Coil springs, tapered vs constant thickness. Advice please.

[StMarks]

Ok, so my little w639 Vito failed it's MOT on a broken rear spring.
No surprise, these things should be listed as a consumable at the rate they seem to fail.!
So, I just ordered a pair & the image is as per my springs ( and all those I've replaced previously). They are a coiled spring, ovoid in shape & with the top and bottom of the spring tapering down to a smaller section of bar.
The ones that have arrived are the same ovoid shape, but are a constant thickness of metal.
My "latent engineer genes" have me thinking that making the spring from a tapered bar must be more expensive, so it must be an improvement ?
However when asked, the supplier simply tells me "these are the correct springs for your vehicle"
When I do a bit of a search, I discover that there are constant thickness springs showing as a more expensive option than tapered.

This is one of the many subjects that I have no decent knowledge of.

So perhaps you'd like to educate me, please ?

 

[ray_hennig]

So perhaps you'd like to educate me, please ?

No idea about your car but when we replaced all springs we calculated the appropriate springs from the MB parts catalogue. This depended on options fitted to the car - aven the radio made a difference. So we had the 2 different part numbers for front and rear and those parts were installed. I didn't really look at them but all is well 8 years later. Ride height is a mite high for a Sportline but that could be reduced by fitting thinner shims. But I'm happy as many roads around here are pretty ropey.

Good luck.

RayH

 

[slade1]

Modern springs are very poor quality.
I spent many years (1960s - 1990s) thrashing around Africa and Middle East broken springs not that common.
Broken shocks, stacks. I ran around for 7 years in a MB Ponton 190B not one broken spring.

 

[StMarks]

I'm sure I agree with you about the inherent quality perhaps not being as good as has been used in the past.

However what I was really hoping for is a little bit of an explanation about why some spring options taper to a narrower gauge of metal at the ends, and some don't. Especially as that must be more complicated & expensive to construct, yet they are not necessarily the more expensive option ?? -That seems counterintuitive to my uneducated mind.???

 

[190]

I can't say for your specific springs but I can make a guess from a fundamental observation about the technical aspects of springs.

The formula to calculate the rate of a spring is :

Spring Rate =(wire Dia to power 4 * 1470000)/(coil dia to power 3*no of coils)

You can see from this that nothing affects the rate of a spring more than the wire diameter because the spring rate is proportional to the wire diameter to the power of 4. My guess is the ends of the springs are tapered so that they will be the first part of the spring to give when weight is applied effectively seating the ends of the spring flat on it’s support.

 

[st13phil]

However what I was really hoping for is a little bit of an explanation about why some spring options taper to a narrower gauge of metal at the ends

If the diameter of the material remains constant but the spring itself tapers down toward the end(s) the reduced spring diameter causes the spring rate to increase in that portion of the spring.

Reducing the diameter of the material as the spring diameter reduces offsets this, giving a linear spring rate.

HTH?

 

[ChipChop]

If the diameter of the material remains constant but the spring itself tapers down toward the end(s) the reduced spring diameter causes the spring rate to increase in that portion of the spring.

Reducing the diameter of the material as the spring diameter reduces offsets this, giving a linear spring rate.

HTH?

Also known as progressive rate springs perhaps? Had some on a BMW i owned years ago.

 

[190]

I was little unclear whether the OP meant narrower on overall diameter or narrower in just the wire dimeter. The phrase "to a narrower gauge of metal at the ends" led me to think it was wire diameter but I had assumed a parallel spring.

Another way to produce dual rate or progressive springs is to have some of the coils more closely spaced such that when these coils close up it leaves fewer active coils which increases the spring rate. So for the initial travel of the spring all the coils are active producing a soft spring rate and then as the number of active coils reduces the spring rate increases.

 

[StMarks]

I was little unclear whether the OP meant narrower on overall diameter or narrower in just the wire dimeter. The phrase "to a narrower gauge of metal at the ends" led me to think it was wire diameter but I had assumed a parallel spring.

Another way to produce dual rate or progressive springs is to have some of the coils more closely spaced such that when these coils close up it leaves fewer active coils which increases the spring rate. So for the initial travel of the spring all the coils are active producing a soft spring rate and then as the number of active coils reduces the spring rate increases.

Sorry if I didn't explain myself clearly. Perhaps a picture of the old vs new will help illustrate the difference between the two designs.

A bit more digging suggests to me that the tapered springs are not listed for the newer models of Vito, yet the newer springs are showing as compatible with the older Vitos.
I'm wondering if the tapered springs are technically better (as outlined above) but have a tendency to fail more frequently & could have been abandoned in the name of brand reliability ??

 

[ray_hennig]

the old vs new

Did these new springs come from MB who looked up the correct reference?

R

 

[NotAnumber]

What is the way to look up the correct part number springs for a car's specific model and options please ?

I replaced my front springs 2 years ago with a pair the supplier claimed were OEM and the right spec for my pre facelift S212. I was pretty sure they made the car sit visibly lower at the front than it had before. The other week an MOT tester put a advisory on the front springs as he thought they were so low a coil must have snapped off. For the sake of peace of mind Im thinking of changing the springs again but making absolutely sure that they are exactly right.

 

[ray_hennig]

What is the way to look up the correct part number springs for a car's specific model and options

Best place to start is an EPC like this one:

https://partsouq.com/en/catalog/genuine

Then navigate to Suspension/Springs.

You'll see some part numbers there but it gets a little confusing. I would get MB to do the hard work.

RayH

 

[NotAnumber]

Thanks Ray