Author Topic: This might come as a shock.  (Read 1325 times)

Online Huzo

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This might come as a shock.
« on: September 20, 2021, 05:17:59 PM »
Rudyís thread was becoming badly contaminated by extraneous off topic chatter.
So hereís the thing.
Tris noted that every time it started to gell for him, it drifted off.
Start by separating the spring from the damper.

Realise that the damper has two functions.
1.
To progressively dampen the oscillations of a given spring when deflected from rest.
2.
To give a top and bottom limit to the maximum and minimum length of the captured spring in the assembled unit.
Next..

The load is supported by the spring, not the damper....(ignore for now any pressurised shocks).
Consider a 200 mm spring at rest on the bench.
Now assemble the shock with no preload.
When you sit on the bike, you are effectively sitting on the spring. That un preloaded spring will compress until it reaches itís compressed length for that load.
Can we call that length 150 mm....(it has compressed 50mm). Remember it has no preload yet.
The spring compressed, (sagged) 50 mm under the combined weight of rider and bike. You will perceive a certain ďsoftnessĒ that you associate with that spring.
Now put the bike back on the centrestand with the rear wheel off the ground...The shock is now ďtopped outĒ against itís top stop, like it was on the bench.

Wind on 25 mm of pre load...Now your spring is 175 mm long.
Roll the bike off the stand and jump back on.
The total LOAD has not changed, so itíll come as no surprise that the spring compresses back to....?...150 mm
The same as before the preload, but it has only compressed 25 mm under the effect of the load to get to that 150 mm
however it will perform the same as the unpreloaded example, because it has still only been compressed to the same length as the first example (150mm).

Hereís the crunch though.
Because you have only compressed the spring 25 mm in the second example, the eye to eye length will be more than the unpreloaded example and therefore the ride height will be greater...(the damper rod is 25 mm further extended).
But the spring performance will not have altered.

For Tris...

« Last Edit: September 20, 2021, 05:27:59 PM by Huzo »

Online Perazzimx14

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Re: This might come as a shock.
« Reply #1 on: September 20, 2021, 05:37:15 PM »
Rudyís thread was becoming badly contaminated by extraneous off topic chatter.


Welcome to WG  :thumb:
It's the ones who've cracked that the light shines through!

Online Caffeineo

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Re: This might come as a shock.
« Reply #2 on: September 20, 2021, 09:04:15 PM »
Thanks for posting that. Took a bit of thought but I think I get "the crunch" now. Spring is compressed the same but the shock rides higher in it's travel due to preload on the spring. I got heavier springs on my trials bike and dirt bike as I am "American size". Makes a huge difference on the trials bike but not so much on the dirt bike???? I am not a fast dirt bike rider so maybe that has something to do with it......
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Offline Bulldog9

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Re: This might come as a shock.
« Reply #3 on: September 20, 2021, 09:46:45 PM »
Rudyís thread was becoming badly contaminated by extraneous off topic chatter.
So hereís the thing.
Tris noted that every time it started to gell for him, it drifted off.
Start by separating the spring from the damper.

Realise that the damper has two functions.
1.
To progressively dampen the oscillations of a given spring when deflected from rest.
2.
To give a top and bottom limit to the maximum and minimum length of the captured spring in the assembled unit.
Next..

The load is supported by the spring, not the damper....(ignore for now any pressurised shocks).
Consider a 200 mm spring at rest on the bench.
Now assemble the shock with no preload.
When you sit on the bike, you are effectively sitting on the spring. That un preloaded spring will compress until it reaches itís compressed length for that load.
Can we call that length 150 mm....(it has compressed 50mm). Remember it has no preload yet.
The spring compressed, (sagged) 50 mm under the combined weight of rider and bike. You will perceive a certain ďsoftnessĒ that you associate with that spring.
Now put the bike back on the centrestand with the rear wheel off the ground...The shock is now ďtopped outĒ against itís top stop, like it was on the bench.

