Re: Re: 1952 Paris built Normale RSJ 834

[Anonymous]

@bazessex wrote:

Oh dear not only am I a presumed member of the Traction Moaners group here on the pesky forum I also am one of those who are risking life and limb for removing the volute springs and using a modern alternative. In the latest issue of FP it is mentioned that the springs are there to solely stop the engine from moving side to side and that the rear block mounting takes the strain. I am new to Tractions but with just a borrowed manual and pure logic I know this is wrong. If someones car is set up in this way no wonder they have noise and vibration. Yes I had concerns re the engine mounting brackets and their ability to do the job being asked but after actually thinking it through and taking a good look I am pretty sure they will be fine.

I wrote the piece in F-P. Thanks for your comments, it’s always good to get some feedback. I don’t really want to cause trouble but I’ve done a bit more research to find out what those volute springs do and what the rear mounting block does. i thought I’d try and get to some numbers.

I was going to change my rear mount block anyway because I’ve had the car a long time and never seen it. So I did that today. The cover came off easily and there was the rubber block sitting perfectly in the middle of the hole – as measured by running my finger round it. I changed it for the new one – per Peter Larsson at CTA Sweden the typical problem is that the rubber hardens over time. The new one felt a bit softer so in it went.

There are wear marks on all four sides of the cover – a similar patch of shiny steel showing through the paint in the middle of each surface.
The clearance between the block and the cover is about 3mm on each side, allowing for where the block sits in the cover – both the block and the cover are tapered.
So the question is how much load is taken by the volute springs when the 3mm clearance is taken up and how much is taken by the block?
That requires knowing about the spring rate and the loads.

I can’t find any specification sheet for the volute spring – but I have found one for something similar made by a company called Don Springs. The smallest one in their catalogue appears to be the same thing – expect the height of the strip material is twice that in the car. This has an effect of doubling the spring rate which is quoted as 10 kg/mm – so from that I can estimate that each spring in the car has a spring rate of about 5 kg/mm. From pushing the engine around without the rubber block in place that I would say it feels something like that – I didn’t measure it though but that’s the figure I’ll use.

So what about the loads? The springs can clearly support their share of the engine statically – the load being shared with the gearbox mount which also controls the fore and aft motion. Clearly the engine moves on its springs when it is running – this will happen as a result of three things – out of balance rotational forces, bumps in the road and torque due to acceleration and braking.

I fired up the engine without the cover on the mounting block. It hardly moved as it is pretty much in line with the gearbox mounting and the engine’s centre of gravity – so any rocking from out of balance forces is going to be on an axis through the middle of the block which therefore plays no part in preventing it. The springs will oppose the rocking but they provide no damping. Solid rubber blocks will also provide damping which would be a good thing.

So what about when the car goes over a bump? Apparently the unsprung part of the suspension can see 5g on a bump but the body itself sees far less because of the suspension. 1g would seem to be a high estimate for the car body to experience but some papers suggest it is a measured figure. If the engine was caused to move at that vertical acceleration it would double in weight and whatever is holding it up will momentarily see twice the load – or it would if the engine mounting was solid. But it isn’t – the two springs combined have a spring rate of 10kg/mm. If the engine weighs 300kg, half of it is on the springs – the other half is on the gearbox mount. So adding another 150kg would cause about 15mm deflection at the springs – which means about 30mm deflection at the rear (because the springs are half way between the rear block and the gearbox mount which is the pivot point). I don’t know if the engine weighs 300kg but it is not far wrong. So – if the springs are fitted and the block is missing the engine would move 30mm – i.e 27mm more than the block permits. If the springs are replaced by stiffer ones such that the displacement is no more than the 3mm gap, or the block is removed, the additional load on the new mounts (and brackets) is 270kg i.e 135kg per side. If you believe these figures that is – you probably shouldn’t really because I haven’t factored in the frequency of oscillation or damping coefficients or the spring rate of the replacement rubber blocks. But the point is it’s a significant number.

The other question was about when the car accelerates or brakes. This will be a bit more accurate because we know the maximum acceleration and braking figures – they are stated in the contemporary road tests – and we know the wheel radius and the distance between the drive shafts, and the engine mounts. We also know the mass of the car. Motor’s road test #4/48 says the car can accelerate from 0 – 30 mph is 6 seconds which is about 2.2 m/s2. At that acceleration I calculated the volute springs would each need a spring rate of about 61 kg/mm to keep the deflection to within 3mm. If none of the acceleration load is taken by the rear block, the load on each side due to the torque increases by 92kg whether the mount is a spring or a rubber block. With the 5 kg/mm springs you would have to take 73 seconds to accelerate from 0 to 30 mph for a 3mm deflection. The car can apparently decelerate under braking at 0.8g but braking loads are not taken through the engine mounts – well the engine braking load is but that must be far less the acceleration load.

So – I think I have shown that, as the car left the factory the rear block is taking a fair load most of the time the car is driving. If the block is taking the load, the springs are not doing much – they can only provide a load of about 22kg at the maximum deflection allowed by the rubber block. If the block is removed, the springs will need to be replaced by rubber mounts that are about 10 times stiffer and that they, and the brackets, will have to support the additional load which will be in the order of an additional 100kg per side. This is not a huge figure compared to the engine mass. But it is quite large compared to the load those brackets normally take.

I don’t recall suggesting that anyone was risking life or limb and since a number of cars have been running around with this conversion for some years, it seems they can take it. I don’t know how much headroom there is though.

I like to refer to this forum in the Citroenian and Floating Power and mention things that are discussed here to encourage people to have a look and join in. Not that I’m that prolific here. I’m not personally bothered about originality – certainly not on my own Normale. I just fitted electronic ignition today – I’m quite excited about that.
Chris