Mc 5 rear spring rates

[tomale]

I have been working on my Mc 5 in the hopes of getting it ready for spring... Lots of stuff to do...  I recently had the rear shocks rebuilt my buddy who owns a shop, He said the springs I have on them have too much pre load.. ie the springs are too long... and still the are too soft for me... I am wondering if someone has a good source for spring rates. I have talked to several shock companies in the past who have no idea what the spring rates are. I want them so that I can set them up my self. It has been my experience that most of the time the springs you get from these guys look in a book and have no idea what they are, nor will they tell you what is on the shocks they sell you.. besides that I have access several sets of NOS Fox springs and it would be cool to use them if they actually are the right length and weight... also any idea how do I tell what springs I do have...  I am not sure of the source and there is no markings on the springs.. maybe by measuring coil size etc. Any help would be great. I am a big guy and am hoping to get the bike set up right for me...thanks...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[joe novak]

Hello,  Somewhere in my vast information log is the formula for calculating spring rates.   I need to dig it out for you.  What I can tell you is that these characteristics (and others) determine the spring rate.  1.wire diameter  2. coil diameter   3. number of coils.   I have compared calculated spring rate values against actual spring rates determined by a spring rate machine.   The values which I had calculated are very close to the spring rate determined by the machine.  Is there an adjustment for the spring preload on your shock body; similar to what a KONI or Girling shock has (3-5 way twist adjustment)?   If the spring rate is not correct, preload is really not the answer.  It is only a crutch.  joe

[Big Mac]

Hey buddy Thom,
Hope things are well, long time no see. Heard you Seattle boys are setting up a Duvall race soon. Hope so.

I just went through a very un-scientific shock re-springing on my MC5 Marzocchis this past week. My spring comparison drill: Take one of the stock too-soft springs and set it on the shop floor side by side to some spring about the same length that you're wanting to compare to. Lean over and put a palm on each one and shove down on both for all you're worth. The stiffer spring will reveal itself. Usually the one with larger diameter coils and fewer turns, it seems.

The purple springs I'd been running were the stiffest Marzocchi springs I'd found, but still had them on full-twist pre-load setting + about an inch of spacer, and continued to bottom hard. At Salem last month, I collapsed the spring retainer clips and springs were bottomed on the swingarm at the lower mount...from bottoming on full coil bind due to spacers I presume.

I dug out some old clapped-out remote reservoir Ohlins I had around for parts ('82 Husky?). They each had a long spring just a little longer than the purple Marzocchi springs. Push test said they were stiffer. These springs were slightly larger inside diameter. I used the Ohlins spring retainers on each end, with a bit of spacer to snug things up. The eye end shoulder is smaller on the Marzocchi compared to the Ohlin, so I found a pair of heavy guage fender washers the right outside diameter and cut/drilled to make right-sized keepers that go in the Ohlin keepers.

Back on the bike, they seem to pass the seat-bounce-in-the-shop test. On 12" travel rear ends, about right is 1" free sag and 4" (1/3) sag with rider. With MC5 at about 9" travel(?), assume 3" rider sag is about right. I'm  twisted back/off the pre-load adjuster quite aways and have about 1/2" free sag and 3" rider sag. Will see how it does on the track but seems like the closest low-tech/low-budget set up for now.

By the way, for the first time since I've had this bike, took the shocks into D&S Cycle and had Mikey Dorn put 30lbs (Marz. bladder type) of nitrogen in each the right way.

The MC5 geometry had the swingarm shock mount closer to the axle than many of the later Euro bikes of '80 or so. My '81 KTM is way further forward, which means much higher leverage on the spring, so that era used heavier stock springs. Ask around for someone who may have extras on the shelf to try out, maybe even their too soft springs off a late-evo Husky or Maico.

Take care, tell Chris I said hi.


Jon McLean
Lake Grove, OR

[joe novak]

I found some info on spring rates.  I suggest that you Google "spring rate calculator", and there are many sites which will calculate the value for you.   Formula is rate= (modulus of spring rigidity) x ((wire diameter in inches) raised to the 4th power)) / divided by ( x (number of active coils) x ((mean coil diameter in inches) raised to the 3rd power)).  One source lists the modulus as 11,250,000 psi.  It is easier to look at the formula than try to decipher my script....   Lots of info on the site.

[Britt Boyette]

If you have springs that are soft and longer than stock, you should be able to trim them to size and that would also make them have a little stiffer rate. That or the trim and spacer method. Anyways, that what Super Hunky told me about soft springs.

