis rail is exposed ,its been a long time but im thinking somewhere below the battery box .PUGS
 
JohnW said:
Very ingenious progressive rate suspension system. Greatly improves behaviour of the 4CV, which had/has pretty firm rear springs. It was designed and I guess marketed by the very interesting engineer, Gregoire. There's a Renault Prairie traytop in Victoria with the variant actually fitted standard to some of those vehicles. I'm not aware of a single 4CV in Oz that has it and there are very few in France. I was told that it cost about half the price of a new 4CV.

It's clever and complicated and works well - I've ridden in a car with it fitted in France.
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John, you could splash out and buy this used setup advertised in December, though likely to be pricey.

https://www.lesanciennes.com/annonce/rech-suspension-gregoire-renault-4cv-a129665
 
JohnW said:
There must be a good 1,000 4CVs in France I'd have thought. I reckon you'd sell several in Australia and we have perhaps a tenth of that number of cars.
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Surely there are 10,000 or more there. There are 3320 Peugeot 404s in the Club Register in France and likely 10 times that in reality. So the 4CV should be far, far higher than 1000.
 
Mike Tippett said:
Surely there are 10,000 or more there. There are 3320 Peugeot 404s in the Club Register in France and likely 10 times that in reality. So the 4CV should be far, far higher than 1000.
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Yes, you are probably right. My friend in Paris estimated 2,000-4,000 based on parts sales. There were 1000 or so Citroen CXs 14 years back.
 
pugwash said:
a little bit of wear on those trunnions represents a lot of movement at the wheels ,a meter or so out ,i recall having my head out the drivers side window reversing my 4cv and being more than a bit surprised at just how far the wheel moved when i let out the clutch ,i have seen those reverse tramp rods. in the day that went front the axle tube to the rear onto the chassis rail ,two bits of flat bar .attached to an eye ,at the chassis end like a spring hanger and a strap around the axle tube with an eye attached .like a trailing arm on a conventional car rear suspension ,but to the rear, were the chassis rail is exposed ,its been a long time but im thinking somewhere below the battery box .PUGS
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Quite so. It's a huge ratio! The "reverse tramp rods" have the problem, at least with most that people have fitted, that the pivot from the wrong place and describe an arc parallel to the line of the car. That stresses the trunnions and would tend to steer the car depending upon how much one side deflects relative to the other. Some have been built really well with the pivot of the rods lined up with the axis of the trunnions.
 
schlitzaugen said:
That was what I meant, I am familiar with the R10 setup, not sure I understand the 4CV setup. What is locating the rear axles in respect to the chassis?
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That's what I was trying to say. They are all exactly the same. It is just that the 4CV and Dauphine locate the whole power back, AND therefore the swing axles, only with the rubber mounting bushes for engine and gearbox. With the R8 they added the trailing arms, pivoting along the line of the trunnion axis, and located the whole unit better. Those trailing arms have Silentbloc bushes at the front ends.

The largest of the trunnion pins seem to last pretty well. The early 4CVs were a smaller diameter, did not last well and had the very hard needle rollers forming indentations along the pins. There's probably a 50:1 magnification of wear from pin to wheel. On early 4CVs, 25 mm of slop at the wheel wasn't uncommon in my experience. I think they improved the steel as well as benefitting from larger diameter pins in the later cars.
 
I don't think we understand each other but I had a look on the 'nets and found my answer. There is nothing to locate the swing arms on the 4CV apart from the trunion as you say, but that is not really controlling for-aft movement. Not really. I mean no more than you'd control a chainsaw at the end of a barge pole. That is what I suspected. Not the best design especially given the rear wheels apply the entire thrust forward. Which means the trunion bushings are constantly under lateral strain. Quite remarkable they hold up at all but I suppose for what? 30-ish HP or so? it is good enough.
 
schlitzaugen said:
I don't think we understand each other but I had a look on the 'nets and found my answer. There is nothing to locate the swing arms on the 4CV apart from the trunion as you say, but that is not really controlling for-aft movement. Not really. I mean no more than you'd control a chainsaw at the end of a barge pole. That is what I suspected. Not the best design especially given the rear wheels apply the entire thrust forward. Which means the trunion bushings are constantly under lateral strain. Quite remarkable they hold up at all but I suppose for what? 30-ish HP or so? it is good enough.
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We sort of do. The swing axles are located by the trunnion pins, full stop on ALL of these models. The whole unit is rigid and the pins only allow up an down motion. One the 4CV, the gearbox is mounted rigidly to the rear cross-member, itself secured in stiff rubber mounts to the chassis rails. The load on the trunnion pins is trivial from torque/power matters but significant from road shock (think of a big pothole). I reckon the early ones behaved badly at a time scale of a decade because the pins are too small and not good enough steel.
The larger pins on the Dauphines seemed not to give trouble (better steel too perhaps) even though they have the same configuration of soft mounts like the R8/10 series. Again no trailing arms. As you'd know, the gearbox unit on Dauphine/R8/R10 is hung from a rubber mounted pin from the cross member and with two side pads in shear. So, with soft mounts at the engine end, it has far more potential to move around than the 4CV.
Hard steel caps contain the needle rollers for the trunnion pins, and they do have seals to keep dirt out. Dirt can only get IN if dirt gets into the universal joint housings and they are typically pretty clean inside (I dismantled one just yesterday from a Dauphine Gordini, never opened from new I suspect, and all is perfect, including the trunnions).
The trunnions have no play in the needle roller system so do control fore-aft movement perfectly well.
So, yes, they are under load all right but quite strong enough for the job. I think chain saw on barge pole isn't an apt description at all.

