Physics Question II - Preload Adjuster

Skunkape said:
Race cars are usually jacking cross weight. Different animal.
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Yes, but it shows weight moving.
vinny said:
This is a dynamic weight shift and is only effective while the G-forces are acting on the car... just like the preload on your rear shock, no weight change, just the way it is managed.
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It moves while sitting still as well ... added preload especially with a ride height change "moves the weight around" you can watch the pounds move from scale to scale when using the adjustments on a car. racers use it to tune the bike but in a bike. it moves differently than a car. Adding preload on a wheel in a car adds weight to that wheel, then starts loading the wheel across from it (as an example I have moved 5% of a cars weight off of one wheel while sitting on a set of four scales ... btw, a car with 35% on the left rear really 'pushes'), adding it to a bike moves the weight to the other wheel and as there are (normally) only two, the weight can only move from on to the other as the center moves. How much? I would have to try it to see.
Again, if the suspension is topped out as this is done, nothing will change.
 
On a cage "moving weight" is the term used, but that isn't what is actually happening. You are simply changing how much downforce is applied from one wheel to another. That can only happen with 4 or more wheels. It would take a lot of time to write up exactly how it works, but a simple way to explain it is to think of a chair. If all 4 legs are equal, weight is evenly applied. But by placing a shim under one leg, you increase the load that leg & the diagonally opposite one has to carry, while reducing the load on two other legs. Try the same thing with a 3 legged chair (OK, table would be more common) and there is still nearly identical weight on all 3 legs. Do it with a 2 legged object, and the downforce hardly changes. That is why preload on a bike is a completely different thing than adjusting the suspension of a race car.
 
nhdiesel said:
On a cage "moving weight" is the term used, but that isn't what is actually happening. You are simply changing how much downforce is applied from one wheel to another. That can only happen with 4 or more wheels. It would take a lot of time to write up exactly how it works, but a simple way to explain it is to think of a chair. If all 4 legs are equal, weight is evenly applied. But by placing a shim under one leg, you increase the load that leg & the diagonally opposite one has to carry, while reducing the load on two other legs. Try the same thing with a 3 legged chair (OK, table would be more common) and there is still nearly identical weight on all 3 legs. Do it with a 2 legged object, and the downforce hardly changes. That is why preload on a bike is a completely different thing than adjusting the suspension of a race car.
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Thanks for saving me the typing. I was going to use the table analogy also. :)
 
Some rough, quick calculations -

If the static CG of the bike is resulting a weight distribution of 65%/35% (455 lbs on front wheel, and 245 lbs on rear wheel), and then adjusting the preload (in a static state, sans rider) results in the rear of the bike elevating 0.5 inch, there would be an increase of weight applied to the front wheel ~ 2.5 lbs (and an equally corresponding decrease to the rear).

If the static CG of the bike with rider results in a weight distribution of 50%/50%, and then the preload elevates the rear 0.5 inch, there would be an increase of weight to the front wheel of ~ 1.9 lbs (with same decrease to the rear).

However, I don't think the preload elevates the rear anywhere close to that figure...also, I think (haven't confirmed it yet though) that the rear suspension on the ST is topped out in a static state (assuming no top box, and nothing in saddle bags, etc). If the suspension is topped out (sans a rider), the preload will not affect the front to rear weight distribution in a static state. If this is the case, the preload would only come into play once the suspension comes off the top (with rider, or other weight applied). The preload would affect the force required for the suspension to come off the top, and would correspond to some amount of elevation difference once the suspension was engaged.
 
okmurdog said:
However, I don't think the preload elevates the rear anywhere close to that figure...also, I think (haven't confirmed it yet though) that the rear suspension on the ST is topped out in a static state (assuming no top box, and nothing in saddle bags, etc). If the suspension is topped out (sans a rider), the preload will not affect the front to rear weight distribution in a static state. If this is the case, the preload would only come into play once the suspension comes off the top (with rider, or other weight applied). The preload would affect the force required for the suspension to come off the top, and would correspond to some amount of elevation difference once the suspension was engaged.
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Precisely, and this is why preload on a bike is only used to set static height with rider on it, to equalize the suspension for riders of different weights. The suspension (and rest of the bike) was designed to operate at a certain ride height, and adjusting the preload equalizes this no matter if you have a 100lb or 200lb rider.
 
okmurdog said:
...also, I think (haven't confirmed it yet though) that the rear suspension on the ST is topped out in a static state (assuming no top box, and nothing in saddle bags, etc). If the suspension is topped out (sans a rider), the preload will not affect the front to rear weight distribution in a static state. If this is the case, the preload would only come into play once the suspension comes off the top (with rider, or other weight applied). The preload would affect the force required for the suspension to come off the top, and would correspond to some amount of elevation difference once the suspension was engaged.
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So if I read this correctly, you are saying that when you take the bike off the centerstand, there is absolutely no rear suspension compression due to the weight of the bike? That's how I would interpret 'topped out', as in changing the preload has no effect on the amount of suspension travel under the weight of the bike, because the preload force exceeds the weight of the bike (more accurately that half of the bike). I don't have a 1300, but on my 1100 and every other bike I've ever owned, I'm pretty sure that the rear suspension compresses somewhat when the weight of the bike is applied. Haven't actually measured it to be sure, but it always seems to sag a bit under the weight of the bike. Does yours not compress at all coming off the stand?
 
