cutting fork springs

[Bonez]

All Iwas wanting to achieve with mine was to lesson the dive factor when heavy breaking, the heavier oil did this perfectly without causing any side effects. Othere than that I would start looking at a set of aftermarket springs, and to top it off a set of Gold valves. But that would cost money.

Are they made from gold?

[2_SL0]

Race Tech's Gold Valve Kits.



http://www.racetech.com

[DEATH_INC.]

I've always cut 'em off the close coils,these are softer and we want to harden the spring,so removing a few of these will get it into the stiffer part sooner(they coil bind first).Plus you'll remove more coils(increase the rate more)for any given length you cut off.
There's a no of factors that determine spring rate;diameter of the coil,diameter of the wire,no of coils and coil pitch(to a lesser extent)and of course wire composition.We can't alter any of these except for the no of coils so this is why we cut 'em.

[erik]

hmmm.

I haven't done any cutting yet, and still aren't sure exactly what I'm going to do, but I'm getting there.
I've been trying to figure out the fork spring rates and having a bit of fun making contraptions to measure them.
First I tried just putting a steel tube in a block of wood to support it, then slipping the spring over it and putting weight plates on top of it and measuring the changes in length. But I couldn't compress the spring enough as I ran out of weights...

So then I thought maybe I'd use a 25litre water container and add more water to that for more weight, and just balance it on top of the weights already on the spring. I ended up borrowing my neighbours bathroom scales and balancing everything on top of them so that I didn't have to measure the water precisely.
But I still ran out of weight, even with the old car rim (9kg) the weight plates, and the 25litre container of water (and it was kinda unstable to boot...).

Possibly I was getting a bit carried away by this point, but I ended up making a contraption to measure the spring compression under different amounts of weight. I had to buy some cheap ($9.75) bathroom scales because I couldn't modify my neighbours ones and I had to get a few other bits and pieces, but the end result works pretty well. It's basically bathroom scales with a couple of bits of wood on top and bottom to stiffen the case, and a threaded rod attached to the bottom case and passing through a hold in the top case. I had to make the dial smaller so that I could position the rod close to the centre of the scales. A couple of bits of pvc tubing and a few washers and a wingnut completed it. (see attachments)

Anyway, I've got a graph of the spring rates now (see the attached gif) and you can quite clearly see the two different rates and the point where the close coils bind and the rate changes.
Just for interests sake at this point I've marked the approximate positions of the total fork travel, the static sag (haven't measured the laden sag yet, will get around to it soon), and the preload.

My next step is to read more suspension setup articles... just as soon as I get a decent nights sleep

Thanks everyone for your input so far, it's informative and helpful

[erik]

Ok, I've got a question:

Do bikes always bottom out the forks when doing stoppies?


The reason I ask is because I tried to calculate the max force the forks would experience, and the figure I ended up with would require a far higher spring rate than what I've seen advertised for other bikes (approx 1.75kg/mm as opposed to 1.05kg/mm highest I've seen advertised).

I guess I should forget about trying to calculate a spring rate to aim for and just try to increase it a bit from what it is.

[Blakamin]

The reason I ask is because I tried to calculate the max force the forks would experience, and the figure I ended up with would require a far higher spring rate than what I've seen advertised for other bikes (approx 1.75kg/mm as opposed to 1.05kg/mm highest I've seen advertised).

did you say something??? calling JRANDOM!!!!!!

[avgas]

Ok, I've got a question:
Do bikes always bottom out the forks when doing stoppies?
The reason I ask is because I tried to calculate the max force the forks would experience, and the figure I ended up with would require a far higher spring rate than what I've seen advertised for other bikes (approx 1.75kg/mm as opposed to 1.05kg/mm highest I've seen advertised).
I guess I should forget about trying to calculate a spring rate to aim for and just try to increase it a bit from what it is.

yes and no, it really depends on the springs and the rate of recoil they have left in them. As mentioned before,basic springs (not progressive) are an exponential based systems. So what actually happens is on the top of the curve u will end up with 3 or more times the initial weight/distance measurement. By preloading the spring however this does compress the "compression curve" and will give a little increase in height during overload (stoppie etc). But this height is just a buffer - so it still "bottoming out".
If you really want perfect forks though a fork kit can go right way.
I acutally run a lighter oil, as i find the forks have better response rate. And i dont end up bunny hopping anywhere anyway.
I have seen a few forks that didnt bottom out while doing stoppies, but they were heavy loaded for lard asses - and prob not that comfy to ride on.
At the end of the day - if u want to spend money on ya bike, those gold kit are really good.

[F5 Dave]

Whoa! I think you are a bit out in the calculations for what you require, did you assume TWO fork springs?

For example my 750 yam uses .8kg springs & that will be a sight heaviers than the 250. In that link about Racetech has a useful spring calculator, choose a bike the same weight as yours (no linkage on the front of course) & make sure the rider weight is in pounds. Choose Street & the recomendations they give are often on the heavy side I'm told.

