Shock Spring Selection

[Ji Gantor]

Thanks evo555,
So what is your bike, what does your manual say the spring rate is, and for what weight rider?

Ji

[Ji Gantor]

After reading some more details about springs and tapping out a few more calculations in theory this formula should work just as well for dual rate springs. After all the spring rate required to set the ride height is the same as I have calculated. The additional rate will be stiffer and help the shock from binding or bottoming out.

The problem with dual rate springs for short or less than 100mm travel shocks is that the spring pitch in the stiffer section is closer together and thus reduces the allowable compression of the spring. A dual rate spring must be part of the design at the time the shock is manufactured and not added later without checking all the facts that I have written about above. 

Ji

[brent j]

Walter my parcel has been given to my mother to hide until the end of next week :'( And I knew you would be watching this

Ji, I've measured my bike. I have a drawing in "paint" but can't upload it and I need a new scanner so I'll write it down.

X  Swingarm length                                                    550
Y  S/a pivot to lower shock mount                                330
    Height from axle to S/a pivot                                   140 (unladen)
T  height from S/a to pivot top shock mount                   330
Z  distance from S/a pivot to top shock mount (rearward) 150
    bottom shock mount above S/a beam                         25

When I weighed my bike initially I had access to a set of certified aircraft scales. I used two of them and even turned the bike around. I've since found bathroom scales are fine but try to put the "other" wheel on a block or simillar to keep the bike level.
OH&S tip Freaky, DON'T LET THE MISSUS KNOW WHAT YOU ARE USING THE SCALES FOR

Ji I'm not a fan of preloading springs. I had heavier (about 122lb/in) springs on the back with minimal preload and now run, I think 115lb/in. I had to add preload to hold the rear up but as the rate is low they are still too soft. Unfortunately the 122's weren't mine.

I will measure my rear springs this weekend and carefully measure the unladen and laden sag, front and rear.
I'll post up the spring rates and sag measurements as soon as I can.

Here's some links from another post to some spring rate calculators I have found invaluable

http://www.pontiacracing.net/js_coil_spring_rate.htm

http://www.proshocks.com/calcs/2spgrate.htm

If you are prepared to spend the $ I've found Tony Foales suspension software is great but I need about two weeks to learn to drive it!

And might I add I AM LOVING THIS THREAD

[Ji Gantor]

Hi Brent,
The 12mm preload I mentioned on my bike is the assembly preload, please pull your shock apart and check what yours is.

Ji

[brent j]

Ji,

I'll check that when I measure the rest.
I'm keen to find out again as didn't note that down last time and I want to recalculate my rear end stuff.

The preload will be what ever I put in last time,

take notes Brent, take notes

Brent

[evo550]

Ji,
just picked up a '86 Husky 250xc, haven't even looked a suspension yet, BUT, refering to my earlier post, I had an '84 Maico 500 with rear suspension issues.
I was having alot of trouble getting anyone even interested in discussing it, until I rang Ballards.
They where able to determine the original spring rate by measuring the coil width, number of coils and distance between each coil. From that the correct spring rate for my weight was determined and problem solved.
Ask for Geoff mention Maico and gees he gets excited.
Good thread guy's.........although it's way over my head.

[Ji Gantor]

Thanks evo555,

That is great input.
I have always found Ballard's team to be very helpful and their gear well priced.


Ji

[brent j]

Don't worry about being "over your head" Evo, I'm still feeling my way around. Believe me I've built some real kangaroos over time

I got interested in suspension for a few reasons.

I have a bad back (like most of us) and good suspension allows me to ride more often

When I started to build up my XT500 I could not find suspension advice. I'd been away from bikes for a lot of years so had to rely on local people. Basically if they couldn't find it on their charts they didn't know.

I also posess the two of the most dangerous qualities known. I'm inquisitive and I'm a tinkerer. Being a fitter and turner helps (or hinders?) too.

Brent

[Freakshow]

so how are you confirming the spring rates of unkown or unmarked springs ?

[brent j]

Freaky my post at 9.15pm has a link to a website (Pontiac of all things) with a really good spring rate calculator. Just enter the measurements and it does the rest.
I like this one as it gives the result in metric and imperial

[JC]

Ji,

126lb springs sounds too heavy for 73 CZ, even considering they were a fairly heavy bike.

[Ji Gantor]

Hi JC,
Yes it is a bit high. What spring rate are you running in your CZ400 1973 ?
As I said that was the first run and I was using a calculator. There are quite a few calculations involved and when I checked back I discovered that I had left the assembly preload out. I have now written an Excel program that works the spring rate out in the blink of an eye. It is great because you can perform what if functions, that really narrows down the spring rate you require. The spring rate wildly varies due to rider height and preload especially on bikes with only 100mm's of rear wheel travel.

Ji
 

[Lozza]

Anyone wanting more insight into this subject I suggest ringing Walter's old mate Greg from ASR in Melbourne pronto. I'll have my 100 pages of knowledge next week, will review when I put it down.

[Ji Gantor]

Hi Lozza,
That is good news I will ring Greg next week.

Thanks Ji

[evo550]

Ji,
Just had a bo peek at the Husky manual, factory shock spring is for a 75kg rider, preload is set at the factory for that weight at 95mm, Rear wheel travel with rear tyre off ground is approx 330mm the standard spring rate is given in sparts parts manual (I don't have one of those).

On the twin shock conundrum, does the spring rate required, need to halved, to split the load between two shocks?