Wind on 25 mm of pre load...Now your spring is 175 mm long.
Roll the bike off the stand and jump back on.
The total LOAD has not changed, so itíll come as no surprise that the spring compresses back to....?...150 mm
The same as before the preload, but it has only compressed 25 mm under the effect of the load to get to that 150 mm
however it will perform the same as the unpreloaded example, because it has still only been compressed to the same length as the first example (150mm).

Hereís the crunch though.
Because you have only compressed the spring 25 mm in the second example, the eye to eye length will be more than the unpreloaded example and therefore the ride height will be greater...(the damper rod is 25 mm further extended).
But the spring performance will not have altered.

For Tris...

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Re: This might come as a shock.
« Reply #3 on: September 20, 2021, 09:46:45 PM »

Online LowRyter

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Re: This might come as a shock.
« Reply #4 on: September 20, 2021, 10:22:24 PM »
Rudyís thread was becoming badly contaminated by extraneous off topic chatter.
So hereís the thing.
Tris noted that every time it started to gell for him, it drifted off.
Start by separating the spring from the damper.

Realise that the damper has two functions.
1.
To progressively dampen the oscillations of a given spring when deflected from rest.
2.
To give a top and bottom limit to the maximum and minimum length of the captured spring in the assembled unit.
Next..

The load is supported by the spring, not the damper....(ignore for now any pressurised shocks).
Consider a 200 mm spring at rest on the bench.
Now assemble the shock with no preload.
When you sit on the bike, you are effectively sitting on the spring. That un preloaded spring will compress until it reaches itís compressed length for that load.
Can we call that length 150 mm....(it has compressed 50mm). Remember it has no preload yet.
The spring compressed, (sagged) 50 mm under the combined weight of rider and bike. You will perceive a certain ďsoftnessĒ that you associate with that spring.
Now put the bike back on the centrestand with the rear wheel off the ground...The shock is now ďtopped outĒ against itís top stop, like it was on the bench.

Wind on 25 mm of pre load...Now your spring is 175 mm long.
Roll the bike off the stand and jump back on.
The total LOAD has not changed, so itíll come as no surprise that the spring compresses back to....?...150 mm
The same as before the preload, but it has only compressed 25 mm under the effect of the load to get to that 150 mm
however it will perform the same as the unpreloaded example, because it has still only been compressed to the same length as the first example (150mm).

Hereís the crunch though.
Because you have only compressed the spring 25 mm in the second example, the eye to eye length will be more than the unpreloaded example and therefore the ride height will be greater...(the damper rod is 25 mm further extended).
But the spring performance will not have altered.

For Tris...

I heard the same thing Saturday when a local rider was checking the suspension & preload on my Ducati.  Nor sure I still understand it.  The bike "feels" stiffer with more preload.  But it does sit up higher with more preload, no question.

BTW- he was impressed that the suspension had so little stiction but the rear shock is too stiff (high speed) compression.  we didn't change anything, it was close enough other than the "Stiff" rear shock.
« Last Edit: September 20, 2021, 10:25:26 PM by LowRyter »
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Online Huzo

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Re: This might come as a shock.
« Reply #5 on: September 20, 2021, 11:43:51 PM »
Like a dog on a bone, just gotta make it hard eh?  :cool:
Itís not hard Bulldog.
But that was a bit prolonged because it allows people who are coming to terms with the concept, to work methodically through and identify where their misunderstanding begins.
You start with the bit thatís easy to understand and work your way through.
Caffeineo in the previous post, indicated that there was a light beginning to appear for him. My post was to help people who are on the cusp of getting it, not people such as yourself who (apparently) already do.
How would you have explained it more briefly without leaving out an important aspect ?
I could have done it in two lines to someone who already understood, but Iíve been trying to learn the stuff Beetle does and I like the way he patiently leads the beginner along.
BTW.
I donít know what the emoji is supposed to convey, can you enlighten me ?
Please donít make it too hard.... :popcorn:

