spring rate decoding

My apologies in advance if this has already been covered, but I am horrible at forum searching.
I was wondering if anyone would like to decode spring rates since the numbers don't seem to match actual load weights.
I am asking because I am finally ready to upgrade the rear suspension on my '79 kz650d2 and have no idea what springs to get. I weigh 140lb, ride back country two lanes (fairly aggressively), gravel and chip n seal regularly, and have a similar weight passenger sometimes. Thanx.

I highly recommend you contact Dave Quinn at Dave Quinn Motorcycles. He can set up a set of Hagon shocks with the correct spring weight (they make a bunch of different styles) that match your bike, your weight, riding style, passenger, etc. He really understands shocks! He set up a pair for me 6 years ago and I love them. Here's the link. Ed

davequinnmotorcycles.com/cgi-bin/webc.cg...GON_TWIN_SHOCKS.html

I've had pretty good luck with RaceTech products and service.

Thanx ed. Helpful as always:) That is probably the route I'll take. They hit my price point exactly. Though was still wondering if anyone knows what the #'s mean. I have seen rates for my bike range from 45/90 to 100/150, so I am guessing that is not rider weight. Call it a flaw, but I get torqued up when I don't understand something that is of interest to me.

If you don't get a satisfactory answer here I'm sure Dave Quinn will take the time to explain it. I found him very helpful and easy to work with. Ed

torquey wrote: Though was still wondering if anyone knows what the #'s mean. I have seen rates for my bike range from 45/90 to 100/150

The key number for a spring is the rate. That is how much force (or weight) is required to compress the spring a certain distance. Generally this would be in pounds/inch, or kilograms/mm. So a 100 lbs/in spring would compress by 1" if you put 100 pounds on top of it, or 0.1" if you put 10 pounds on it. The larger the number, the more force is required to compress the spring, the stiffer the ride. So given the choices above, 45/90 would be on the soft side, and the 100/150 would be firm...

The fact that you give two numbers (45/90) suggests that the springs you were looking at are dual rate springs, or progressive rate springs. Basically, the spring rate increases as you compress the spring more. That is, the force required to compress the first inch is less than the force required to compress the final inch. To make this they generally have one section of the spring with a low rate, and once with a high rate, and you can often see that the two ends are not wound the same when you look at it. The idea is that the initial soft spring gives a cushy ride, but when things really get crazy and the wheel moves a lot it stiffens up the suspension.

The Z1-R came with dual rate springs. All they did was stack two different springs together, and put a plastic spacer between them to keep them in line.

There are a ton of other numbers too if you were actually ordering a random spring, like coil diameter, wire diameter, and how far you can compress it before the coils bind. But often when you're talking about a single application, all you care about are the rate (lbs/in) and the free length of the spring.

How you translate the bike weight, rider weight, riding style, and specific shock and swingarm geometry into an ideal spring rate is super complicated. At the very least it requires a bunch of trigonometry and fancy maths. But beyond that it requires experience to know how to translate "comfortable" or "sporty" or "twisty canyon" into a target number to put into the math. I can't help you with any of that.

torquey wrote: Thanx ed. Helpful as always:) That is probably the route I'll take. They hit my price point exactly. Though was still wondering if anyone knows what the #'s mean. I have seen rates for my bike range from 45/90 to 100/150, so I am guessing that is not rider weight.

I was told the first number is the weight in pounds to deflect the spring down one inch, the second number is the ADDED weight it takes to push down another inch..... hence why they are called progressive, they get stiffer as they are compressed.

The weight numbers are measured at the shock post so do not correspond exactly to the riders weight. A passenger's weight is a lot closer to be centered over the rear shock than the rider's weight is.

I think the stock weight for my 750 was about 80/120 which seems to be perfect (I weigh 175). If you ride passengers, 100/140 might be a better choice?

Thanx folks. Weight to compress one inch makes sense. So a 100lb/in non-progressive spring with three inches of travel would take 300lbs to fully compress. That opens up a lot of options when looking at progressive springs. Oh, and yes, DOHC, I did get those #'s from progressive springs. Good call.