Fork springs

[Mark Annan]

I think it is time for some new springs in my forks.  I can use up about half the travel just giving it a little "hold the front brake and push down on the bars" bounce test.  It will sag about 3-4 inches from full extension with just the weight of me (160 lbs) and the bike.  The initial travel is very soft and then it gets very firm almost instantaneously.  I currently have a mix of 7.5 and 10 wt oil in them.  They are 32mm Ceriani's on a 73 Sixday.  I'll be using it for MX.

Anyone know what the original forks spring rate was, and the free length?  Any recommendations about springs?  Anyone have any experience with Progressive Suspension replacement springs?

Thanks,  Mark

 

[Larry Perkins]

Mark, David Boydsten at AMS has some good ones.

 

[Mark Annan]

Thanks for the info.  I'll check with him.  I'll check with Mat at Speed & Sport also.  I'd like to get them from someone that supports the events I ride in.  I bet they both carry Progressive Suspension springs.

 

[Steve Minor]

Try these 2 sites too....but please understand I've not used either buisness, I just happen to have their web site addresses.

http://www.frankmain.qpg.com/
http://www.vintagedirtbike.com/

 

[Mark Annan]

finally had a chance to pull my springs out and have another look at them.  They are progressive wound.  Free length is 512mm (20 inches), 25mm diameter (1 inch).  I did a crude spring rate test (bathroom scale& a measuring rod ).  The initial rate is approximately 19 lbs/inch, the rate at 6 inches of compression is about 25 lbs/inch.

Anyone know what the stock rate is?  These sound pretty soft to me, especially when compared to the rear shock spring rates.

 

[Mike Lenz]

Progressive makes 30 to 40 lb springs

 

[tomale]

Mark, how did you measure the spring rate using bathroom scale and a measuring rod?

Thom Green,I own and ride a 76 250 MC5 MX which I bought new.

[Mark Annan]

I use the rod to guide the spring, and measure how far I am compressing the spring.  The scale simply reads the force (in lbs) that the spring is exerting when it is compressed.

Here is how to do it.  Take a piece of rod and slip it inside the spring.  The rod needs to be longer than the spring (long enough to hold onto) and strong enough to support the spring as you compress it.  The rod should fit fairly close to the ID of the spring but not so close that there is a lot of friction as the spring is compressed.  I used a 1/2" rod in my test of a spring with an ID of 17mm.  

Stand the rod and spring upright and mark where the top of the spring is on the rod.  Slip the rod out and make more marks 1 inch apart, starting from the first one you made.  Now slip the rod back into the spring and stand them both up on the scale.  Now comes the tricky part.  Hold the rod upright while pushing down the spring.  An assistant to hold the rod really makes it much easier.  I used a short piece of copper tube to hold onto and push down on the spring.  It would be better to have a bar with a hole in it (so it makes a T on the rod) to use.  A piece of 1 1/4" or 1 1/2" water pipe with a hole through it so it slides up and down the rod easy, with a handle on each side of the rod, should work well.  To make the measuring marks easier to see you could make your first mark on top of whatever you use to push down the spring.

Starting at 0 (no load on the spring) push the spring down 1 inch and take a reading on the scale.  Push it down another inch and take another reading.  Keep it up until you have completely compressed the spring, or you run out of "push", which ever is greater.  Try to get readings every inch at least equal to the forks travel.  Do several runs at it and average the readings at each mark, you will likely have a variance of a couple of pounds at each reading.  The first inch is pretty easy.  1 Inch of compression = the reading on the scale= the initial rating of the spring.  The next reading you will divide by 2.  XX lbs on the scale/2 inches of compression= spring rate at 2 inches.  Do this at each reading.  You will want to do it for each inch of spring compression because many springs are not completely linear in their spring rate.  It might only take 20 pounds for the first inch of compression, but it make take 30 lbs for each additional inch of compression near the point of complete compression.

Be careful when you are compressing the spring.  If it suddenly gets released it can really fly.



Edited by - Mark Annan on 06/02/2003  12:01:19 AM

[tomale]

Thanks mark, I think I understand. I will give it a try.

Thom Green,I own and ride a 76 250 MC5 MX which I bought new.

[LINDEMAN]

Hi Mark.For reference I use 23 lbs springs in my 250 forks for mx.And 21 for hare scrambles.The stiffest I have ever needed are progressive 25/35 for my yamaha 650 which weighs over 350lbs.I weigh 175. I would question your scale test since I have had three sets of used 32mm penton springs professionally tested.They ranged from 16-19lbs.I would guess you should maybe try a fresh 19 or 20 lb.rate.I think straight rates work best overall except for very heavy bikes which can use the bottoming help of a progressive rate.If you were happy with the performance other than the sag?Then I would just cut about 1.5 inch of your old springs on the soft coil end, which will give them a slightly stiffer rate and then preload them to your weight.You might also try 20wt. oil at 10-20cc over std. for extra bottoming resistance.Nothing like a cheap fix to make a vintage racer happy!

 

[Mark Annan]

Thanks for the information.  I haven't had a chance to really test the bike yet.  I'm just getting it finished up and ready to go.  I just put 1 inch of preload in (there was 0 before).  It got the sag to a more acceptable level.  I'll have to get the bike in some rough stuff to do any further diagnosing.  I'll report back my findings.

Mark

PS, I talked to AMS and S&S, they were out of stock on springs.



Edited by - Mark Annan on 06/03/2003  11:46:34 AM