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Linear or Progressive wound front springs?

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by QuarterWit, Oct 2, 2007.

  1. Hey fellas!

    My front end on my Yamaha SR is as soft as shite, and bounces around uncontolably at speeds, resulting in the dreaded tank slap. It's not very surefooted, and after having a knowledgable friend of my look at it - just as i suspected, the front end is muchas too soft and has a disproportionate amount of rebound in it.

    So, a few things are going to happen to 'er - sadly it's even too soft for a heavier weight oil. First of all, some springs and some heavier weight oil and if it's still too soft, some air caps. If I still haven't solved the problem it's getting some racetek gold valves slipped in and away I go.

    After that I'll looking at a set of ohlins piggybacks some for the rear. (When i sell my first born to raise funds.)

    Anyway, asking around for some advice has been a nightmare. It seems everybody has a strong opinion if i should get linear or progressive wound springs for the front, but nobody can really back up the theory with anything but loose rhetoric. But I've heard both from people's who's experience does count.

    So, weigh in - which way should i go, and in layman's terms, why?

    Cheers in Advance!
  2. The spring controls dive (and the oil weight/valving on some shocks) and keeps the mud guard off the tyre. the oil and valving control rebound.

    So what is your shock doing? compressing too much or rebounding too quick?

    have you had the springs out to make sure they havent broken? have you replaced the oil, have you checked the valves for broken petals?

    Your getting tank slap? (rear end all over the place)or do you mean head shake? (front end wobling)
  3. Compressing too much, too easily and too quickly.

    I've been getting tank slap as in the front end going lock to lock in a manner of a second and smashing my thumbs against the tank. From my understanding, it's because the front end is too soft and changes the steering geometry.

    Nothing wrong with the rear end at the moment, just want to (eventually) clean it all up.
  4. Lots of things trigger a tank slap, I suspect the springs only amplify the problem and are not the cause.
    I'd get stiffer springs if they are too soft for sure, but I'd also check head and wheel bearings, and check to ensure front rim is true and balanced.
    I cured a head shake problem on the GTR by tightening the rear swingarm pivot bearings..........

    Regards, Andrew.
  5. Any SR could use stronger fork springs than standard, particularly because they soften as they get older.

    You may be able to get progressive springs which are the best for road use because they will provide a soft springing action when there is not a lot of load on the front wheel (under acceleration) and a firmer springing action when the brakes are on hard (to help resist bottoming out). Most 1980's and 1990's standard bike springs are step-progressive with 2 different rates. "Progressive" brand are smoothly changing rate from one end to the other. Race springs are single rate.

    However you may find it hard to buy progressively wound springs for the SR. I would take a current spring to a wrecker and ask if he has any cheap springs that would be stronger to replace it. Ideally, springs out of a heavier bike from a similar era, eg a GPZ900, GS1100 etc. Make sure the outer diameter is similar so they fit in your forks. It'll be a cheap improvement.

    Then, put 20w oil in the forks, and put enough spacers on top of the spring that the forks compress 1"-2" when you sit on the bike at a standstill.

    Made a WORLD of difference to my CX500, along with 20w oil. Anything more fancy than that is a waste of money because the frame and rear shocks and fork structure is not up to the task. Once you've replaced the rear shocks with anything better (eg IKON/KONI or them ohlin$), then rebuilt your frame and swingarm with bracing, THEN you bolt on a completely different set of forks.

    Don't blow your money on gold valves. They're too good for an SR (unless you're spending 15,000 to make the nicest SR in the world, in which case you should bolt on new forks and frame).
  6. What you are describing here is not a spring problem but a damping problem.

    I'd be putting heavier weight oil in to start with. It's cheap and easy.

    Are you running out of travel?

    Your spring may well be too soft, but it would be a poor assumption without putting heavier weight oil in first.

    To answer the question about progressively wound springs, you need to understand that forks are rising rate due to the air in them anyway. So a progressively wound spring increases the rate of rise. i.e. it gets stiffer quicker. So you can go from compliant suspension to almost locked up quicker then you would like.

    If you get some advice from an expert you should be able to get close enough with linear rate spring. And yeah I wouldn't be investing in gold valves etc for you bike unless there is a big disparity between your compression rates and your rebound rates. Even then that can be solved with a drill bit or some washers.
  7. Maybe. You are making one big assumption, though, without checking all the available data. You could be quite wrong.


    Trevor G
  8. The bike is in very good condition,

    I'm not running out of travel or anything, there's no 'chunk' sound/feel I've felt on other bikes that run out of it. I'm pretty sure the problem can't be solved with some heavier grade oil. A mate of mine who knows about such things had a look and said my best hope was some air caps, but even then, I'll want to come

    I've decided to take the lazy way out (is it really the lazy way though?) and pull the forks and start from scratch. 20wt oil with some ikon RD springs and some 330mm Ikon shocks for the rear. (up 15 or so mm on the stock SR)

    I'm pretty much running a softer version of a friend's racing SR, that has an 18" wheel up front, racetek gold valves, some obscenely heavy oil and a few other things.

    Thanks for your advice fellas. Wait till i do it myself and watch the disaster unfold...

  9. You and I are both too lazy to read the suspension setup "sticky" at the head of this forum. Maybe it's not very good so I will give you the facts on how to work it out for yourself.

