Understanding Preload

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Brian P said:
Ride height (not preload!) on the rear is adjusted by changing any of the following depending on the bike:
- The overall (uncompressed) center-to-center length of the rear shock. Some shocks that have built-in ride height adjustments allow this to be changed, but this adjustment is NOT on the spring seats, it's on the shock rod itself.
- Center-to-center length of rear suspension "dogbones", or equivalent modifications that change the geometry of the linkage in some way.
- Thickness of spacers at the upper shock mount, between the top of the shock linkage and the chassis.
- Spring perch height.
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Fixed!

Adjusting spring perch height certainly changes ride height. Of course that adjustement changes other things too, that's not a meaningful observation as there are almost no adjustments that affect only one element of the suspension settings in isolation of all the others.

Adjusting the spring perch height CAN NOT affect preload, because there is no preload on a properly set up bike. This is demonstrable two ways;

- Add any amount of weight to the bike. Does the suspension compress? Yes, because there is no preload.
- Adjust the spring perch height. Does this change the static load on the suspension? No, because there is no preload.

So there is no preload on the bike, how can you adjust it?
 
Brian P said:
Ride height on the rear is typically not adjusted by the threaded spring collars. Those adjust ... preload! and therefore sag. They are analogous to the threaded preload adjusters on front forks or the length of spacers above fork springs on forks that don't have external adjustments.

Ride height (not preload!) on the rear is adjusted by changing any of the following depending on the bike:
- The overall (uncompressed) center-to-center length of the rear shock. Some shocks that have built-in ride height adjustments allow this to be changed, but this adjustment is NOT on the spring seats, it's on the shock rod itself.
- Center-to-center length of rear suspension "dogbones", or equivalent modifications that change the geometry of the linkage in some way.
- Thickness of spacers at the upper shock mount, between the top of the shock linkage and the chassis.

Adjusting ride height and adjusting preload are two separate things!

And it's true that nobody directly adjusts "preload" with a target value for the "preload". One adjusts the "preload" in order to get the "sag" correct. Then one adjusts ride height front and rear to get the geometry of the bike correct so that it steers properly and has proper grip coming out of corners (mostly rear) and has sufficient cornering clearance.

Measure the sag, compare with target value, adjust preload, re-check. Ride the bike to observe steering feel to establish if ride-height changes are warranted. Then you adjust the RIDE HEIGHT - not the preload! - in order to get the steering response correct.
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Gee, you'd think you're a smart guy engineer or something, being able to put this in terms that lay-people can understand.. :-)
 
I have a cbr F4i, and weigh about 215lbs. After I turned the preload adjusters all the way in the rider sag was still about 50mm. I orderd 1kg/mm
springs, and now withe the the preload adjusters in the middle the sag is about 30mm. The bike feals way better, and does not bottom out under
medium braking. The stock springs on that bike are only 0.66kg/mm. This is the best change I have made on this bike.

Dave
 
This is a very difficult concept for most people. Best way I've found to describe the concept is that you want two thirds of the suspension available for braking and compression over bumps, BUT at the same time you want a third available for rebound or "getting light" over the tops of hills. So if it's set for static sag in this manner and it bottoms out under braking or on rough roads, you need stiffer springs because more "preload" will take away from that top third. And conversely, if you can't get that top third of static sag with rider without "preload" then you need softer springs. Don't even mention ride height. Not easy to grasp, and the magazine riders do not help the situation.
 
I've just got to say how encouraging this is! All of these posts and not one person arguing that preload stiffens the suspension. Never had that experience before. Maybe GTA riders are way smarter than average?

If you want a good laugh, read the comments at the end of this article on the MotorcycleUSA website. I noticed the typical errors about preload in the article and mentioned it. What ensued was a cyber lashing, wherein I was informed that I didn't know what I was talking about.

I was logged in as "CanaDuc".

http://www.motorcycle-usa.com/103/1...2012-Ducati-Streetfighter-848-First-Ride.aspx
 
fastar1 said:
Adjusting the spring perch height CAN NOT affect preload, because there is no preload on a properly set up bike. This is demonstrable two ways;

- Add any amount of weight to the bike. Does the suspension compress? Yes, because there is no preload.
- Adjust the spring perch height. Does this change the static load on the suspension? No, because there is no preload.

