Short question, does cutting a spring in half double it's poundage ?
FF: Forks Modified Steering Stem
- Thread starter greg brillus
- Start date
Chris, I think the short answer might well be "Yes" but it is probably not that simple. From what I have read on the internet about springs/rates it seems that the rate changes almost universally by the amount of coils you remove, and generally the first coils do not count, as the end is nearly butted up to the next coil of wire which causes this first coil to be inactive. I must say that I have been quite active in the "Cutting" of springs to get a desired result, and as David rightly discovered, it is easier to start with a short spring and simply pack it up if necessary. Though I will say that in all of the new stem installations I have done so far, all have had pre-load on the springs, even Neal's Comet had about 20 mm of pre-load on "Cut down springs" with what I would say have about 45 lb rates which gives approximately the same rates I have in my twin but my springs have about 50 mm of pre-load. Though Neal's Comet has an original Koni shock absorber which has less travel than the Thornton I have on my twin. Cheers...........Greg.
Hi Greg
The problem I'm running in to is I have the normal ride beautifully soft and pliant and the dampers are nicely absorbing the shock when hitting large potholes and speed humps, but they are going down to the bump stops under hard braking, my next plan of attack is to install shortened C inners so they only come in to play the last inch of travel, if that's not enough I was thinking of cutting them in half and installing tubular spacers to bring them to the same height, which would in the first instance give me a 20lb/in boost and I hoped 40lb/in in the 2nd case.
Chris.
The problem I'm running in to is I have the normal ride beautifully soft and pliant and the dampers are nicely absorbing the shock when hitting large potholes and speed humps, but they are going down to the bump stops under hard braking, my next plan of attack is to install shortened C inners so they only come in to play the last inch of travel, if that's not enough I was thinking of cutting them in half and installing tubular spacers to bring them to the same height, which would in the first instance give me a 20lb/in boost and I hoped 40lb/in in the 2nd case.
Chris.
Chris, I have actually just installed a set up on a Shadow that has brand new spring box cases on it. I noticed that the new cases when assembled with say a pair of the 36 lb springs from Norman, that the cases are about 10mm longer end to end than original cases. I chopped the springs down by about 25 mm each and even when installed the front end was basically topped out. Now shortening these springs by say 25 mm will automatically up the spring rate to near 40 lb's. My suggestion to you is try shortening the springs by somewhere between 25 and even 50 mm each, then use packers to return the bike to the correct static height we have talked about and try it again. If the springs were say 50 mm shorter the spring rate would be about 45 lb's each and this would make a big difference as far as the bike near bottoming under heavy braking. Or else just replace one spring with an original and give that a try. I change the spring on the left side, as this is easy using the tommy bar as a windlass, you can get a result quite quickly. The ride quality will not change by much, but the ability for the suspension to not bottom out is very important. I can apply my front brakes very hard and the suspension still follows the bumps in the road surface. Cheers...........Greg.
Hi Greg
At the moment I have one of Norman's 36lb springs and an original in so I'm at 45-46lb area anyway, I don't want to go cutting up any of my "main" springs as I don't have any spare.
It's not "hitting" the bottom but more like slowly sagging into the bump stop, this is on a downhill stretch I use with the front brake only,
Shadow drums, alloy brakeplates, longer brake arms and heavy duty cables gritting my teeth hoping it doesn't go from under me.
Riding sensibly I don't hit the bump stop but I'm testing things.
Have you tried any other dampers than the Thornton ? I find the Koni is too harsh over the small stuff.
At the moment I have one of Norman's 36lb springs and an original in so I'm at 45-46lb area anyway, I don't want to go cutting up any of my "main" springs as I don't have any spare.
It's not "hitting" the bottom but more like slowly sagging into the bump stop, this is on a downhill stretch I use with the front brake only,
Shadow drums, alloy brakeplates, longer brake arms and heavy duty cables gritting my teeth hoping it doesn't go from under me.
Riding sensibly I don't hit the bump stop but I'm testing things.
Have you tried any other dampers than the Thornton ? I find the Koni is too harsh over the small stuff.
The short answer is yes but you must count the active coils properly. Google "spring rate calculator " and you will find calculators ready made so that you can enter your spring dimensions and find out what rate your spring will have when shortened.
