Preload - maybe a dumb question

[RoSPA_man]

Hi

When you set more preload either with ESA or manually to "stiffen" the suspension, you are screwing the spring closed, making it shorter etc. If so, then why does the rear of the bike rise, or at least appear/feel like it's risen if the spring length is shortened?

 

[dirtrider]

Hi Hugh

 

Basically you are moving the upper spring seat down so this does raise the rear of the bike.

It does compress the spring more as you add preload but seeing as the bike sits with some spring compression neither topped out or bottomed out the rear of the bike just raises up as the spring seat moves down.

Keep in mind this doesn’t change spring rate just loaded height.

 

 

 

[Twisties]

If you consider a single rate spring with a rate of 150 lbs per inch and then look at the weight of the bike on that spring, say 300 lbs, you can see that the spring will compress 2" and the bike's sag will also be 2". If you now pre-load the spring by compressing it 1" you will see that, all else the same, the spring now compresses 1" under the 300 lbs load. The spring is actually still compressed 2" total, but the bike's sag is only 1".

 

You want sag adjusted to prevent bottoming or topping out the suspension. Generally 1/4 to 1/3 of the suspension travel should be taken for sag.

 

 

[RoSPA_man]

If you consider a single rate spring with a rate of 150 lbs per inch and then look at the weight of the bike on that spring, say 300 lbs, you can see that the spring will compress 2" and the bike's sag will also be 2". If you now pre-load the spring by compressing it 1" you will see that, all else the same, the spring now compresses 1" under the 300 lbs load. The spring is actually still compressed 2" total, but the bike's sag is only 1".

 

You want sag adjusted to prevent bottoming or topping out the suspension. Generally 1/4 to 1/3 of the suspension travel should be taken for sag.

 

 

Great explanation Twisty - I admit that I had to think about it though!. The key to it is that the 1" of preload is not "wasted". Instead, it is, as the name suggests, preloaded and ready for use, not preloaded and wasted!

[RoSPA_man]

Hi Hugh

 

Basically you are moving the upper spring seat down so this does raise the rear of the bike.

 

Hi DirtRider

I'm not saying you're wrong but I just don't get that logic I can't see how if you move the upper spring seat down (i.e. bring it closer to the bottom of the spring, i.e. shorten it, how that would cause the rear of the bike to rise?

[Joe Frickin' Friday]

Hi DirtRider

I'm not saying you're wrong but I just don't get that logic I can't see how if you move the upper spring seat down (i.e. bring it closer to the bottom of the spring, i.e. shorten it, how that would cause the rear of the bike to rise?

 

Jan's example lays things out pretty well. Let's take it to both logical extremes.

 

Suppose you have a rear shock whose spring has a spring constant of 150 lb/inch. The rear of the bike loads the shock with 300 pounds. Further suppose that this shock is unique in that you can remove all of the preload from it; in this condition, the slightest load on the shock causes some compression, but with no load on it, the shock is at its fully extended length, just barely making contact with its internal mechanical stop. After removing all preload from the shock in this manner, you take the bike off of the center stand, and find that the rear spring compresses a full 2 inches (300 lb divided by 150 lb/in).

 

The shock is riding somewhere near its fully-compressed condition like this. If you take off for a ride and go over a speed bump, you're likely to bottom out the suspension when you hit the bump. If you go over a dip in the road, the wheel will follow the downslope into the dip nicely, but you'll probably bottom out when you hit the far side. It's a spine-jarring ride like this, and it's easy to overload the suspension components and bend hard parts.

 

Now crank up the preload. Crank it way, way up, until the spring has been squashed by a full two inches. It's now pushing the two ends of the shock away from each other with 300 pounds of force; the only thing keeping the shock from flying apart is its internal mechanical stops, which are bearing that 300-pound force from the spring. Now drop the bike off of the centerstand. As long as the bike is just standing there, the shock/spring won't compress at all. The spring is already pushing the shock's ends apart with 300 pounds of force, and now the bike is pushing the ends of the shock together with 300 pounds of force; the mechanical stop has just been relieved of its 300-pound force, but the suspension hasn't really sagged at all. In other words, the rear of the bike is now sitting two inches higher than it was in the previous (zero-preload) case.

 

The shock is now riding at its fully-extended position. Go for a ride. Hit a speed bump, and it will soak the bump up nicely, but when you get to the far side, the wheel won't follow the downhill slope very well, since it can't extend any farther than the fully-extended position that it takes on during level cruise. The wheel is likely to become airborne over relatively minor bumps, and you may find yourself pitched up out of the saddle slightly from time to time as the suspension stops pushing the bike up (when it hits its full-extension stop) and the seat padding pushes the bike away from your butt.