« Last Edit: September 20, 2021, 11:47:31 PM by Huzo »

Online Huzo

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Re: This might come as a shock.
« Reply #6 on: September 20, 2021, 11:50:46 PM »
Thanks for posting that. Took a bit of thought but I think I get "the crunch" now. Spring is compressed the same but the shock rides higher in it's travel due to preload on the spring.
Almost mate.
The SPRING is compressed TO the same length, not compressed BY the same amount.
That would imply that in the second example, it was compressed 50 mm.
This is why an explanation has to be free of ambiguity. I know what you meant to say and youíd have been correct, but these points have to be nailed down to definitions. Thatís why Iím criticised for being picky and I know it looks like that.
If we all just agree with BS itíll be warm and fuzzy, but at the end of the day weíll still be wrong.
« Last Edit: September 20, 2021, 11:57:55 PM by Huzo »

Offline jacksonracingcomau

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Re: This might come as a shock.
« Reply #7 on: September 21, 2021, 01:04:34 AM »
Without wishing to rain on anyoneís parade, research into these subjects may help in understanding motorcycle suspension and the effect of adjusting preload
1/ Progressive springs
2/ Rising rate linkage (n/a v85 but relevant to most monoshock Guzzis)
3/ Angle of shock absorber (very relevant to v85 and noticeable  difference on 21 V7 compared to prior)
To understand the latter read up on 60ís Velocettes with adjustable top shock mount position.

Happy reading, no debate, it is what it is

Offline tris

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Re: This might come as a shock.
« Reply #8 on: September 21, 2021, 01:30:35 AM »
Cheers Huzo - this is good of you  :bow: :bow:

Firstly

......Hereís the crunch though.
Because you have only compressed the spring 25 mm in the second example, the eye to eye length will be more than the unpreloaded example and therefore the ride height will be greater...(the damper rod is 25 mm further extended).
But the spring performance will not have altered.

For Tris...

GOT IT ... I think  :grin:
In my words
If you changed the spring for a solid tube (spacer) the preload adjuster just changes the relative position between the top and bottom mounts as they slide on the damper that controls the max and minimum stroke (ignoring the damping effect)

So when I raise the preload to carry my lardy ass the prime objective is to maintain the suspension (say) midway through its travel (I know that's a simplistic position)

Is that right?
« Last Edit: September 21, 2021, 01:45:01 AM by tris »
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Offline tris

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Re: This might come as a shock.
« Reply #9 on: September 21, 2021, 01:41:17 AM »
Without wishing to rain on anyoneís parade, research into these subjects may help in understanding motorcycle suspension and the effect of adjusting preload
1/ Progressive springs
2/ Rising rate linkage (n/a v85 but relevant to most monoshock Guzzis)
3/ Angle of shock absorber (very relevant to v85 and noticeable  difference on 21 V7 compared to prior)
To understand the latter read up on 60ís Velocettes with adjustable top shock mount position.

Happy reading, no debate, it is what it is

To be honest this bit I have more understanding of (I THINK) and simplistically

1) Spring gets stiffer the more it compresses
2) That's the application of different length levers
3) That's to do with force vectors

Assuming that is correct, it all falls apart if you don't get the basics which is where I am (was?)  :thumb:
« Last Edit: September 21, 2021, 01:50:07 AM by tris »
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Online Huzo

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Re: This might come as a shock.
« Reply #10 on: September 21, 2021, 02:02:43 AM »
Cheers Huzo - this is good of you  :bow: :bow:

Firstly
GOT IT ... I think  :grin:
In my words
If you changed the spring for a solid tube (spacer) the preload adjuster just changes the relative position between the top and bottom mounts as they slide on the damper that controls the max and minimum stroke (ignoring the damping effect)