So there is no preload on the bike, how can you adjust it?
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Again, this is simply wrong.

The bike compresses when you add weight because the weight of the bike itself overcomes the spring preload. When the bike is resting it is at an equilibrium point where spring force balances bike weight, any additional weight will cause more compression to a different equilibrium point. If you have the forks or shock off the bike and run your experiment again you will find different results.

At full fork extension the spring is still compressed from its free length. The spring is exerting a force to extend the fork but the mechanical stops in the fork oppose it. For the sake of argument lets say the spring is exerting a 50N force at full fork extension (this is the numeric value of....... wait for it... spring preload). Any external force less than 50N applied to compress the fork will not actually compress it at all. It will relieve some of the force exerted by the mechanical stops but nothing else.
 
caboose483 said:
Again, this is simply wrong.

The bike compresses when you add weight because the weight of the bike itself overcomes the spring preload. When the bike is resting it is at an equilibrium point where spring force balances bike weight, any additional weight will cause more compression to a different equilibrium point. If you have the forks or shock off the bike and run your experiment again you will find different results.
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Do you mean with the springs off the bike? Either way, I'm not sure what that's meant to illustrate. I'm saying that any bike with a setup that is in the general ballpark will not have any preload. Like I said; "...a properly set up bike..." Your scenario seems to presume a bike that couldn't even be ridden out of the garage.

At full fork extension the spring is still compressed from its free length. The spring is exerting a force to extend the fork but the mechanical stops in the fork oppose it. For the sake of argument lets say the spring is exerting a 50N force at full fork extension (this is the numeric value of....... wait for it... spring preload). Any external force less than 50N applied to compress the fork will not actually compress it at all. It will relieve some of the force exerted by the mechanical stops but nothing else.
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Again, there is no real world scenario in which anyone would want to set up a bike with the shocks at full extension.

Nobody's going to be playing with "preload" on a bike that collapses on it's shocks or that is fully extended on it's shocks.
 
I think you should read either

- Sportbike Suspension Tuning by Andrew Trevitt or
- RaceTech Suspension Bible

before you post again in this thread. You are so far out in left field you're in the parking lot cooking hot dogs.
 
YellowDuck said:
No, even with progressive springs, the springs are operating in the same range (i.e., with the same compression) regarless of the preload setting, since they are supporting the same weight regardless. Notice that in Figures C and D, the spring length is the same. This is also true with progressive springs.
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Correct, I was just blindly referring to the amount of spring compression with each kg of weight added, pointing out that this is no longer a linear relationship.
 
caboose483 said:
You have absolutely no idea what you're talking about.
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LOL!

I think I'm able to detect some restraint in your reply, based on the thousand other times we've ended up in this impasse.

Anyways, I probably need to clarify (again) my earlier response...

If a shock is at full extension and it can be fixed with what we presently call a "preload" adjustment then yes, it can help solve that problem. You will note however that the problem, a fully extended shock, manifests preload and the solution, a shock with some range of rebound travel, REMOVES the preloading of the shock.

Similarly, if a shock is fully compressed and the problem can be fixed with what we presently call "preload" adjustment then the solution once again will end up with the REMOVAL of the preload forces from the damper or spring (if coil-bound).

This is what I'm saying.

In a properly set up bike, preload won't even exist. Preload will be as close to zero as possible throughout the operating range of the suspension. Once the suspension is within that range what we presently call "preload" adjustment only serves to change ride height and bump/rebound travel (aka sag). So if you don't want to call it ride height adjustment then call it sag adjustment, but "preload" adjustment is absolutely incorrect.
 
fastar1, I'll be blunt. You're wrong. The rest of us in this thread are all on the same page and know what we are talking about (and yes, on GTAM, that's rare, but it's good to see in this thread).

I am not going to get into "spring perch height" because almost all modern motorcycle suspensions have a "spring perch" that travels up and down with whatever method is used to affect the "ride height" independently of the preload. (The spring is wound around or assembled inside the damper or fork and the damper as a whole has both a "spring perch height" built into it and a "ride height" adjustment for the overall length of the assembly - the "spring perch" travels with the shock or fork as an assembly - unlike many cars which have a spring that is separately mounted from the damper.)