 

Make more sense?

[dirtrider]

Hi Hugh

 

Basically you are moving the upper spring seat down so this does raise the rear of the bike.

 

Hi DirtRider

I'm not saying you're wrong but I just don't get that logic I can't see how if you move the upper spring seat down (i.e. bring it closer to the bottom of the spring, i.e. shorten it, how that would cause the rear of the bike to rise?

 

Morning Hugh

 

OK lets for the sake of argument take the bike out of the equation.

 

You just take the spring and set it on the garage floor, then you put a 300 pound weight on top of that spring. Are we OK with this so far?

 

That weight has just compressed that spring XX amount of displacement (lets say 3”of travel for this analogy).

 

Now you lift the weight and place a 1” spacer between the weight and spring top. Then you set the 300 pound weight back on top of that spring.

 

That 300 pound weight just compressed the spring the EXACT SAME 3” as it did without the spacer only now the weight sits 1” higher than it did.

 

This is just what happens when the spring is in the motorcycle. The only time this doesn’t hold true is IF the shock geometry is such that it changes shock angle or working geometry, and you always get a little of this in a real world suspension environment but not usually enough to matter that much though.

The other time it doesn’t hold true is if the preload adjustment causes the shock to hit it’s up or down constraints such as causing the shock to top out with the bike’s weight on it. Once the shock tops out with bike’s weight on it all bets are off as the controller then is the shock extension limit not the spring itself.

 

As long as the bike’s sprung weight is sitting on top of that spring and the suspension is not topped out or bottomed out then the spring acts just like above with the spring sitting on the garage floor.

 

 

[basilr]

These are all really good explanations and very informative.

 

I have a 2005 RT without ESA and now that I understand about the preload (the one I crank up or down from under the seat), what exactly is it I am doing when I turn the screw on the bottom of the rear shock from "H" to "S" or somewhere in between?

 

Also, how important is it to have these two adjustments coordinated and how do I know when I have it right?

[dirtrider]

Afternoon basilr

 

When you turn that screw marked H or S that is changing the re-bound dampening. The H stands for “harder” dampening and the “S” stands for “softer” dampening.

 

What that basically does is change how fast the spring (and rear of motorcycle) can return to the up position after it is quickly compressed when hitting a bump or hole.

 

It works just about like that pneumatic cylinder on you screen door. If you are familiar with how that works, if it is set it too “hard” the screen door takes for ever to close, if it is set too “soft” the screen door slams shut whenever you let go of it. Ideally it should allow the door to close as fast as possible but not slam shut.

 

OK, back to your shock adjustment. You want the rear wheel to stay on the road and allow the suspension to easily move up and down to absorb the road bumps and humps. If the rear wheel returns from compression too fast it tends to overshoot and there is lack of wheel control. If the rear wheel returns to the up position too slow it won’t be all the way back up before you hit the next bump so the suspension can pack down or allow the suspension to keep lowering slightly as you ride a very bumpy road. This can also make the rear wheel not stay in contact with the road on square edged bumps or pot holes. Some of this can also be related to forward speed as the faster you go the quicker the wheel gets stroked up and down on a bumpy road.

 

The above is just a quick and dirty on how the shock rebound dampening works for bumps and straight line handling. The rebound dampening can also be part of the curve and cornering handling but that is way above the simple stuff here.

 

Here is the deal on the H and S in relation to the spring preload setting. As you increase the spring preload that normally just raises the rear of the bike so unless you adjust so much that the bike fore/aft pitching changes or you run out of shock travel and get to top out or bottom out not much changes. But normally you increase the rear preload to compensate for added rear loading such as a passenger or rear baggage. What that does is lower the rear of the bike even with the added spring preload so that in effect makes the spring more difficult to compress farther as it is already compressed father than it was at non-loaded street trim. With the spring compressed farther it has more force pushing the rear of the bike up so the spring wants to return to rest quicker.

 

What does this mean? In simple terms it means as you add rear spring preload to a bike that has no added rear weight you really don’t need to do much with the rebound adjuster. Maybe if anything add just a little more rebound dampening as you change the spring and shock working angle slightly, not much but slightly.

If you increase the rear preload adjustment because you added rear weigh such as a passenger or luggage then you also need to add more rear rebound dampening (H). How much is kind of a trial and error thing. If the rear bounces around then you need more rear rebound dampening. When you get too much rear rebound it will start to get harsh in the rear.

