Effects of Spring Length

You are correct, the rate of a spring doesn't change with preload until the thing coil binds. Your statement was '10mm makes no difference'. 10mm on a shock spring will change the sag immensely.

 

Yes, you are correct. My statement was very poorly worded. The point I was trying to make was that adding 10mm of preload to a spring was no different than cutting a spacer "properly". In other words, it's just how you adjust sag, there is no magic benefit from "cutting a spacer properly", if you have to use 10 mm of preload to get proper sag, it's all good. Preload is how we fine tune sag, not cutting spacers.

 

My last 07 was a 750 had AK-20s and a Penske shock, this one has Ohlins 20 mil and a triple clicker shock. I don't think the difference is the hydraulics though. I had ridden other GSXRs of the same year with the Ohlins bits and they didn't feel near this good or have a tendency to oversteer. It is for real a night and day difference, but according to accepted logic the bike is set up all wrong (fork tubes, eye-eye shock length, etc.)

I'm not going to change a thing, but I want to understand what exactly has happened here. And not running any preload (through the external adjusters) in the front caught my attention, which is why the spring topic came up.

 

RM's got it right. You might want to consult someone at GMD Computrack with these questions as well. They have so-called 'sweet numbers' they've developed for various bikes/rider weights/preferences. Once we begin talking about rising-rate linkages, hi & lo speed damping adjustment, I am lost. That's one reason I'm into vintage. As for visualizing spring dynamics, I read somewhere that a coil spring is just a torsion-bar, rolled up. The spring material still behaves in much the same way. The rate is determined by wire diameter (thicker=stiffer), wire temper/tensile strength(higher=stiffer), & length(all other factors being equal, shorter=stiffer). I've also heard from many sources that fork springs which yield good sag & rate/compression action without the addition of preload are what you want, ideally.

 

If Rick has it right, why not consult him?

People who do this for a living take their time to come on here and post and answer questions. Why not give them a shot at earning your business? If they are this helpful for free imagine how nice the relationship will be as a paying customer.

 

Im no expert but there's installed preload on a shock spring.Thats diffrent then adding or backing off preload once the shock is on the bike (setting sag etc)..Also depends on the brand shock your using and how its valved and sprung.I would drop T-Man or lenny Albin a line for the correct annswer to your ??

 

I missed this post..He said what i wanted to say

 

Forgive me for resurrecting an old thread, but I want to bounce my current understanding of this topic off the forum.

My impression is that preload does have in impact on how "soft" or "stiff" the fork feels, but only in the initial portion of the stroke. I'm basing this off Paul Thede's explanation which he touched on in the linked Sport Rider article.

If you graph 2 spring rates with the same target sag, the softer spring will require more force to compress below the intersection point (rider sag) and less force above it.


http://www.sportrider.com/tech/146_9510_tech/index.html

For the sake of argument, you could assume that the bike/rider combo in the graph requires 30kg per fork (60kg total) to achieve 30mm of front sag w/ rider. Both the 0.5 kg/mm and 1.0 kg/mm spring can be dialed to give the targeted sag, but the too-soft spring will bottom sooner and will also require more force to compress from full extension, such as what might occur when you hit a bump while WFO or over a pothole/severe dip.

In effect, the softer spring will feel less compliant in the top of the range due to too much initial spring force, and will also feel more harsh in the bottom of the stroke due to bottom-out or due to the fork compressing to the point where the effective spring rate becomes super-progressive due to oil-level.

The stiffer spring would have the potential to be more compliant at the top of the stroke, and also potentially at the bottom of the stroke if the oil level can be safely reduced, thus allowing the bottom of the stroke to be less progressive.

I keep thinking I get this, and then read where someone says "preload has no impact on how stiff the fork feels... it only affects ride height". This would make sense to me if the installed length of the spring were also equal to the free length, but since fork springs always have some amount of preload, installed or otherwise, there will also be some amount of initial force on the spring. Therefore, additional preload will always have an impact on the initial compression of the fork (but not the actual rate of compression).

The way I imagine it is... the bike is WFO and the front is skimming the ground. The forks are fully extended and then hit a bump which generates 20kg of force. If there is more than 20kg of initial force in the spring and there is no load on the fork, then the bump will be insufficient to compress the fork and will deflect the bike instead, as though the front-end has a rigid fork.

To complicate matters (in a potentially good way), most forks use top-out springs which lower the effective spring rate at the top of the stroke, but it would seem to me that this is not always going to solve the too-soft/too much preload issue in all cases, and that too much preload would potentially cause the main spring to override the top-out spring, which would bring the rate change increasingly close to the zero axis on the graph.

Is this correct?

Here's another page I found on the topic:

http://www.peterverdonedesigns.com/05fork.htm