    The Rough Guide to Spring Setting

    1) With the bike upright and no rider or passenger the front and rear suspension should be close to the top of the travel, or fully extended.

    2) When you sit on the bike the suspension, front and rear, should compress between 25% and 30%. If the forks have 6" of travel they should compress about 1.5", and the rear shocks, with about 4", should compress maybe 1".

    3) You will have to measure the extended length, and then see what it is when you sit on it. This is slightly different to mx or enduro bike setup with long travel suspension, which often have static sag (no rider) built in.

    If you try to build in static sag on an older, short travel suspension bike like an SR you will end up bottoming over every bump!

    4) Next, push down/up/down on the forks a few times with the front brake on and see how far they compress. If you can compress the forks more than 2/3 of their potential travel the springs will be too soft.

    If you can only compress the forks about half way, and they do not have preload spacers or the preload "jacked up", the springs are too stiff.

    How do you know how much travel your forks have? Either read the manual or:

    5) Remove the fork caps, remove the springs and gently lower the forks until they will travel no more. Measure the distance from the lower yoke to the top of the dust rubber, or the fork leg.

    Extend the forks fully and measure this distance again. Subtract the small figure from the large one and you have the fork travel.

    If the spring seems right according to the above guide, move on to the damping guide.

    Rough Guide to Correct Damping

    A stiffer spring requires more damping, a softer spring requires less damping. Once you have the damping sorted, changing preload (the amount of pre-tension applied to the spring, either with a screw or cam arrangement on the rear or spacers or screw adjustment on the front) does not affect or require a change to damping.

    I'll say that again: Changing spring preload does not alter the amount of damping required.

    Here's how you test:

    1) When you compress the front or rear suspension, the suspension unit should compress and return to its original position, and go no further.

    2) If the suspension unit has too little damping, when you compress the unit it will return and go past its starting point, and then drop back down to its starting point, or maybe even go further.

    3) If the suspension unit has too much damping, when you compress it it will not readily return to its original starting point, or will do so noticeably slowly.

    It can take a little practise to push down and then release enough, before applying just the original weight again, to see this work properly. Don't give up, it really does work this way.

    Another test for too much damping: push rapidly up and down on the suspension - if you can get it to gradually go further down, that is because it is returning too slowly and the repeated compressions will eventually, on the road anyway, result in the suspension bottoming out.

    Just about every 4 wheel motorbike has way too much rebound damping, especially Polaris. But Yamaha and Honda are no better. Try one next time you get near. I haven't tried on any of the chinese ones, though.

    Testing Compression and Rebound Damping

    1) With the spring removed, it should be a lot easier to compress the unit, than to extend it. Somewhere in the order of 3 or 4 times more rebound than compression damping.

    2) "Gas" type rear units operate under high pressure with a nitrogen sac which is compressible, to allow for the varying volume available within the suspension unit (almost always a rear shock) as the shaft goes in and out. It is much harder to test for damping in these, and in general I don't like 'em.

    Give me a good oil-control shock like a Koni or Ikon any day - they are demonstrably better damper units.

    Servicing Suspension Units

    Forks are easy. Change the oil for a start. Most forks start out with 5 or 10 weight oil. If the damping is too light, use 15 wt fork oil.

    That, for example, is what we used in the end in our VTR250 to get it right.

    If heavier oil does not seem to affect the damping in your forks you will have a broken shim or wave spring, or perhaps a damaged or broken control ring. Or even a blocked valve.

    The fork spring in the '99 VTR250 is a little on the weak side - I fitted aircaps, which increase both the preload and the spring rate. They only need a psi or two to raise the front end by 15mm and stop the bottoming "shock" as you drive across gutters into a servo, for example.

    Rear units.

    Konis and their modern replacements, Ikons, are rebuildable by the factory, or by the user if you are keen. Before you go that far, try adjusting the damping.

    Some of the later units have a knurled ring up near the top mount. The higher the number, the greater the damping.

    Earlier Konis have an adjuster which is only accesible by removing the spring. You then push the damper rod all the way in and turn it gently until it drops another mm or so as it engages with the adjsuter. You have about 5 half turns of damping adjustment.

    From memory, anticlockwise increases damping, but that will become obvious as you try it afterwards.

    Non-rebuildable rear units.

    Most Japanes (and Korean, Brazilian, Chinese and Thai) shocks are nominally non-rebuildable - they are welded up in manufacture.

    However, there are some clever suspension specialists around who cut them open, replace damaged components and then weld them shut again.

    We had that done to our no-longer-damping rear VTR250 shock just recently. Interestingly enough, RAD Shocky Repairs in Brisbane filled this unit with 15 wt shock oil, too. It is just adequate, but so much better than before. Cost us around $100 including freight.


    Trevor G


    And understandably so, too! A soft spring needs lighter oil, a heavier spring needs heavier (higher viscosity or weight) oil.

    It won't make much of a difference either way, really, unless you get the correct rate and length spring.

    The correct rate progressive spring will be softer initially but then stiffen up as the wider-spaced coils take over. Progressive is always nicer as long as they are the correct rating and length. Too much preload and you will not notice the progression!
  10. for sure, but it's the place to start. One thing at a time and start with the most likely. It also helps that it's the easiest and cheapest.