The rebound dampening and spring preload can also change the way a bike squats or stands up in a curve as well as effect fore/aft weight transfer into and out of the corners or under acceleration or braking . This is much more advanced than just compensating for weight added or preload adjustment so for this you should read some good credible bike tuning information then plan on playing hard to evaluate the changes you are making.

 

[Hall Vince]

Thanks for the heads up on the thread Hugh.

 

I'm Watching thread, as I'm fiddling my rear Wilbers trying to get it working for my liking.

 

Wilbers uses variable rate springs. I think it's this I can't seem to get comfortable with It feels far to soft with my 25~30% of dynamic and way to hard if I set it with 10% of static. I don't seem to be able to get the 10% static and with me sat on the bike 30% Dynamic it ends up more like 20%.

Spring rate is apparently correct for me + luggage and the bikes weight wihin a few kilos.

 

I'm currently just backing off the preload till if feels more like I think it should. It came with very little preload, when I rolled the bike of the stand the rear sank about 40mm before I sat on it orginally.

I wound it up till I had the 10mm static sag, but have been backing it off a turn at a time then riding.

No remote preload adjuster on mine so it's a bit of faff doing it.

\v/

 

[dirtrider]

Hi Vince

 

I feel for you on adjusting the dampening on those variable rate springs. I hate variable rate springs myself as you are trying to tune to a bunch of different spring rates with basically one or two fixed dampening settings.

 

Problem is, if the sag you are looking for starts coil binding the softer coils in that variable rate spring you are chasing a quickly changing spring rate as the spring compresses farther. The only way I have ever got a decent tuning on variable rate springs is to start experimenting and riding. In my eyes the book just gets the bike rideable and then it’s trial and error until you get it where you need it.

 

I have a set of variable rate springs in the front of one of my dirt bikes and to get the full stroke compression and partial stroke rebound dampening where I want it it’s just way too harsh at low inputs or short stroking. Did I mention I hate variable rate springs?

 

 

 

 

[Redbrick]

Lots of good info here.....Check the latest edition of OTL magazine from BMW RA (September 2010) if you can...They have an article "Super Simple Suspension" that is interesting......

[basilr]

Thanks Dirtrider,

 

I will definitely be bookmarking this thread. It's, like you say, all you really need to know at this stage of the game.

 

Thanks again. This is a really good site.

[RoSPA_man]

Hi Hugh

 

Basically you are moving the upper spring seat down so this does raise the rear of the bike.

 

Hi DirtRider

I'm not saying you're wrong but I just don't get that logic I can't see how if you move the upper spring seat down (i.e. bring it closer to the bottom of the spring, i.e. shorten it, how that would cause the rear of the bike to rise?

 

Morning Hugh

 

OK lets for the sake of argument take the bike out of the equation.

 

You just take the spring and set it on the garage floor, then you put a 300 pound weight on top of that spring. Are we OK with this so far?

 

That weight has just compressed that spring XX amount of displacement (lets say 3”of travel for this analogy).

 

Now you lift the weight and place a 1” spacer between the weight and spring top. Then you set the 300 pound weight back on top of that spring.

 

That 300 pound weight just compressed the spring the EXACT SAME 3” as it did without the spacer only now the weight sits 1” higher than it did.

 

This is just what happens when the spring is in the motorcycle. The only time this doesn’t hold true is IF the shock geometry is such that it changes shock angle or working geometry, and you always get a little of this in a real world suspension environment but not usually enough to matter that much though.

The other time it doesn’t hold true is if the preload adjustment causes the shock to hit it’s up or down constraints such as causing the shock to top out with the bike’s weight on it. Once the shock tops out with bike’s weight on it all bets are off as the controller then is the shock extension limit not the spring itself.

 

As long as the bike’s sprung weight is sitting on top of that spring and the suspension is not topped out or bottomed out then the spring acts just like above with the spring sitting on the garage floor.

 

 

Firstly Dirtrider, I really appreciate the lengthy and useful replies.

 

I understand excatly what you are saying in your "spring on the garge floor example". So, to check that I get it, let me paraphrase what I think you are saying and if I am right, it will unfortunately raise another question!

 

Here goes: I have always assumed that when I get my 'C' spanner and increase preload- ie by tightening the collar clockwise, that I was compressing and therefore shortening the spring- and as a consequence the overall shock length. However, what you seem to be saying is that in fact my actions are not shortening the spring, but are in fact doing 2 things a)creating a "spacer" above the spring i.e. lengthening the overall shock by NOT shortening the spring (at least not much) and b)lengthening the shock by "releasing" more of the threaded part of the shock - ie the bit above the collar that I am tightening down. Therefore the bike's height rises. Logically that is the only way the bike height can increase.