The derivative Hino Contessa suspension has a fully spherical joint instead of trunnion pins, and trailing arms to locate the swing axles. More sophisticated, but heavier and more expensive to make. I don't think Renault was interested in redesigning that floor pan to do anything more than accomodate those centrally mounted trailing arms. As I said I think those trailing arms are really to stop the power pack from rotating in the horizontal plane and they do nothing effective for the trunnions. It's really 1930s technology with lipstick on the pig at a much later date. :)

I don't know, but suspect at least some of the various attempts to add control arms were to control badly worn trunnion movement on the earlier cars. When they are bad they are really bad!!
 
Yeah, I understand now why you said the pins locate the swing axles but in my opinion they locate it as much as the foundation locates Burj Khalifa. The bottom sits there but you have no idea what the other end is doing.

I am pretty sure that was why it was needed on R8 and 10 to use control arms otherwise with potholes like you say, your wheels would be left behind. Or overtake you if you had the power. from memory the arrangement on the R10 rear control arms is pretty similar to that on the R12 at the front (and other Renault FWD cars), where again, you might think there would be no need to control the upper suspension arm but it seems the engineers thought there was.
 
schlitzaugen said:
Yeah, I understand now why you said the pins locate the swing axles but in my opinion they locate it as much as the foundation locates Burj Khalifa. The bottom sits there but you have no idea what the other end is doing.

I am pretty sure that was why it was needed on R8 and 10 to use control arms otherwise with potholes like you say, your wheels would be left behind. Or overtake you if you had the power. from memory the arrangement on the R10 rear control arms is pretty similar to that on the R12 at the front (and other Renault FWD cars), where again, you might think there would be no need to control the upper suspension arm but it seems the engineers thought there was.
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We might have to agree to differ. The pins are strong and there is absolutely no fore and aft wheel movement possible relative to the transaxle unless the bearing system (needle rollers in a hardened steel cap) has failed. the swing axles are quite robust, as is the mounting. It's certainly an antiquated design, I agree. The later trunnions don't seem to fail, or if so not often. As I said, the early ones do fail and the wheel movement has to be seen to be believed. I repaired mine (early 4CV ones) 31 years ago with bronze bushes that turn inside the caps but are a press fit on the damaged pins. Perfect. I have several later swing axles in the cupboard and all trunnion pins are fine.

The control arms merely keep the transmission properly in line, no more. Remember, they are rubber mounted themselves, so if the swing axles needed the arms to keep their orientation correct relative to the transaxle, it wouldn't work due to deflection of the Silentbloc bush.

We still haven't caught up for coffee! :)
 
Hi :)
Can I put my :2cents: in here. Back in the day of my 750s the rego man knew what to look for and went for the rear wheels first up ! Then suggested i do something about it before coming back ! A friend that was a Renault/Citroen mechanic told me to get some nylon bushes to replace the needle rollers, a known fix, and that would get it through the inspection OK which it did. But the nylon did hammer out so you put those silly radius arms on to help a bit. Even then I could see the geometry was wrong. That was the way it was in the 60s. Bump steer from the rear, as well as on/of power steer. Lucky they only had 29 HP. All my 750s had flogged out trunnions but I only bought cheap cars.

To move on to the R8/10 design. I believe the overall design parameters had moved on from cheap and changed to comfort and noise reduction, but recycling the old cheap swing axle design. So having the rear mechanicals all hanging on rubber as a unit and able to absorb the road shocks made a remarkable difference to the NVH and set a new standard for a small car. The trailing arms did actually provide a foward point at the pivots that controlled the whole back end alignment and kept the rear wheels and axles pointing correctly but able to move back and forwards a bit. This also reduced the shock loads on the trunnions and they then lasted without problems. Even in R8G rally cars.
A very clever design in my opinion that was largely unnoticed because there was no other cars doing similar R&D on NVH.(except Peugeot). I went to a couple of motor shows in Sydney that had films of some of their design ideas and testing which was all new ideas to me at that time. Even crash testing was being done which was the first I had heard of that stuff. Renault Australia was interested in selling cars in those days:cool:
Jaahn
 
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