Custom Fighters - Custom Streetfighter Motorcycle Forum banner

Any of you fellows know how to calculate spring rates?

6K views 21 replies 8 participants last post by  sebwiers 
#1 ·
I'm converting a Honda CX500 into a monoshock suspension using a Honda CBR954RR Showa shock. I'm hoping you can help me find an appropriate spring for this project. I don't know what information you might need to help me sort this out, or if sorting this out is even a thing you can help me with, but I'll try and include as much as I know.

The shock is mounted 120° to the swingarm. The swingarm is 19 inches long from pivot to axle. The lower shock mount is located 4 inches from the pivot. Curb weight of the bike is as of yet unknown, but I'm expecting it to be close to 500lbs with me on it.

Any help you could lend on this matter would be great appreciated.

 
See less See more
1
#10 ·
Is the basic shape of it... Rear shock will carry around 300lb, multiply by 5 due to leverage and get 1500lb spring? A blackbird has about a 1000lb spring. A cb-1 has about 1100lb. Those are the highest rates I know offhand.
 
#11 ·
I chose this shock, excepting that it's also one I already owned, because it had the same amount of stroke per wheel travel in that positioning as it did on the CBR. We should only have to worry about valving if the shock will be moving faster or more aggressively. "Should". I guess I'll find out as time progresses.

I actually have a Ninja 650 shock coming...will probably be here today...that I'm going to slap on to test spring rate. It's got a 17kg spring on it vs. the CBR's 14.7kg. If that spring rate is appropriate then Summit Racing has an appropriate Eibach spring of the same weight that'll fit the CBR shock.
 
#12 ·
The approach I took with my bike (when doing dual to mono conversion) was just to buy a used shock made for a bike of the same weight with roughly the same travel, and THEN figure out a mounting position that allowed the shock to fully extend / compress when my swing arm went through its travel. IMO this is easier and more reliable than trying to calculate what spring rate is needed.

I think you will find that a single shock for a 500lb bike has to have a rather beefy spring, especially if given a high wheel rate (IE, leverage of wheel vs shock). In fact, as wheel rate goes up (shock throw goes down), the required spring rate goes up as its square. So, if you had two springs and now have 1, that means you need double the rate. Then if you take the wheel rate from 1.5/1 to 3.0/1 (double the wheel rate), that's another x4 to the needed spring rate... for a total of x8!

The shock I ended up buying (made for a BMW Montauk, which weighs 550 lbs and has ~4 inch travel rear) was ABSURDLY stiff (over 1000 lbs / inch) so required me to set up a mount with a high wheel rate. On the BMW, this wheel rate is the result of a long swing arm with the shock mounted near the pivot; on my bike it is the result of the shock being in a "layed down" position (leaning forward from axle). It ended up working very well, with the bonus that the "layed down" arrangement is naturally progressive, which suits my riding style.
 
#13 ·
Yeah like I said I picked this shock from my collection because the wheel travel per shock travel was the same as it is on this bike set up as a direct link. It will need to be a heavier spring but not much heavier than what the 954RR was already using. Most modern bikes use a big lever on the link and thus have a lighter spring. The 954 has an almost symmetrical link which means a heavy spring. It uses a 14.7kg/mm spring while most bikes use closer to 10kg. The spring I'm going to try is a 17kg/mm spring and I'm fairly sure it's going to be over sprung.
 
#14 ·
You need to take in consideration the shaft travel of the shock leverage ratio rider weight and position of the shock. The 954 has 57mm shaft travel and 135mm wheel travel which gives it about a 2.37 to 1 leverage ratio. If you can keep that ratio the same on your build it could work OK. It looks at the leverage ratio you currently have you would end up with over 220 mm of travel. You would have to mount the shock much further back to make it work and you may not have room to do that. Here is a Penske chart that shows some Honda spring rates.
I am building a CB600F which uses a heavy spring and has a direct no linkage mount may be a good option for a shock.
 

Attachments

#15 ·
It doesn't exactly that simply because there is another level in the mix in the linkage...but that is essentially what I did. I measured the desired wheel travel on the CX and basically dialed it in at just a hair more than what the 954RR had and then mounted the shock in a location that gave it full compression/expansion for that wheel travel. Because I'm missing that extra lever, though, my spring is too light...but that really should be the only issue I had.
 
#18 ·
Hah so I got a Ninja 650 shock just to try out. It has a 17.3kg/mm spring, but the shock is maybe 8mm longer eye to eye than the CBR shock.

The CBR shock has a 14.3kg/mm spring.

With the CBR shock I have about 2" of sag, but then it basically bottoms out...almost acts normal, but is just a touch slushy.

With the EX650 shock I'm sitting about 1.5" higher, but nearly 4" of sag, and feels even softer.
 
#19 ·
so how bout this shit

The CBR954RR Showa shock with a 14.6kg/mm was too soft. So I order a shock from a Kawasaki 650 Ninja because they have a 17.4kg/mm spring and I could try that for $30.



The 650 Ninja shock felt even lighter. I had nearly double the sag with that shock.



For kicks, though, I swapped the 17.4kg/mm spring from the Ninja shock onto the Honda shock and...it's perfect and I don't get it. About 30mm sag and feels nice when I jump on it.



 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top