Rear Suspension Pre-Load

jfheath

jfheath

John Heath
Joined
Mar 18, 2006
Messages
3,142
Age
71
Location
Ilkley, W Yorkshire, UK
Bike
2013 ST1300 A9
Occasionally I come across a comment - (rarely on this forum however) - which is related to adding pre-load to rear suspension - and it is sometimes described as 'making the suspension stiffer', or 'compressing the spring'.

My own opinion is that both of these assertions give the wrong impression, and that adding pre-load simply raises the rear of the bike.

This is to compensate for the fact that an additional load has already compressed the spring and reduced the ride height. The pre-load adjustment is required to retore a reasonable ride height, to place the suspension piston within the midle area of its range and to prevent the suspension from 'bottoming out'.



The diagrams are my interpretation of what happens to the suspension.
Comments / criticisms welcome - but if you disagree, please explain. I want to understand.

Rear Suspension Pre-Load 1 2 3 4 5.gif


Fig 1 - Weight of bike and rider. No preload. Spring partly compressed, ride height is OK and damper piston is clear of the top and bottom.
Fig 2 - Extra weight is added. Spring compresses. No preload. Suspension is in danger of bottoming out on the damper piston. Ride height needs to be increased.
Fig 3 - On centre stand, Swing arm drops. No weight on suspension. Suspension is at maximum extension.
Fig 4 - Preload is added (red ring). With the suspension at full extension, the lower and upper mounting points cannot be moved further apart. So adding pre-load compresses the spring.
Fig 5 - With pre-load added, bike is removed from centre stand. The load is same as in Fig 2 so the spring compression is the same as Fig 2. (Hook's Law - Same Weight = Same Compression).

Ride height is now OK. Damper piston is clear of top and bottom (as in Fig 1).

Result - ride height is increased, spring compression is the same as before the pre-load was added. (See Fig 2).
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If the bike is not put on the centre stand when pre-load is added, what happens is slightly different, but the result is the same:


Rear Suspension Pre-Load 1 2 5.gif

Fig 1 - Bike and rider, no preload, ride height is OK (as before)
Fig 2 - Extra weight added. Spring compresses. Ride height is now too low.
Fig 5 - Preload is added while the weight is still on the spring. There is no extra weight to compress the spring further and the rear of the bike is now moveable. So the rear rises.

Note that the amount of spring compression is the same as in Fig 2. Also the ride height has been restored to the same 'OK' point as Fig 1 before the load was added.

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Following from the comments from SMSW and dduelin later on in this thread:

Rear Suspension Topped Out.gif

On some bikes without weight of the rider or pillion, the suspension is at the maximum extent of its travel. (Pic 3)
When pre-load is added the suspension unit cannot extend any further, so the spring compresses. (Pic 4)
The compressed spring is able to support some of the rider's weight without compressing further. The rest of the riders weight causes the spring to compress a little and the entire suspension unit to lower the bike slightly. (Pic 5)

In fact, the total spring compression is identical to what it would be if the weight of the rider was placed directly onto the spring - but in this case, due to the pre-loaded spring, the amount of movement from the suspension unit being 'topped out' is much smaller. When the rider gets off the bike, the suspension is at the very top of its travel. This doesn't happen with the ST1300. When the bike is on its wheels, the weight is supported by the spring which is already beginning to compress.

Whichever way you look at it, providing that the unit doesn't top out or bottom out when riding, if the rider gets off and adds a little more pre-load, all that happens is that the rear of the bike will be a little higher when the rider remounts.


Whichever way you look at it, adding preload compensates for the extra load of rider, pillion and luggage. Increase the laod, you need to add pre-load to the suspension to compress the spring and move the piston somewhere to the middle of its range to prevent it from hitting bottom or hitting top when riding over bumps.

What it doesn't do is make the suspension stiffer. For that, you need to buy different springs.

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Over to you ..... !
 