To the person who replaced their springs with a higher spring rate, the spring rate is indeed something that will affect the rider-aboard sag and it will affect how much the forks compress under braking or in response to cornering loads. This is a further level of complexity and selecting the correct spring rate is a whole different topic of discussion ...
 
When one of the previously-illustrated fork spring spacers is in place but before screwing in the fork cap, with the front end of the bike off the ground so that the suspension is completely extended, there is most certainly "preload" on this spring. The fork cap has to be pushed down to compress the spring before the threads of the cap will engage, and screwing it in preloads the spring even further.
 
davkan said:
I have a cbr F4i, and weigh about 215lbs. After I turned the preload adjusters all the way in the rider sag was still about 50mm. I orderd 1kg/mm
springs, and now withe the the preload adjusters in the middle the sag is about 30mm. The bike feals way better, and does not bottom out under
medium braking. The stock springs on that bike are only 0.66kg/mm. This is the best change I have made on this bike.

Dave
Click to expand...


Per RaceTech's website, a 2002 Honda CBR600F4i came with:

- Stock Front Spring Rate: .660 kg/mm (stock)

- rear shock spring rate: 14.2 kg/mm
- Ideal Rider Weight for the Stock Spring: 165 lbs or 75 kg

--------------------------------------------------------------------

Recommendations for a 215 lbs. rider without riding gear:

Recommended Fork Spring Rate: 0.978 kg/mm

Recommended Shock Spring Rate: 15.375 kg/mm


Hope this is helpful
Installing the correct springs for the rider's weight will make such a big improvement for about $300 - $400 that most people are just so surprised and pleased.

Every bike is different!
 
Brian P said:
fastar1, I'll be blunt. You're wrong. The rest of us in this thread are all on the same page and know what we are talking about (and yes, on GTAM, that's rare, but it's good to see in this thread).

I am not going to get into "spring perch height" because almost all modern motorcycle suspensions have a "spring perch" that travels up and down with whatever method is used to affect the "ride height" independently of the preload. (The spring is wound around or assembled inside the damper or fork and the damper as a whole has both a "spring perch height" built into it and a "ride height" adjustment for the overall length of the assembly - the "spring perch" travels with the shock or fork as an assembly - unlike many cars which have a spring that is separately mounted from the damper.)
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That's right, and I haven't said anything to the contrary. The matter at issue here is; when the spring perch height is adjustable what is actually changing?

It seems even the premise of this discussion isn't understood. I'm willing to keep trying to explain if anyone else is willing to try and understand.
 
fastar1 said:
That's right, and I haven't said anything to the contrary. The matter at issue here is; when the spring perch height is adjustable what is actually changing?

It seems even the premise of this discussion isn't understood. I'm willing to keep trying to explain if anyone else is willing to try and understand.
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Spring preload is constant throughout your suspension travel.

You don't uderstand the mechanics of spring preload or the physics of spring compression.
 
I've been reading all of this and I really think you guys are talking past each other. I think what fastar1 is saying is simply that adding preload doesn't change the tension in the spring when the suspension is actually compressed below the top-out point (which is essentially always), so he objects to the term "preload" because to him it implies that changing it somehow changes the "load" (tension) in the spring, which it doesn't.

BrianP and caboose483 aren't getting this line of reasoning because they are so well acquainted with the concept of preload and technical useage of the term that they have no problem realizing intuitively that it refers only to the tension in the spring with the suspension topped out. They say that preload is a constant regardless of where the suspension is in its stroke, which of course is correct - but that's hard to grasp for many riders, even if they do understand what a preload adjustment does.

In terms of how a preload adjustment actually affects the suspension, as far as I can tell everyone is saying the same thing.

fastar1 is told he "has no idea what he is talknig about" when I think the truth is that others "have no idea what he is talking about" because he is using terms a bit differently - speaking a slightly different language, but with the same meaning.
 
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YellowDuck said:
I've been reading all of this and I really think you guys are talking past each other. I think what fastar1 is saying is simply that adding preload doesn't change the tension in the spring when the suspension is actually compressed below the top-out point (which is essentially always), so he objects to the term "preload" because to him it implies that changing it somehow changes the "load" (tension) in the spring, which it doesn't.
Click to expand...

Yes it does.
 
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