 

If that is correct (and I readily accept I may not have interpreted what you said correctly) then that would imply, just like your example, that I am increasing the length of the shock (effectively adding in a spacer), BUT NOT changing the characteristics of the spring i.e. the spring is no more compressed than before tightening the collar? But we know that preload changes the characteristics beyond the effect that a spacer would account for. What am I missing?

 

 

[dirtrider]

 

 

I understand excatly what you are saying in your "spring on the garge floor example". So, to check that I get it, let me paraphrase what I think you are saying and if I am right, it will unfortunately raise another question!

 

Here goes: I have always assumed that when I get my 'C' spanner and increase preload- ie by tightening the collar clockwise, that I was compressing and therefore shortening the spring- and as a consequence the overall shock length. However, what you seem to be saying is that in fact my actions are not shortening the spring, but are in fact doing 2 things a)creating a "spacer" above the spring i.e. lengthening the overall shock by NOT shortening the spring (at least not much) and b)lengthening the shock by "releasing" more of the threaded part of the shock - ie the bit above the collar that I am tightening down. Therefore the bike's height rises. Logically that is the only way the bike height can increase.

 

If that is correct (and I readily accept I may not have interpreted what you said correctly) then that would imply, just like your example, that I am increasing the length of the shock (effectively adding in a spacer), BUT NOT changing the characteristics of the spring i.e. the spring is no more compressed than before tightening the collar? But we know that preload changes the characteristics beyond the effect that a spacer would account for. What am I missing?

 

 

Morning Hugh

 

You pretty well have it.

 

This all holds true UNTIL the shock becomes topped out or is extended all the way to top of travel. At that point you do start shortening the spring and compressing it more.

 

I’m not sure you are missing anything as simply raising the rear of the bike transfers more weight to the front wheel. Also keep in mind the shock/spring ratio is not 1:1 with the rear wheel as the spring/shock is leaning at a slight angle and mounted about half way between the rear wheel and rear swing attachment to the bike. As the rear of the bike is raised or lowered this working geometry is changed slightly. Also raising or lowering the rear effects front fork or front suspension working angles.

 

 

 

[ShovelStrokeEd]

Maybe I can help.

 

The bike is supported by the shock. Just look in there, it is connected, at the bottom to the swing arm and at the top via an eye bolted to the frame.

 

The spring, a part of the shock, wants to extend the shock to its full length. The more you load the spring via the preload collars, the more force it will exert to make the shock go full length.

 

Now, add the weight of the bike, which tends to make the shock shorter. That force is resisted by the force of the spring wanting to extend the shock.

[RoSPA_man]

Hi Vince

 

I feel for you on adjusting the dampening on those variable rate springs. I hate variable rate springs

 

 

...they're not variable - they're linear

[RoSPA_man]

Hi DirtRider

I'm not saying you're wrong but I just don't get that logic I can't see how if you move the upper spring seat down (i.e. bring it closer to the bottom of the spring, i.e. shorten it, how that would cause the rear of the bike to rise?

 

Jan's example lays things out pretty well. Let's take it to both logical extremes.

 

Suppose you have a rear shock whose spring has a spring constant of 150 lb/inch. The rear of the bike loads the shock with 300 pounds. Further suppose that this shock is unique in that you can remove all of the preload from it; in this condition, the slightest load on the shock causes some compression, but with no load on it, the shock is at its fully extended length, just barely making contact with its internal mechanical stop. After removing all preload from the shock in this manner, you take the bike off of the center stand, and find that the rear spring compresses a full 2 inches (300 lb divided by 150 lb/in).

 

The shock is riding somewhere near its fully-compressed condition like this. If you take off for a ride and go over a speed bump, you're likely to bottom out the suspension when you hit the bump. If you go over a dip in the road, the wheel will follow the downslope into the dip nicely, but you'll probably bottom out when you hit the far side. It's a spine-jarring ride like this, and it's easy to overload the suspension components and bend hard parts.

 

Now crank up the preload. Crank it way, way up, until the spring has been squashed by a full two inches. It's now pushing the two ends of the shock away from each other with 300 pounds of force; the only thing keeping the shock from flying apart is its internal mechanical stops, which are bearing that 300-pound force from the spring. Now drop the bike off of the centerstand. As long as the bike is just standing there, the shock/spring won't compress at all. The spring is already pushing the shock's ends apart with 300 pounds of force, and now the bike is pushing the ends of the shock together with 300 pounds of force; the mechanical stop has just been relieved of its 300-pound force, but the suspension hasn't really sagged at all. In other words, the rear of the bike is now sitting two inches higher than it was in the previous (zero-preload) case.