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Ok, first inquiry. If preload is adjusted buy the black knob on the left side under the seat of bike, what are we adjusting by the set screw at the bottom of the shock? Is there a diagram for that?

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Dave.David said:
Ok, first inquiry. If preload is adjusted buy the black knob on the left side under the seat of bike, what are we adjusting by the set screw at the bottom of the shock? Is there a diagram for that?

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The screw at the bottom of the shock is to adjust the speed of the shock/spring rebound after absorbing bumps. It is adjustable so with different loads, the tire will always remain in contact with the pavement after absorbing hits.
 
But does our preload adjuster on the ST1300 compress the spring...? Or just add fluid to the bottom space in the shock...??

ToddC
 
ToddC said:
But does our preload adjuster on the ST1300 compress the spring...? Or just add fluid to the bottom space in the shock...??

ToddC
Click to expand...

As I understand it, and others who have taken the shock apart will chime in to correct me if I am wrong, the preload adjuster (as we screw the knob in) pushes oil into a hydraulic cylinder that sits atop the spring - much like the red collar in John's sketch. The difference is that instead of a collar with detents or threads on the shock body, we have a hydraulic cylinder. As the cylinder expands, the top of it presses up on the shock body and the bottom of it pushes down on the spring. Does it compress the spring or push up on the shock body to raise the rear of the bike? Aye, dat's da rub. John and I have discussed this at length, and I think we are talking semantics - heads I win, tails you lose. What is happening is we are adding 'preload' to the spring. If the bike is unloaded and we add 100 lbs of preload, it will take 100 lbs of weight on the seat to return it to its unloaded condition (assuming the shock was not either topped or bottomed out).
 
John thanks for the diagram and clear explanation. I knew the end result but never really grokked the physics. It's not rocket science but it may as well be as far as I'm concerned!
 
This kind of talk is why I joined this forum, thank you all for the pictures, grafts, and smart words.

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jfheath said:
.......confusingly called 'pre-load' ......
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With the shock off the bike, the preload is the load (force) it takes to start compressing the shock.

The hydraulic preload piston travels about .375". So if you crank it all the way down with the OEM 900 lbf shock, you increase the preload by about 350 lbf, it will take 350 lbf more to start compressing the shock (Which is a lot less in equivalent cargo weight, due to the leverage of the linkage).
 
Preload sets ride height with the best practice to have the shock operating in it's middle 1/3rd of travel to allow the rear wheel to absorb bumps with compression and drop-offs and depressions handled with available extension. Rider Sag is the amount the shock compresses under weight of bike, rider, and all cargo & passenger.

Sag under weight of bike, rider, and all cargo ideally should not exceed 33% of available travel. The ST1300 has 123 mm of rear suspension travel so 33% is about 41 mm. Preload is used to set sag for various configurations given that the spring fitted to the bike is not readily changed for the day's ride. Preload adjustment can handle a passenger, full luggage, etc. Riders of different weights are also able to set personalized sag by adding preload with the adjuster. Heavier riders will need more preload than lighter riders.

We use two types of sag to set suspension. Rider sag is amount of suspension compression from fully extended under weight of bike with rider and all cargo/passengers ready to ride full of fluids. Free sag is sag just the weight of the bike alone - though ready to ride with full loads of gas, oil, luggage (empty). The relationship of rider sag to free sag is an indicator of the spring rate and if the spring rate is good for the particular rider. The idea being that if you can set rider sag with a certain amount of preload via the adjuster then remove the weight of rider and the shock does not top out (fully extend) but retains 5-10 mm of free sag then the spring rate is in the ball park for the rider's weight. If the shock w/o weight of rider has less than 5 mm of free sag or completely tops out then he needed too much preload to set sag therefore the spring is too soft. My own experience with the stock spring is that it is only adequate for riders under 160 lbs. I have about 7-8 mm of free sag and need 9 clicks of preload to hit target sag of 40 mm and 12-14 clicks to 36 mm (same as the forks). I'm 152-155 lbs in street clothes and about 175 in riding gear.
 
dduelin is quite right in his explanation of setting sag, but for those questioning why preload raises the height of the bike there is another explanation needed. In the simplest form preload cancel weight. If you have 1000# /in spring and you had a 1" of preload it will take more the 1000# before the spring will compress on the suspension. So when you add preload you are cancelling out the weight of the bike acting on the spring, there for the spring doesn't compress as much with the bikes weight and rides higher. This is a very simple explanation for those questioning why it affects ride height.