Regards Mac
I did manual calculations, but I think there are only a few things to be aware of. You need to know the diameter of the wire, which is easy to measure. Multiply that times 11,250,000 Lbs./in. Squared, which is the modulus for spring steel.
This will be divided by: 8 times the number of active coils (count all the coils and subtract one for the number of active coils), multiplied times the mean coil diameter (you can put the tips of your caliper at the center of the flattened and ground end coil or do the math).
The "free length" of the spring seems obvious, but in spring parlance the spring should be "set" after manufacture. This means that the spring should be compressed to coil bind once before you measure the free length. Just be aware that the free length may change slightly if the spring is not set first.
The inactive number of coils is usually 1/2 per end, that is the part of the coil that is touching down on the spring box and thus, not active. NASCAR fans often hear that the pit crew has put a "wedge" in a spring. By jamming in a rubber wedge the crew makes more of the spring coils inactive and this increases the spring rate. It can be yanked out if track conditions change and a softer spring is required.
Wallace Racing has a lot of good calculators: http://www.pontiacracing.net/js_coil_spring_rate.htm
David
This will be divided by: 8 times the number of active coils (count all the coils and subtract one for the number of active coils), multiplied times the mean coil diameter (you can put the tips of your caliper at the center of the flattened and ground end coil or do the math).
The "free length" of the spring seems obvious, but in spring parlance the spring should be "set" after manufacture. This means that the spring should be compressed to coil bind once before you measure the free length. Just be aware that the free length may change slightly if the spring is not set first.
The inactive number of coils is usually 1/2 per end, that is the part of the coil that is touching down on the spring box and thus, not active. NASCAR fans often hear that the pit crew has put a "wedge" in a spring. By jamming in a rubber wedge the crew makes more of the spring coils inactive and this increases the spring rate. It can be yanked out if track conditions change and a softer spring is required.
Wallace Racing has a lot of good calculators: http://www.pontiacracing.net/js_coil_spring_rate.htm
David
A few probably pedantic observations on accurate spring calcs:
The formula is k = Gd4/[8nD3]
Where:
k = constant, pounds of load per inch of deflection
G = modulus of rigidity of spring material, pounds per square inch
d = wire diameter, inches
n = number of active coils, which is the number of coils subjected to flexure (always less than the total number of coils)
D = mean coil diameter, inches = Outer Diameter - Wire Diameter
As the rate is dependant on the fourth power of the wire diameter, you need to know this very accurately as any error will be greatly magnified. Forming the spring may cause the wire to be out of round and any coating will also cause an error. What I do is measure in several places then try to make an educated guess what gauge wire was used and use that as the probably correct number.
The rate is dependant on the third power of the spring diameter, so again measurement accuracy is important, and you are likely to find the OD is greater at the ends than in the rest of the spring.
Lastly David says total active coils are 1 less than total coils.
Most online calculators I have seen assume 2 less, and a few assume 3 less.
This clearly varies with the spring pitch among other things.
Using a "2 less" online calculator for one of the AVO coilover springs resulted in a rate higher than the manufacturers spec. Close examination suggested that 2 whole inactive coils was an overestimate. Changing this number to 1.5 coils inactive gave a better match to the spec.
The formula is k = Gd4/[8nD3]
Where:
k = constant, pounds of load per inch of deflection
G = modulus of rigidity of spring material, pounds per square inch
d = wire diameter, inches
n = number of active coils, which is the number of coils subjected to flexure (always less than the total number of coils)
D = mean coil diameter, inches = Outer Diameter - Wire Diameter
As the rate is dependant on the fourth power of the wire diameter, you need to know this very accurately as any error will be greatly magnified. Forming the spring may cause the wire to be out of round and any coating will also cause an error. What I do is measure in several places then try to make an educated guess what gauge wire was used and use that as the probably correct number.
The rate is dependant on the third power of the spring diameter, so again measurement accuracy is important, and you are likely to find the OD is greater at the ends than in the rest of the spring.
Lastly David says total active coils are 1 less than total coils.
Most online calculators I have seen assume 2 less, and a few assume 3 less.
This clearly varies with the spring pitch among other things.
Using a "2 less" online calculator for one of the AVO coilover springs resulted in a rate higher than the manufacturers spec. Close examination suggested that 2 whole inactive coils was an overestimate. Changing this number to 1.5 coils inactive gave a better match to the spec.
ER, I only needed a rough answer.