So when I raise the preload to carry my lardy ass the prime objective is to maintain the suspension (say) midway through its travel (I know that's a simplistic position)

Is that right?
Well if itís not then Iím not....(but I am).
The simplistic approach is always the best, if you have not introduced variables or inconsistencies into the mix.
Yes Tris, that is correct.
Once you solidify that into your thinking, youíll be aghast at some of the people youíll hear saying...
ďThe bike was too soft in the arse end through turn 4, so I wound on a bit of preload to stiffen it up a bit..Ē
Theyíll do what they think is right then swear black and blue that it was a good idea..(better than the alternative).  :rolleyes:

Online Huzo

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Re: This might come as a shock.
« Reply #11 on: September 21, 2021, 02:12:49 AM »
I think the error is made in this type of scenario.
If your forks are going to bottom out, the tendency may be to apply some pre load. This will indeed reduce the tendency to bottom out, but not because the fork travel is reduced (stiffer ride), it is because you have pushed the bottom slider further away and given the staunchions more room to compress before the crash at the end.
The ride quality wonít have changed, but the whole thing is happening further away from the bottom stop and the tendency to bottom out is reduced.

Offline chuck peterson

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Re: This might come as a shock.
« Reply #12 on: September 21, 2021, 06:25:54 AM »
Great explanation, thanks
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Re: This might come as a shock.
« Reply #13 on: September 21, 2021, 08:34:24 AM »
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Offline tris

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Re: This might come as a shock.
« Reply #14 on: September 21, 2021, 09:43:35 AM »
In between working  :wink: I've been pondering this!

ďThe bike was too soft in the arse end through turn 4, so I wound on a bit of preload to stiffen it up a bit..Ē

Should he have
a) gotten some springs at a higher rate to cope with his weight
b) wound the compression damping up to slow the rate the suspension got to full travel?

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Online Huzo

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Re: This might come as a shock.
« Reply #15 on: September 21, 2021, 11:03:34 AM »
In between working  :wink: I've been pondering this!

Should he have
a) gotten some springs at a higher rate to cope with his weight
b) wound the compression damping up to slow the rate the suspension got to full travel?
(a) Certainly yes
(b) Iíd venture to say so

Offline Tom

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Re: This might come as a shock.
« Reply #16 on: September 21, 2021, 01:15:19 PM »
Itís not hard Bulldog.
But that was a bit prolonged because it allows people who are coming to terms with the concept, to work methodically through and identify where their misunderstanding begins.
You start with the bit thatís easy to understand and work your way through.
Caffeineo in the previous post, indicated that there was a light beginning to appear for him. My post was to help people who are on the cusp of getting it, not people such as yourself who (apparently) already do.
How would you have explained it more briefly without leaving out an important aspect ?
I could have done it in two lines to someone who already understood, but Iíve been trying to learn the stuff Beetle does and I like the way he patiently leads the beginner along.
BTW.
I donít know what the emoji is supposed to convey, can you enlighten me ?
Please donít make it too hard.... :popcorn:

 :popcorn: :popcorn: :popcorn: :popcorn:
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Re: This might come as a shock.
« Reply #17 on: September 21, 2021, 09:28:51 PM »
I read somewhere that Henry Ford grabbed the first model T off the line and took it for a spin across a lumpy field.  His debrief note read "Needs shock absorbers."  Some say he specified hydraulic, some say he was not specific.  It doesn't matter.  The point is that springs and shocks are two different components of a suspension system, and they have been forever.  With that said, I'd like to offer how I understand them.  Maybe it will help.  Keep in mind that I'm speaking in generalities.

A lot of vehicles have springs and shocks as discrete parts of the suspension.  My van for example, had a leaf spring in the back and a shock absorber somewhere on the rear axle.  I can change one without affecting the other.  The van front has a coil spring and a remote shock at the edge of the control arm.  My Miata has the shock and spring incorporated into a "strut" at all four corners.  Unlike the van, if I need to service either my shock or spring, the entire strut must be disassembled.  So they made the shock length and spring preload independently adjustable. 