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mlheck said:
dduelin is quite right in his explanation of setting sag, but for those questioning why preload raises the height of the bike there is another explanation needed. In the simplest form preload cancel weight. If you have 1000# /in spring and you had a 1" of preload it will take more than 1000# before the spring will compress on the suspension. So when you add preload you are cancelling out the weight of the bike acting on the spring, there for the spring doesn't compress as much with the bikes weight and rides higher. This is a very simple explanation for those questioning why it affects ride height.

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Michael, I took the liberty of correcting your typo in boldface.

Bottom line, here, is that a spring has a given, linear compression rate. I understand MidLife to be saying that the stock spring compresses 1" under a 900 lb load. Whatever this is, (insert the correct number if I'm mistaken), said spring will support 900 lbs and collapse 1", 1800 lbs and collapse 2". The progression is linear.

If the preload ONLY adds height to the top of the shock and does not compress the spring, i.e. raises the rear of the bike, then the compression and sag of your static bike weight plus the rider will be, said 1" for a 900 total load, and 2" for a combined 1800lb load. In other words, the only way to get the bike to carry an increase in load is to preload the spring. We do this by shortening the spring - either with a screw collar or hydraulic cylinder. Now, as Dave said, you need to add more weight than before in order to get the spring to compress at all.

John's pictures show this. Picture 3 & 4 - unloaded shock with and without preload. The preload compresses the spring. Pictures 1 & 5, the shock is on the bike and in 5 preload is added. The spring is compressed. The preloaded shock (5) will require more weight to compress the spring than the same shock without preload (1). Since the spring is compressed in 5, the unladen bike will be higher in the rear until our rider sits on it. The preload adds height to the rear of the bike by compressing the spring.

Adding preload ALWAYS compresses the spring. If the spring can push the bike up higher when not loaded and not hit full extension, it is still compressed more than the spring with zero preload. Even at full shock extension, a preloaded spring cannot expand as much as the zero preloaded spring. Look at the pictures.

I'm starting to repeat myself. Going to go soak my head.




(These numbers for the spring rate sound high to me given what the sag is, but there are some considerations. If the shock were perfectly vertical, then a 900 lb spring rate would do what I am saying - drop the bike 1". Incline the shock 45 deg, and you will not get the same vertical drop for the same load sitting on the bike's seat. This is not meant to be a distraction to the discussion, but there is some multiplier here based on the shock's inclination off the vertical and there might be a linkage that increases or decreases force on the shock on the bike. I've not taken the suspension apart)
 
SMSW said:
Michael, I took the liberty of correcting your typo in boldface.

(These numbers for the spring rate sound high to me given what the sag is, but there are some considerations. If the shock were perfectly vertical, then a 900 lb spring rate would do what I am saying - drop the bike 1". Incline the shock 45 deg, and you will not get the same vertical drop for the same load sitting on the bike's seat. This is not meant to be a distraction to the discussion, but there is some multiplier here based on the shock's inclination off the vertical and there might be a linkage that increases or decreases force on the shock on the bike. I've not taken the suspension apart)
Click to expand...

There is a record somewhere of somebody measuring actual spring rate and it came close to the stated nominal 900 lb/in.

But the bike is undersprung and some run higher rate springs (in the 1100-1300 lb/in range).

Due to the shock being close to the swing arm axle, the load on the spring is higher than the load on the saddle, because of the leverage.
 