dwalby said:
So if I read this correctly, you are saying that when you take the bike off the centerstand, there is absolutely no rear suspension compression due to the weight of the bike? That's how I would interpret 'topped out', as in changing the preload has no effect on the amount of suspension travel under the weight of the bike, because the preload force exceeds the weight of the bike (more accurately that half of the bike). I don't have a 1300, but on my 1100 and every other bike I've ever owned, I'm pretty sure that the rear suspension compresses somewhat when the weight of the bike is applied. Haven't actually measured it to be sure, but it always seems to sag a bit under the weight of the bike. Does yours not compress at all coming off the stand?
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Okay, I just went out to the garage and done a quick and dirty check...yes, I believe the suspension is topped out when the bike is in the static state. You can verify this yourself quite quickly (if you had a ST1300) - take a look at the position of the swing arm relative to a fixed point on the bike while on the side stand (I looked at the center of the axle on the right side of the bike relative to the bottom of the muffler). Adjust the preload full in and full out and note the relative change to the muffler (I think you'll see no difference). Now, with the preload full out (no preload applied) put the bike on the center stand and look at the same point (again, I don't think you will see a difference...I didn't see a difference in mine). As a double check, I left the bike on the center stand and tried to pull up on the rear end of the bike - I couldn't get it to move. Therefore, it appears the suspension is most likely topped out in the static state.

The preload changes the relative position of the spring on the rear shock - if the shock is already fully extended (topped out) regardless of the position of the preload (bike in the static state), further change of the preload (ie, more compression on the spring) will not change the position of the suspension.

When I move the bike off the center stand, I have always been on the bike...and, yes the suspension does compress. However, I am on the bike...and if the suspension is setup correctly, it should compress. I've only taken the bike off the center stand once while standing beside it, and I nearly dropped it...I haven't repeated it since ;-)
 
fubartes said:
Let's hear it in your terms BB. :bow1:
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LOL, you don't want to hear it in my terms--I'm an engineer. ;)

Simple version... Adjusting the preload produces only extremely small change in wheelbase length, and adjusting the preload does not change the location of the CG of the bike nor the total weight of the bike. The weight through each tire is determined by the bike's CG location along its wheelbase, therefore any changes in front and rear tire load due to preload adjustment is a very, very, very small fraction of the bike's weight.

I expect that at most, perhaps a pound or so of weight would change between the scales from preload disengaged to fully engaged.

I was going to make a detailed calculation based on actual measurements from my bike, but I've run out gumption.

-----

Now, to continue the Physics Question, here's the harder question:
Which scale will slightly increase as preload is increased?
:popcorn
 
BakerBoy said:
LOL, you don't want to hear it in my terms--I'm an engineer. ;)

I was going to make a detailed calculation based on actual measurements from my bike, but I've run out gumption.
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Any true engineer would have done the research, logged the results and wait for everyone to make their "man on the street", under-educated guess. ;)

BakerBoy said:
Now, to continue the Physics Question, here's the harder question:
Which scale will slightly increase as preload is increased?
:popcorn
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This SHOULD be easy since you've tipped your hat. When preload is increased the wheelbase gets shorter, so the weight/COG moves forward?
 
The stock ST has a F/R weight distribution of 46.3%/53.7% according to a Motorcycle Consumer News review.

My stock sprung 2005 has a free sag of 7 mm on the rear. A properly functioning rear suspension needs 5 to 10 mm of free sag. Free sag is that measured with no rider and cargo in a laden state ready to ride with preload set to the rider's preference - generally 25 to 33% of rear suspension travel or 30 to 35 mm with the weight of rider and cargo. If it has less or none the suspension tops out when rebounding or when the bike is ridden fast over a rise and the bike loses weight (momentarily less than 1 G) on the suspension. If it tops out that is not good - the bike should never top out or bottom out - the bike should always work within the limits of available suspension travel. The 5 to 10 mm range after preload is set also indicates the spring rate is in an acceptable range for the weight of bike and rider.
 