All of these examples do the same thing (simply put) -- limit the travel of sprung (body) vs unsprung (wheel and axle) weight, and control the bounce and roll.  The strut on my Miata though saves a lot of weight compared to the other designs, and fits into a much smaller space.  My bike does the same thing as my Miata.  It incorporates the spring and shock into a single, light, small assembly called a "coilover."  That is short for "coil spring over its shock absorber."  We install them as a single unit, but they are in fact two separate components sharing a common center.  And like the Miata, my EV has independently adjustable springs and dampers.

The limit of travel in the suspension is generally governed by the travel limits of the shock (or suspension stops).  A shock with 4" (useable) travel is going to compress 4".  Beyond that extension, the wheel leaves the road.  at the end of compression, you "bottom out" or scrape a fender.

But the shock only dampens the freefall/rebound of the bike onto the "bottom out" point.  And that's not where you want to ride.  What holds the sprung weight (bike) off of that bottom point is the spring.  The spring has a lot more than the 4" of travel that the shock does, but it's not going to freefall to bottom.  It finds equilibrium for whatever static (resting) load it's got, and dynamically (in motion) compresses/extends from that point.  You're going to have to compress it a bunch to get it to the bottom of shock travel.  And that's where preload comes in.

Let's say that the bike at "curb weight", which is full of fluids and gas but has no gear or rider aboard, compresses the springs 1" (25%)  That initial compression is called "sag".   Now the rider sits on the bike and the spring sags another inch (50%).  His pillion mounts and and a third inch goes away.  The shocks now have one inch of travel.  Those potholes are going to hurt.

We can't add travel to the shock post.  That has hard, physical limits.  So to allow more travel for the shock, we need to make the spring sag 1" at the load we intend (in this case, the rider and pillion.)  We do this by increasing the preliminary load on the spring (preload) to match the intended load.  That way, when the riders climb aboard, the spring is compressed at the curb weight 1", and the shock independently retains its travel.

But in my example, I had two riders and no gear.  If I add gear or subtract the rider, I might have to crank the springs up or down for optimal performance. I'm usually loaded heavy on bad roads.  So I tune to that and only bother to readjust if I'm getting annoyed.

Online Huzo

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Re: This might come as a shock.
« Reply #18 on: September 21, 2021, 10:01:14 PM »
Thatís an informative addition RK.
Someone may get more out of that than my example/s. Over the journey Iíve found that at times, I can struggle furiously over some concept and then along comes someone and blurts it out in a different way and suddenly...
Bingo...!
Itís a beaut subject and the only reason I laboured with it was that there were a lot of people struggling like I was.

Nice to hear from you by the way.... :bow: :thumb:

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Re: This might come as a shock.
« Reply #19 on: September 22, 2021, 12:11:44 AM »
I heard the same thing Saturday when a local rider was checking the suspension & preload on my Ducati.  Nor sure I still understand it.  The bike "feels" stiffer with more preload.  But it does sit up higher with more preload, no question.

BTW- he was impressed that the suspension had so little stiction but the rear shock is too stiff (high speed) compression.  we didn't change anything, it was close enough other than the "Stiff" rear shock.
As soon as your butt hits the seat as you climb aboard, you get a signal as to the perceived ďfirmnessĒ of the suspension.

As an exercise.
Wind all the preload off your shock and jump on the bike, feet up with someone holding you level.
Have an assistant measure the length of the SPRING.
Now climb off and wind on a realistic amount of preload (say, 10mm).
Hop back on the bike with your feet up and with your scantily clad assistant holding you level, have your other assistant measure the SPRING length again.
It will be the same...(it will therefore perform the same).
Now put the tape measure away and stop looking at your scantily clad assistant..(donít be tempted to try measuring anything else...) :wink:

I hope the preceeding may serve useful to anyone who made no sense of my earlier offering.
« Last Edit: September 22, 2021, 12:13:03 AM by Huzo »

Offline tris

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Re: This might come as a shock.
« Reply #20 on: September 22, 2021, 01:28:19 AM »
I suspect that part of my confusion was the use of the word "Preload".

