Anyone out there have a stock shock (spring) off the bike?

[BrianM]

I'm looking for 3 quick measurements.  Wire diameter, spring outside diameter and total number of coils.

Mine's on the bike, but I got:

.42 inches (wire diameter)
2.8 inches (spring OD)
and 6 coils (best guestimate)

I just want to verify these without pulling my shock...

Thanks.  The same info from any of the other shocks that've been fitted would be nice too (I'll provide spring rates off these numbers).

[tomacGTi]

Brian, I'll see what I can do seeing that the old shock is sitting in a box in the basement.

Where do you want me to start counting the coils?

-Randy

[BrianM]

The very end (on the flat surface that rests against the spring stop).  I basically want the Total legnth of the wire used in the spring and can get that from the total number of coils.  

And thanks much, keeps me from pulling the shock in 40f* weather, still too bloody cold here in Wyoming to be picking up metal wrenches.  At least the snow is staying about 500' above town.

[erik]

I got 10.1mm wire diameter (0.4") measured with calipers.

about 72-73mm coil OD (2.83-2.87")

and about 6 active coils if I'm counting them from the right place.

http://i38.photobucket.com/albums/e118/edorp/bandit%20shock/IMG_0644.jpg


The 1991 (I think...) gsxr1100 shock I've got on it now has a wire dia of 12mm, coil OD of about 80mm and about 7.5 active coils.

http://i38.photobucket.com/albums/e118/edorp/IMG_8693.jpg

I think the gsxr shock is a bit too stiff, but the preload is cranked up almost to the max to get the laden sag to about 30mm (I was around 107kg including gear when I set that up). I'm not sure if the original spring was too soft or if it just didn't have enough preload available, the laden sag was about 60mm with the max preload and me at that weight on it. There's only about 105mm total travel so it was bottoming out really easily.
I think a longer spring, possibly of similar or only slightly greater stiffness than the standard spring might be the way to go as it would allow more preload which should prevent it from bottoming out.
That's what I reckon anyway, I'm no suspension expert. ;)

[tomacGTi]

Eric you beat me to it. I was a bit lazy at 11.30 last night to root around in the basement.

I would try adjusting the compression adjustment on your GSXR shock. FWIW, I weigh in at 160lbs (72k or so) with gear and I also have the preload down about as far as your picture.

I set the sag (30mm) via preload and fine tuned the rebound and compression to where I thought it performed the best on corner exit and entrance. I wasn't too concerned with how plush it was.

-Randy

[BrianM]

Okay, so the B4 shock has 8 turns (tip to tip) makes it a Much more reasonable 400in/lb spring.  That's along the lines of what I was expecting as Suzuki's typically have 400~500in/lb springs.

The GSXR shock is a 500in/lb spring....  which should be about right for someone of my size (250lbs) if my educated guess is correct.  But I may be off a rate one way or the other (my newly aquired Fox has just over a 500in/lb spring so we'll see).

Erik, you're right about the preload thought.  In a perfect world, you should only need to preload the spring about 10~15mm to get a sag of roughly 25~33% of the total travel.  Normally, that means you'd need a stiffer spring ~ but because it's a non-serviceable OEM shock, it's very likely that the oil is worthless and you're getting no damping which means that the shock will compress and rebound easily.  

I'm just a touch over your size, have a background in MC suspension and a shock with a 500in/lb spring...  so I'll report back.  It might just be that you want to try and get that shock serviced (if it can be, it may be possible) to get fresh oil and a 300psi nitrogen charge in there.  But Thank you!  You've answered my question and taken a bit of worry off my shoulders (I was getting the B4 shock spring coming in at 800in/lbs!!)

If anyone else wants to know spring rates of other shock springs, just post the same info.  For the 180lb standard, you'd probably want a 450~475in/lb spring.

[gsxr400 racer]

cant i just read what it says on my penske spring lol Maybe we will have to discuss my suspension some day i have done some funny things over the years if anything you may get a good laugh

[erik]

Eric you beat me to it. I was a bit lazy at 11.30 last night to root around in the basement.

I would try adjusting the compression adjustment on your GSXR shock. FWIW, I weigh in at 160lbs (72k or so) with gear and I also have the preload down about as far as your picture.

I set the sag (30mm) via preload and fine tuned the rebound and compression to where I thought it performed the best on corner exit and entrance. I wasn't too concerned with how plush it was.

-Randy

Different time zones so it wasn't so late here. :)

The preload on the gsxr shock in that photo is different to on the bike.
http://s38.photobucket.com/albums/e118/edorp/bandit%20shock/?action=view&current=IMG_8786.jpg
It's cranked up a bit further on the bike, I should probably adjust it since I've lost about 15kg since I set it up, but I don't think it's that critical.
I've tried adjusting the compression and rebound damping a bit, at first I set them at half way, then I tried reducing them and just increasing them to where I thought they were ok. I still think the spring is too stiff. But it seems to work well with a pillion on the back!

Brian, why do you say the preload should be 10-15mm in a perfect world? As I understand it, the spring rate should be      chosen, then the preload should be set so that the laden sag is right. Apart from the sag, the only thing the preload affects is the suspension at full extension - higher preload will mean stiffer suspension (less responsive) at full extension eg. when braking heavily, the rear wheel will be more likely to bounce/skip than follow the road if the rear preload is high. But I think it's more important to have the spring rate and laden sag correct than to worry about the suspension near full extension.

I'm curious what formula you're using to calculate the spring rates. I've got a formula in a mechanical design textbook:

spring stiffness (in N/mm) = G*d^4/(8*D^3*n)

where G = modulus of rigidity (MPa)
d = spring wire dia (mm)
D = mean coil dia (mm) (eg from centre of wire to centre of wire)
n = number of active coils

using that formula and 6 active coils, the  bandit spring comes out at 70N/mm (400lb/in) and the gsxr spring with 7.5 active coils is 88N/mm (500lb/in).
I played around with different values for G and n, I used 80000MPa for G which is in the middle of the range (79300-82700) listed in the book for steel.

If you're using 8 coils in your bandit spring calcs, maybe the formula you use takes out the first and last coils?

I'm not sure how accurate my formula is. Some Z750S fork springs I measured at 1.08kg/mm came out at 0.96kg/mm based on the formula (with G = 80000MPa) so it's possible it gives low values but I'm unsure.

[BrianM]

Eric ~ spring rate and preload are directly connected ~ meaning that if you've chosen the correct spring rate, you'll be setting inital preload (off the bike, after a rebuild/refresh) to somewhere around 10mm.  That goes for both front forks and rear shock.  

If you're dealing with preloads more than about 5mm outside of that point, then you're looking at the wrong spring rate and ills that come with wrong springs (more preload will cause a too-soft spring to feel harsh when it should feel compliant, and wallowy when it should feel firm ~ too little preload on a too-stiff spring is the lesser of the 2 ills and usually just means the shock/forks feel too harsh nearly all the time.  The key here is that you never use the full travel).  

Anyway, the spring rate calculator I use was created by Tony Foale, and it automagically deducts 1.75 turns from the total spring legnth to get the "Active coil" measurement (which is the general consensus on steel springs).  It's the same formula, just in a pretty package.  You can DL it from Tonys site http://www.tonyfoale.com/ (oops, he's redone the layout and I don't know where his downloads are ~ try http://www.tonyfoale.com/progs ).

Cheers

[erik]

Interesting. I've got a couple of books on motorcycle design/setup - Sportbike Performance Handbook and Motorcycle Design and Technology. I worked my way part way through the first one but I've got to read more of them, just have to find the time...