W123 racing setup - serious questions within - PeachParts Mercedes-Benz Forum

Secondaries · #1 09-13-2013, 09:04 AM

Quote:

Originally Posted by Stretch

Ummm - it is a bit difficult for me to follow exactly what you are up to but I think you might have missed the point that the stiffness or rate of a coil spring is proportional to the number of active coils.

An active coil is a coil that is not touching another.

Look back at the link I gave to that vacuum pump thread to see the equations you need - also please note different springs were fitted with different stiffnesses / rates for different chassis and engine weight combinations. You should ideally pick out a couple of springs that you think you might be able to find and then start from scratch with the measurements and length adjustments.

Stretch - aftermarket springs are available in known rates at varying lengths - 9.5, 10, 11 ,12, 13, 14 inches seem to be the most common. When I refer to using a 10" or 11" spring, I am not talking about cutting coils, I am talking about totally different pieces.

Behold! An 11" 600 lb/in spring.

DeliveryValve · #2 09-13-2013, 12:58 PM

Quote:

Originally Posted by Secondaries

Stretch - aftermarket springs are available in known rates at varying lengths - 9.5, 10, 11 ,12, 13, 14 inches seem to be the most common. When I refer to using a 10" or 11" spring, I am not talking about cutting coils, I am talking about totally different pieces.

Behold! An 11" 600 lb/in spring.

Nice find on the spring. Can't comment on the Math, though I think it looks good.

Looking forward to seeing the progress...

BTW - looking at the picture of the spring I see it is flatten on the ends. This will work on the top, but if you plan to use it with the stock shape MB bottom perch/plate, you may have to cut a coil off to have it seated properly.

.

Secondaries · #3 09-13-2013, 05:27 PM

Quote:

Originally Posted by DeliveryValve

Nice find on the spring. Can't comment on the Math, though I think it looks good.

Looking forward to seeing the progress...

BTW - looking at the picture of the spring I see it is flatten on the ends. This will work on the top, but if you plan to use it with the stock shape MB bottom perch/plate, you may have to cut a coil off to have it seated properly.

.

You are correct! I did notice that, but I am looking for other spring manufacturers that make closed/unground bottom ends.

Stretch · #4 09-13-2013, 03:07 PM

Quote:

Originally Posted by Secondaries

Stretch - aftermarket springs are available in known rates at varying lengths - 9.5, 10, 11 ,12, 13, 14 inches seem to be the most common. When I refer to using a 10" or 11" spring, I am not talking about cutting coils, I am talking about totally different pieces.

Behold! An 11" 600 lb/in spring.

No I get that - but - you need to know how many turns of the coil you have for each spring.

When you have a spring that has say 10 free coils that are not touching each other you've got a stiffness that is equal to say X N/mm

Compress the spring to fit it in the car and you might find that you've only got 8 free coils that don't touch each other. The stiffness will be INCREASED to say XX N/mm

If you then want to change the height again for a race situation then you may only have 6 free coils that don't touch each other so you've then got a stiffness of XXX N/mm

The over all length of the spring and the number of turns and the gauge of the wire and the mean diameter are all important considerations (assuming you know what stiffness you want from a stability calculation)

[Also if you want a height change on the front springs you'll end up with some pretty messed steering geometry / camber / castor on a standard W123 front suspension design]

Secondaries · #5 09-13-2013, 05:24 PM

Quote:

Originally Posted by Stretch

No I get that - but - you need to know how many turns of the coil you have for each spring.

When you have a spring that has say 10 free coils that are not touching each other you've got a stiffness that is equal to say X N/mm

Compress the spring to fit it in the car and you might find that you've only got 8 free coils that don't touch each other. The stiffness will be INCREASED to say XX N/mm

If you then want to change the height again for a race situation then you may only have 6 free coils that don't touch each other so you've then got a stiffness of XXX N/mm

The over all length of the spring and the number of turns and the gauge of the wire and the mean diameter are all important considerations (assuming you know what stiffness you want from a stability calculation)

[Also if you want a height change on the front springs you'll end up with some pretty messed steering geometry / camber / castor on a standard W123 front suspension design]

Well, linear springs have equal spacing and thickness between all active coils. They should compress equally and one shouldn't touch another until they are all touching. These aren't progressive springs.

Stretch · #6 09-14-2013, 05:17 AM

Quote:

Originally Posted by Secondaries

Well, linear springs have equal spacing and thickness between all active coils. They should compress equally and one shouldn't touch another until they are all touching. These aren't progressive springs.

True but a coil spring is only linear when the load is applied evenly along the length of the spring. That isn't the case in a W123. Have a look at your existing springs if they are still fitted to the car. It is quite common to see them bulging a bit. This isn't a problem for the W123 under normal circumstances but if you are putting in new springs then basic things such as counting the coils and working out how likely it is that coils will touch is important.

Secondaries · #7 09-14-2013, 11:59 AM

Quote:

Originally Posted by Stretch

True but a coil spring is only linear when the load is applied evenly along the length of the spring. That isn't the case in a W123. Have a look at your existing springs if they are still fitted to the car. It is quite common to see them bulging a bit. This isn't a problem for the W123 under normal circumstances but if you are putting in new springs then basic things such as counting the coils and working out how likely it is that coils will touch is important.

You may be correct about the bind. They make swiveling weight jack cups to prevent bind. They are not to much more expensive than the standard bits so I might spring for them instead.

charmalu · #8 05-19-2014, 12:27 PM

Is the race still on???

Charlie