On that site I gave with the spring article there is another on measuring springs using a lever & tape measure + scales, though I am impressed with your elaborate spring holders. Personally I'd just make them a bit heavier & try it.

[scroter]

i wouldnt bother trying to calculate your current springs weight its progressive therefore different weight at one end than the other. cutting is extreme, if you do, dont forget to flatten the end youve cut. i still would reccomend linear rate springs as the air pocket left above the fork oil is like a progressive spring anyway, eg. add more fork oil will help with bottoming out without increasing the stiffness of the initial stroke. ie you should only notice it when hard on the brakes try it it works.

my suggestion contact someone in the know like ohlins or ray clee(whitepower) or suspension tech, they should be able to tell you the correct weight springs for your weight and the correct fork oil level to stop bottoming out. in my experience robert taylor(ohlins nz) usually advises rather stiffly sprung setups but then he will usually let you send springs back for either softer or harder springs if they dont suit.

[erik]

I'm pretty sure I got the calculations right, but I guess that the damping plays a large role in preventing the suspension from bottoming out. But since I don't know how to take that into account, I can't really go any further with calculations.

I think it is probably just time for me to adjust things and test it. I reckon I'll try increasing the preload just by itself at first. I read that preload should generally be set so that the laden sag is about 30 to 33% of the full suspension travel for streetbikes (http://www.strappe.com/suspension.html), and as it is my zeal is at almost 45%. After that I'll have a go at cutting the springs...

[erik]

Well, I cut the springs with an angle grinder last sunday. That was before I learned it was illegal to heat motorvehicle suspension springs...
Maybe I wouldn't have done it if I'd known before, not sure...
So caz it's not entirely legal, I won't mention who it was that helped me bend the coils flat. But thanks for your help

Before putting them back in the bike, I tried measuring their rate to see how they compared to the expected new rate and found they almost perfectly matched the predicted rate (see attached pic)
But, after compressing the spring, the last coil that had been bent flat stayed bent in the compressed position!
I guess I should've expected that since by heating the coil, the steel lost it's temper.
Now I'm worried that because the end isn't flat, it'll make the springs rub excessively on the insides of the fork tube. It wouldn't have been so much of a problem if I had cut and heated the top end, since it doesn't move in the tube, but because I did the bottom end, it's sliding up and down against the tube the whole time, so it'll probably cause significant wear and maybe excess friction in the forks.
I'm guessing it wouldn't be too good to turn the springs upside down either, as that would put the close-wound coils in the moving end and since they bind against eachother, and there are a whole lot more coils, it would probably rub against the insides of the forks quite a lot too.
That said, I'm pretty sure it wouldn't have helped much by cutting the top, close-spaced, coils because that would have only increased the soft rate of the spring before the coils bound.

So this morning I went looking for springs at a couple of bike wreckers on Barry's Pt road. Action bike wreckers had a whole heap of springs and let me look through them, and I found a spring that looked like it would be perfect for the job, but couldn't find it's partner!
Atomic bike wreckers said they only had complete forks, not springs by themselves.

I've heard of a place that can re-temper springs, so I'll try phoning them on monday (they seem to be shut now) to see if it's possible and how much it costs so that I can compare it to the price of new aftermarket springs.
I've heard it's about $120 for a set of car springs, if it's significantly cheaper for the fork springs, I'll give it a go, otherwise I think aftermarket springs will be the best option.

It's quite disappointing since I have really enjoyed measuring the spring rate and stuff. :disapint:
Oh well... at least I'm learning stuff and having fun.

[erik]

Or I could just be thinking and worrying too much. A mate said he reckons he'd just put them in and try it.

hmm...

[Mr Skid]

Or I could just be thinking and worrying too much. A mate said he reckons he'd just put them in and try it.

hmm...

Yeah! Go for it! What's the worst that could possibly happen??

[DEATH_INC.]

Sounds like you over-heated the coil(should only be heated to 'cherry'red and gently bent).I would've cut the close end,but you've done it now.
You could either hit the spring with a grinder and remove anything that sticks out from the od of the spring,or just put it in upside down,it won't hurt anything.
Give 'em a go,you need to see out your experiment to learn what you've done so it'll be better next time.Don't forget to replace what you've cut off with a spacer too,and I always add a splash more oil too( a capful or so).
If you're talking about Snells or Archers for re-tempering,don't waste your time....

[erik]

Sounds like you over-heated the coil(should only be heated to 'cherry'red and gently bent).I would've cut the close end,but you've done it now.
You could either hit the spring with a grinder and remove anything that sticks out from the od of the spring,or just put it in upside down,it won't hurt anything.
Give 'em a go,you need to see out your experiment to learn what you've done so it'll be better next time.Don't forget to replace what you've cut off with a spacer too,and I always add a splash more oil too( a capful or so).
If you're talking about Snells or Archers for re-tempering,don't waste your time....

Thanks for that, I'll try putting them in upside down.
Re: the heating, yep, I think we heated it above cherry red. I'll be more careful next time.

I was talking about Snells, a friend that's into cars told me maybe they could help. How come it's a waste of time?