Britt Boyette
1976 125 MC5

[joe novak]

I agree with Britt.  Things that increase spring rate that we can easily change is the length of the spring.  Yes, if you cut the spring (decrease the number of active coils), it will increase the spring rate.  The prefered method of cutting coils is to cut them, rather than torch them.  Heat changes the property of metal.  If you do decide to use the torch, heat it quickly and cut asap.   Have you asked around to see if you can borrow a loaner spring set to find the ones which work well?  Since you had your shocks rebuilt, we assume that those will be the shocks which you will use, and just need to find the correct springs.   I suggest you use the formula to determine your present spring rate, then use the formula to determine how many coils need to be removed.   We would hate to cut the spring too short to get the required rate, if necessary.   If you give me the three measurements, I can do the math for you:   spring diameter, coil average diameter, and number of active spring coils.   Is the spacing between the coils about the same all along the length?   Is the coil diameter the same along the length?

[tomale]

thanks guys, part of my problem... at least as far as this goes is that there is only one other bike the same year in this area... and as of right now it is not being raced.... but the idea using others springs might be helpful...
Joe, the Fox shocks do not have an adjustable pre load adjustment. thanks for the formula... I will have to have my wife help me with math stuff.
Britt, yea, the springs are too soft and too long. but I am afraid they will be too soft even if I did cut them...
Thanks Mac, Kris says hi..
we are doing pretty well. been a hard year... but getting better...It has been 14 months since I raced my last race. I can hardly believe it has been that long... Can't wait till I can race again... Maybe in May
I have seveal project I am working on.
It has been even longer since I raced my MC5 but that is going to change.
would be cool if I knew what the stock spring rate was as a reference point....any ideas?

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[Richard]

I may be thinking of another shock spring that I checked some time ago but I think that the purple springs were 135 lbs per inch. You could check by placing the spring on a bath room scale and compressing the spring one inch and reading the scale.

Also, I believe the fox shock spring preload is adjusted by removing the shock body spring retainer and you should see several groves in the lower body. In one of these groves there should be a wire clip that can be placed in any of the groves. The spring retainer holds the clip in place.

Best regards, Richard Beck

[tomale]

Hi richard, Yea if you use the shorter springs there are some adjustments by changing the clip position. but when you the longer springs, there are only one groove. thanks, for the weight on the spring... I think I have one purple spring.. I will check it out.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[tomale]

I got to thinking about the purple springs and why I only had one. when I bought the 78 400 I seem to remember it having a broken spring. Mac gave me a set of purple springs... Here is the point, the purple springs were a bit soft for me as well so I may need even a stiffer spring, I say may because the spring rate of the 78 is probably different for my 76.. just a quess but it makes sense to me....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[Big Mac]

Thom, your other broken purple spring is sitting on my bench.

Believe I ended up with it when we were swapping black and purple springs awhile back. The broken-end spring and the mate you have would be good experiments for cutting/shortening a set and see if it stiffens up enough. I also have some stiff/short versions from Works and Ohlins that could work for dual, and you can experiment with.

See http://www.superhunky.com/articles/Spring.php for some ideas.

[Mike Lenz]

See spring info from another posting of mine:
Posted - 12/03/2009 :  9:39:56 PM    
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Progressive has shock springs that will work on the Marz or most anything else for mc-5's. 03-1364B 140/190 lb progressive for us heavyweights and 03-1365B 120/170 lb for you regulars.
Also:
03-1368B 105/150lb
03-1367B 95/140
03-1370B 90/130
or get this
03-1371B 270/310!!!
Also
03-1386B 65 lb
03-1387B 75
03-1389B 90
03-1391B 110
03-1392 120
They also have some narrower springs for those non race restorations
02-1351 thru 02-1354
The B means black. They also have chrome and some blue.
You have to find a progressive dealer.
I would advise the progressive rate springs.
Dont buy em all, I still need some more!

Race Tech is making vintage fork springs now. Anyone looked into them?

Renthal Mini High Bars do not set you back in the seat at all, even with the swept back mounts. They have a modern straight bend, work real nice with the swept back mounts. Feels like your sitting on a modern bike.
 

[tomale]

thanks Mike, I was a wondering when you would weigh in.....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[tomale]

Mac, thats what happened to it... I think I should go find that other spring...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[garrettccovington]

I went to the Cycle World Cycle show in Long Beach on Dec 6th and Race Tech had a Vintage Tech (Mathew)and a complete vintage display set up. You can call them up and they have cartridge emulators, revalving kits, fork springs and can recoat everything.  You got the money, they can do it. Very nice display. Actually they can do just about anything front or rear.  They are only about 15 miles from my chino hills house.  As I recall it was about $700 bucks to do the works on my 72 6-day front end and around $1500 bucks to make a set of rear shocks.  I decieded to go with 1/2 PVC pipe and 20 weight for $1.98.

G

72 six-day