The 1000lb/in number I used has no reference to the actual ST shock. It was used to make things easier. I am one of the riders that has upgraded my spring to a 1200lb/in spring. The original or a upgraded spring are installed with more preload initially than are preload adjuster can achieve. I'm at OH-STOC right now and data access is spotty. I'll post more after I've had a chance to read this completely.

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jfheath said:
So although adding more pre-load will 'attempt' to push down on the spring, the spring is already supporting all of the weight that is on it. It will only compress if there is more weight, and there isn't any. So if the suspension unit cannot push the spring down, it pushes the upper mounting point up.

So the result of adding / removing pre-load to the suspension simply adjusts the height of the bike ensure that the suspension and damping are operating in the optimum range.


Yesterday I went out and proved this to myself. I wound the pre-load all the way off and measured the distance between the upper collar and lower collar of the spring. It was 165mm. I measured the height of the rear of the bike above the ground. It was 755 mm. I then wound on full preload, and took the same measurements. The spring was the same at 165mm. The ride height had increased by 25mm to 780mm.
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This is getting ridiculous. You are focusing on ride height and I'm focusing on the spring. We are talking about the same thing.

From your pictures, Pic 1 shows the stock bike with a light rider and no preload. Pic 5 shows the same bike with a heavy rider who has cranked in preload. The ride height is the same, but the spring for the heavier rider MUST be compressed more to keep the ride height at the same level. With the heavier rider, if he releases the preload (backs it off) his bike sags to Pic 2.

Of course, if said rider gets off the bike and fiddles with the preload, the bike will pogo - go up with more preload, go down with less. And in this case the spring length will not change with additional preload because there is no additional weight. But the purpose of preload is to carry additional load with the shock dampener in the middle of its movement range - this is another way of saying the purpose of preload is to adjust the ride height. You cannot adjust the shock to the middle of its movement range (preload adjustment) without adjusting the ride height. We are talking about the same thing from different directions.

In order to carry more weight, the spring has to be compressed more than for a lighter weight. This is accomplished with more preload. And doing so moves the rear of the bike up.
 
Again I say thank you for all your insight and info, so let's get real life here. If you are light there is lots of info and not much math needed.
Real life, we are a growing race and no sign of fast food going away. Yes, we know, let's not fat shame here.
For us over 250, 275 and even 300 lbs guys, usually with large women passenger's, any settings suggestions? I'm wondering if knowing the preferred riding height of both fenders I could adjust preload to achieve that height while Rider is on bike? Or is that too simple?

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Dave.David said:
Again I say thank you for all your insight and info, so let's get real life here. If you are light there is lots of info and not much math needed.
Real life, we are a growing race and no sign of fast food going away. Yes, we know, let's not fat shame here.
For us over 250, 275 and even 300 lbs guys, usually with large women passenger's, any settings suggestions? I'm wondering if knowing the preferred riding height of both fenders I could adjust preload to achieve that height while Rider is on bike? Or is that too simple?

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There are no setting suggestions in this instance as the stock spring is inadequate for 300 lb riders. Swap the stock 900# spring for a 1300# spring.
 
dduelin said:
There are no setting suggestions in this instance as the stock spring is inadequate for 300 lb riders. Swap the stock 900# spring for a 1300# spring.
Click to expand...
Do you know how one would find those? What to Google? A idea of what is a fair price? Thanks Dave

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I would contact:
Mike Hardy
mike@traxxion.com
Sales
Traxxion Dynamics, Inc.

Give him an email with your weight, pillion weight, type of riding and bike, along with what type of shock you are using (OEM, Penske, Ohlins, etc...).

They'll make the recommendation of a spring.

I'm using a 1300# spring. They recommended a rebuild of my shock and re-valve it but I didn't want to spare the down time.

PS I think spring and collar (needed to fit spring to OEM shock) was under $200 with shipping.
 
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