Shocky spring length

motoddrob

motoddrob

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Location
S/W Australia
Howdy all,
About to send off the WP shock for a service.
Having discovered a crack in the weld on the frame top shock mount and a comment from Phil that the shock may be undersprung causing it to bottom out. I got out the original DeCarbon shock the frame kit was supplied with. I sort of remember back then Phil saying the spring was a WP unit and as it's white could very well be. So I removed the spring from the DeCarbon shock for a comparison with the one on the WP shock.
The DeCarbon spring was 25mm longer in length, same number of coils and the coil diameter is 10mm on the shorter and 11mm on the longer. I can't see any markings on either so have no reference to their ratings.
Thinking logically which spring would be the heavier one?? Over to you Cam for a explanation on the dynamics of springs, cheers in advance
 

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My guess would be thicker spring would be stiffer rate than thinner (assuming same windings per inch).
Saying that the longer one looks to have less turns per inch, and is it also the thicker one?
Less turns per inch would seem to me to indicate the angle of the windings is closer parallel to the direction of travel, making it stiffer.
More windings per inch would make it less parallel to the direction of travel and make it have a lower spring rate.

Or it could be a load of bollocks.
 
You can use a known weight (10kg?) on each spring and take exact measurements of how much they compress. Alternative compress each a fixed amount (1cm) onto a set of accurate scales.
 
According to 'Motorcycle Tuning - Chassis' by John Robinson, on p52:

spring rate s = Gd4/8nD3

G = shear modulus of the material
d = coil wire diameter
n = number of active coils
D = mean diameter of the coil.

Easy peasy!

bazzee
 
Rob,try compressing the shock fully without the either spring,take a note of the dimensions available for the springs in that length and measure both springs and take away the gaps between the coils.I overdid the preload once to get a sag figure,should have upped the spring weight but didn't and it went spring bound before bottoming and broke a link doing some jumping.Spring bound is not good,big loads get transfered to the shock mounts.How is the bottoming rubber,any damage.What are you sag figures loaded,you on it with the bike on its wheels and you off it still sitting on its wheels.Does the shock have a rising link or is it cantilevered.
 
bazzee said:
According to 'Motorcycle Tuning - Chassis' by John Robinson, on p52:

spring rate s = Gd4/8nD3

G = shear modulus of the material
d = coil wire diameter
n = number of active coils
D = mean diameter of the coil.

Easy peasy!

bazzee
Click to expand...

No worries Bazzee, just reaching for my notepad and pencil as I type, give me 5 mins and I'll let you know the answer :-\




My calculations the answer is 42!!

Hey Vince, I can tell you the wheel has 110mm of travel. As for the DeCarbon, it's siezed :o The suspension is rising rate. I wouldn't mind trying a heavier spring, I'm thinking about the cracks that bottoming out has caused. It's been the same shock and spring fitted for the last 25 years. Had had a few rebuilds along the way.
As you know Oz roads are pretty fucked so it's copped a hammering, probably done over 90k kays, It replaced the DeCarbon the kit was supplied with.
 
Typically bikes are designed for the average 80 to 90kg bloke.If your heavier or lighter then you go up or down a rate.Not so easy with your custom setup.My recollection is set the sag at 1/3 travel with you on the bike.The unloaded sag should be 10 to 15mm or so.More than 15mm and the spring is to light,less than 15mm and its to heavy.Thats a simple rule of thumb but with a rising rate its a bit more complicated.Keeping high in the travel is better than lower.Thats springs,damping is a whole other subject.After a few years running 7.5 wt fork oil in the front I am going to try some 15wt at 130mm.Lately I have noticed a bit of kicking from the front,springs overwhelming the damping I think.Its fun trying new things.
 
Vince, I've always used 15w in the forks with the gold valves.

I don't think I'm gunna get that longer spring on the WP without some sort of spring compressor.
I might have to buy a spring :o I'll talk with the guy doing the service on the shock.
I run 80kg and the misses is 50kgs so not too heavy a load :D, I reckon it's the shit roads that's the problem
Got to somehow stop it bottoming out
 
Hi Rob - I would guess the shorter one is heavier rate. Put the pair of them in a sash cramp (with wooden plates) and wind it up - the one that close up the most is the weakest. - Regards - Phil
 
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