okmurdog said:
Okay, I just went out to the garage and done a quick and dirty check...yes, I believe the suspension is topped out when the bike is in the static state. You can verify this yourself quite quickly (if you had a ST1300) - take a look at the position of the swing arm relative to a fixed point on the bike while on the side stand (I looked at the center of the axle on the right side of the bike relative to the bottom of the muffler). Adjust the preload full in and full out and note the relative change to the muffler (I think you'll see no difference). Now, with the preload full out (no preload applied) put the bike on the center stand and look at the same point (again, I don't think you will see a difference...I didn't see a difference in mine).
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I think the flaw in your methodology was measuring the distance with the bike on the sidestand, as opposed to being stationary and vertical while off of the center stand. Putting the bike on the sidestand unloads the suspension significantly, so gives you an incorrect reading of the static suspension compression. On my 1100 I measured all three options: center stand, sidestand, stationary and vertical. The center stand and sidestand numbers were similar to yours, almost identical. Stationary and vertical showed a compression in the range of 8-10mm or so, similar to what dduelin measured on his 1300 (7mm). That is the true static sag value.
 
dwalby said:
I think the flaw in your methodology was measuring the distance with the bike on the sidestand, as opposed to being stationary and vertical while off of the center stand. Putting the bike on the sidestand unloads the suspension significantly, so gives you an incorrect reading of the static suspension compression. On my 1100 I measured all three options: center stand, sidestand, stationary and vertical. The center stand and sidestand numbers were similar to yours, almost identical. Stationary and vertical showed a compression in the range of 8-10mm or so, similar to what dduelin measured on his 1300 (7mm). That is the true static sag value.
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Like I said - it was a down and dirty check.

If some bikes do have a few millimeters of sag in the static state ( and I mean static as in nothing in the side cases, etc)...then it wouldn't necessarily hold true that everybody's bike should have a few millimeters of sag in the static state, as this is a function of load on the bike (weight in side boxes; top box (if any), and weight of rider, [and passenger]) if the suspension is already matched to the rider and all associated ancillary load for use in the dynamic state...using the preload to match accordingly.


Sent from my iPhone 4s using Tapatalk
 
okmurdog said:
Like I said - it was a down and dirty check.

If some bikes do have a few millimeters of sag in the static state ( and I mean static as in nothing in the side cases, etc)...then it wouldn't necessarily hold true that everybody's bike should have a few millimeters of sag in the static state, as this is a function of load on the bike (weight in side boxes; top box (if any), and weight of rider, [and passenger]) if the suspension is already matched to the rider and all associated ancillary load for use in the dynamic state...using the preload to match accordingly.


Sent from my iPhone 4s using Tapatalk
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But every bike should have some free sag for reasons already posted. If in doubt about this consult any reference or authority to motorcycle suspension design and tuning.

Free sag is the amount the bike compresses its available travel under the weight of bike and normal load configuration WITHOUT rider(s) and cargo, cargo being items not normally carried on the bike WITH the preload set so when rider(s) and cargo are loaded the bike sags to the target set by the rider. The amount of free sag is an indicator of how much preload is needed to offset the weight of rider(s) and cargo. If the rear spring rate is too soft and massive amounts of preload are necessary to prevent excessive sag with the rider(s) and cargo loaded then the bike will have little to no free sag with the rider(s) weight removed because all of that preload then tops out the suspension. That is why free sag is important if any of this stuff matters at all.
 
dduelin said:
But every bike should have some free sag for reasons already posted. If in doubt about this consult any reference or authority to motorcycle suspension design and tuning.

Free sag is the amount the bike compresses its available travel under the weight of bike and normal load configuration WITHOUT rider(s) and cargo, cargo being items not normally carried on the bike WITH the preload set so when rider(s) and cargo are loaded the bike sags to the target set by the rider. The amount of free sag is an indicator of how much preload is needed to offset the weight of rider(s) and cargo. If the rear spring rate is too soft and massive amounts of preload are necessary to prevent excessive sag with the rider(s) and cargo loaded then the bike will have little to no free sag with the rider(s) weight removed because all of that preload then tops out the suspension. That is why free sag is important if any of this stuff matters at all.
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I agree that suspension settings are extremely important...however, the title of this thread wasn't about debating suspension settings ;) - I know I opened that door a bit with my response; however, the crux of my topic was originally debating whether the suspension is indeed topped out in a static state, and if so - how it might relate to weight balance (or fore and aft weight distribution) if the pre-load is adjusted. I still think it is possible that the suspension is indeed topped out in a static state (stock bike, nothing in side bags, etc)...if not topped out, how much preload does it take to top it out?...might check it this weekend if I don't have anything else to do (yeah right).

Now...does anybody happen know the distance of the centerlines of the swingarm? :D
 
okmurdog said:
I agree that suspension settings are extremely important...however, the title of this thread wasn't about debating suspension settings ;) - I know I opened that door a bit with my response; however, the crux of my topic was originally debating whether the suspension is indeed topped out in a static state, and if so - how it might relate to weight balance (or fore and aft weight distribution) if the pre-load is adjusted. I still think it is possible that the suspension is indeed topped out in a static state (stock bike, nothing in side bags, etc)...if not topped out, how much preload does it take to top it out?...might check it this weekend if I don't have anything else to do (yeah right).

Now...does anybody happen know the distance of the centerlines of the swingarm? :D
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Fore and aft? You ex-Navy?
 
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