 

The shock is now riding at its fully-extended position. Go for a ride. Hit a speed bump, and it will soak the bump up nicely, but when you get to the far side, the wheel won't follow the downhill slope very well, since it can't extend any farther than the fully-extended position that it takes on during level cruise. The wheel is likely to become airborne over relatively minor bumps, and you may find yourself pitched up out of the saddle slightly from time to time as the suspension stops pushing the bike up (when it hits its full-extension stop) and the seat padding pushes the bike away from your butt.

 

Make more sense?

 

Now this makes more sense than it did 2 days ago! Thanks.

[Hall Vince]

Hi Vince

 

I feel for you on adjusting the dampening on those variable rate springs. I hate variable rate springs

 

 

...they're not variable - they're linear

 

Yes hugh is correct.

I understood they were variable due to info on the Revs racing site, I think I misread or it refers to other shocks.

We had confirmed advice from Wilbers themselves that the springs are Linear.

 

I am also a closer to having mine set up as I wanted, just by knocking off a few turns of preload. Bike handles far better now, and more like how I expected.

 

Just waiting for conformation that the spring I have is correct for the weight I normally carry. I don't want to be fiddling with taking the rack off and a side panel every time I want to add preload if I load up a pannier.

I never did with the ESA, just left it on one person + luggage and altered damping when I got to the smooth twisties.

I was kinda hoping once I had the Wilbers set I could leave it.

 

Oh, and would Mr Newtons Third Law help explain why the bike rises when you shorten the spring?

It works for me, assuming your spring rate is powerfull enough to suport the bikes weight.

\v/

 

[RoSPA_man]

In case it adds anything to the communal fund of knowledge or in case anyone wants to comment, here is what I have just written to Dirk at Wilbers:

 

 

Hi Dirk

 

I have one last round of questions- sorry - I finally got to adjusting everything with the help of a friend (who will also be going for Wilbers soon I’m sure)

 

Here’s what we did:

 

We took out all preload, all compression and rebound damping front and rear (but I made a note of the settings). For info, all were 12 clicks (of 22)actually. So, after doing that, as expected, the bike felt “soft” and very ‘bouncy’ which was a great start point.

Rear Static Sag:

a. Measured “A” with bike on the centre stand i.e. wheel fully extended in the air= 443mm.

b. Then “B” i.e. the sag of the bikes weight and it was 411 i.e. sag of 35mm which was too much!

c. We increased the preload in stages to 8.25 revolutions on the hydraulic adjuster to get the sag to about 10mm.

With static sag set OK at this 10mm of sag , I then measured the dynamic sag with me +kit etc. on the bike. I wanted it to be about 1/4 or 1/3 of the total travel of 148mm (theoretical) or about 128mm (real-world if we allow for the bump stop on the shock). That would mean about 35mm -40mm of sag. And yes, with the same preload of 8.25 revolutions, I get 36mm of dynamic sag. Result !!

 

Front Static Sag: . I used cable ties to establish with direct measurements of their displacement that static sag was 21mm- simply by measuring the movement of the cable ties (from fully extended hanging front wheel) to being off the stand and got 21mm. The same process repeated with me on the bike gave me 35mm of dynamic sag.

Because the front travel is quotes as 117mm (or 97mm real-world) and if I want the same ¼ or 1/3 of that travel, I need static sag of about 31mm (i.e. 107*.29 – i.e. middle of travel and middle of ¼ and 1/3).

 

Conclusion: My front preload needs to be increased to reduce the sag to 30 or 31 mm (Although I could probably cope with it as is)

 

Problem: I managed to get the lock collar of the front shock loosened but it is hard to tighten the preload collar against the spring in such a confined space and with the shock tending to rotate etc.. What I propose to do is to remove the front shock and to increase the preload by suitably jamming the body of the shock to prevent rotation and tightening down the collar on the bench.

 

Question: Can you tell me, for this linear spring with a rating of 63 N/mm and of course bearing in mind it’s BMW Telelever in case that matters, what is the correlation between one 360 degree turn of the collar and the consequent reduction of sag? To be clear, if the current sag is 36mm and I tighten the collar of that spring by (say) 2 revolutions, will it reduce the sag by 5mm? 10mm ? If I have that information, I can make one adjustment and refit the shock.

 

Many thanks in advance

 

Hugh