Shock Spring Selection

[Ji Gantor]

I know that I started this topic to determine rear shock spring rates but now I have written a program to predict rear shocks, unless more data is discovered I feel we have covered it enough to be close to what is required.

It will not be very difficult to modify the program to allow front fork springs to be analysed as well.
When I get time I will copy and rearrange the program to predict fork springs. Once this is done may be we can figure out what Brent was getting at.

Brent's hypothesis;
The front and rear suspension should be balanced.
Both ends of the bike should be in harmony for the best ride and traction. To improve cornering, jumping, take off at the starting gate...........

We would have to determine frame geometry to be able to get anywhere near the answer but we have to start some where.


 Ji

[Ji Gantor]

I am back after an 8 day trade show.
I will start on Brents rear shocks this week.
Brent can you supply the front end geometry of your bike.

Ji

[brent j]

I've really enjoyed this thread although I've not put much into it after the start.
And I've had to dust off my maths brain for some of it and that's been a shock to the system.

I think a starting spring rate can be determined and to my mind that would be static but fully laden (with rider) There are too many variables to try to work out landing from jumps etc unless you land from the same jump at the same speed etc etc.
I still think that getting the springs to hold the bike up and using the damping to control it is the way to go.

I've found with my 500's that not much of what I do with the rest of the bike affects the forks too much but the back is a different story.
My shocks are length adjustable and I tried different lengths over time to try different handling. With the bike low in the rear the swing arm will squat and the rear suspension will preload requiring more spring rate but then that's too harsh with no power. Conversely with the back jacked up the bike will rise and take the load off the rear springs. With changes in shock length and spring rates I also had to make changes in damping to compensate. At one stage I would be changing the damping to try and compensate for a different spring rate and also for a different suspension reaction. These two things could be opposing or compounding depending on what I had done.

I can say that I got it horribly wrong some times, quite a few times to be honest. Now I seem to get it right more often that wrong but that's not saying too much.

Thanks guys for for a lot of info and for making me think again!

Brent

[Ji Gantor]

Hi Brent,
I should get some time this week to check out the data you sent me.


Ji

[paul]

[paul]

[paul]

walter  you explain the pics   

[brent j]

I want one of those spring rolling machines for my shed!

I used to build machinery like that way back when

[Lozza]

Pictures of the goings on inside the Sports Bar would be FAR more interesting

[BAHNZY]

I have a PE250N with a set of piggy back Ohlins fitted. Taking a guess the rear springs have been set up for a 70-80 kg rider and probably not well set up given that there is an alloy spacer fitted between the spring and the top spring saddle/retainer. When i throw my 110kg frame on the bike i compress the the rear suspension better than 1/2 way through the stroke and obviously need to correct the situation.
A quick call to Cramers and i find out that a set of springs (rate and free length to be confirmed) will cost more than what i think is reasonable. The Ohlins run dual springs and i am told that the valving in the shock is set up to work with dual springs and that i should not fit a single spring. So the question, how do i get a spring that has the correct spring weight to suit my weight and the bike but also match the shock in relation free length that when compressed and fitted to the shock won't end up with coil bind before the bump stop get's to do it's job?

[BAHNZY]

Using Ji's image i have the following specs.
Z = 60mm
Y = 335mm
X = 540mm
T = 270mm
The shock is an Ohlins Piggy Back that is;
395mm eye to eye
290-310 between the spring platforms
and if i am measuring correctly has a stroke (without bump stop) of 140mm
And with gear i'm around the 115 kg mark.

Suggestions as to the best spring?

[BAHNZY]

Any Suggestions? Need to get something sorted for the Harrow weekend.
Thanks in advance.

[JohnnyO]

Bahnsy i've got single ohlins progressive rate springs on the Ohlins from my RM250T and the same springs on my 490 Maico bought new from Cramer. They would be pretty close to what you need. The part number is 180-46. The spring length is 270mm, shock length is 385mm.

[evo550]

Banshy,
You can determine your existing spring rate by using this calculator, then best ring a suspension shop (maybe Walter) with the details and weight existing springs where designed for, from that they can work out what rate suits your weight.
I have tried to get Cramers to help with Ohlins rates before, but they weren't interested unless I had a part number.
Your best bet is a suspension shop.
http://www.racingsuspensionproducts.com/spring%20rate.htm
p.s. the flat "half" spring at the very top and bottom of the spring can be counted as 1 active coil.

[LWC82PE]

To calculate if you will run into coil bind, first measure OD of the wire of the spring, then times the thickness of the wire X the number of coils. this gives you the solid/compressed length of the spring. if you have dual springs do this for the short and long spring and dont forget to add the thickness of the plastic spring divider. Then measure the compressed length of shock (with out bump stop) between the top and bottom spring retainers. That measurement will vary depending on what preload clip groove you have it on but then if you do it from maximum preload you know you will always be safe. Then if the compressed length of the spring/s is LESS than the distance between spring retainers/seats then you have no problems with coil bind. if it is more then that is not so good and the spring will bottom out on its self and before maximum shock travel and it will overload the top spring retainer and would most likely break.

Old Ohlins springs are identified by spring colour and coloured markings on the springs so if you know the paint is original the spring rate can be identifed by that.