Am I right to say that the objective is NOT to pre load the spring it is to just shift the mounting point of the spring

If the shock wasn't there limiting movement then ride height would simply increase

However we have a shock limiting the movement so the spring is compressed, and mechanically compressing the spring by 1 inch is the same as applying a mass that compresses the spring by one inch

Physically ride height can never exceed the max travel of the shock however the spring doesn't know that so doesn't react until the travel created by the travel created by the mass exceeds the compression applied by the "pre load" adjuster (I'm assume simple static loads at this point)

Am I still on the right track?

Cheers

Tris


PS
My second-hand Roamer came with Hagon Nitros fitted and 17GKg springs IIRC
I wrote to Hagon last night to see if I have the right springs for my mass :grin:




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Online Huzo

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Re: This might come as a shock.
« Reply #21 on: September 22, 2021, 04:26:35 AM »
I suspect that part of my confusion was the use of the word "Preload".

Am I right to say that the objective is NOT to pre load the spring it is to just shift the mounting point of the spring

If the shock wasn't there limiting movement then ride height would simply increase

However we have a shock limiting the movement so the spring is compressed, and mechanically compressing the spring by 1 inch is the same as applying a mass that compresses the spring by one inch

Physically ride height can never exceed the max travel of the shock however the spring doesn't know that so doesn't react until the travel created by the travel created by the mass exceeds the compression applied by the "pre load" adjuster (I'm assume simple static loads at this point)

Am I still on the right track?

Cheers

Tris


PS
My second-hand Roamer came with Hagon Nitros fitted and 17GKg springs IIRC
I wrote to Hagon last night to see if I have the right springs for my mass :grin:
Mate I think so, preloading the shock is purely and simply to stop the spring having to squash so much before it will equalise to the load.
If your shock compressed 50 mm with your load in an unpreloaded condition, you could theoretically get off, wind on 49 mm of pre load and the spring would only have to squash another 1 mm to carry you when you jumped on.
But the first time you exited a depression, it would obviously top out.
Think of YOUR spring on YOUR bike with YOUR weight as an example and that spring will always compress to the same length (X), loaded at 1g.
Winding on preload will just move the bottom perch that the spring sits on, higher up the damper body and as long as the rod does not top out, the top of the spring is free to retreat away from the bottom perch as it winds up always maintaining that (X) length.
Hereís one..
If you stand a coil spring on a sheet of wood on the floor and you place a load on the spring, it will obviously compress.
Then if you grab the piece of wood and lift it upwards, the top of the coil will move upwards at the same rate as the bottom because there is no impediment to itís motion.
Same with a coil over spring like we have.
Winding on pre load if you were sitting on the bike with the shock loaded, would just result in the top of the coil retreating upwards, again as long as the damper doesnít top out.

Offline tris

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Re: This might come as a shock.
« Reply #22 on: September 22, 2021, 07:11:35 AM »
Thanks H!

It's all making sense now from a spring perspective. Just need to have a think some more about damping  :grin:

In other news.

Hagon have come back to tell me I need 18Kg (/mm?) springs as opposed the 17Kg on the bike now

Just need to find out how to get them changed out
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Online SIR REAL ED

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Re: This might come as a shock.
« Reply #23 on: September 22, 2021, 07:19:26 AM »
When are we going to get to the "shocking" part?
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Online Huzo

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Re: This might come as a shock.
« Reply #24 on: September 22, 2021, 07:36:11 AM »
When are we going to get to the "shocking" part?
Well...?
The thread hasnít been nuked and a fight hasnít broken out yet. Thatís a bit of a shock isnít it ?

Offline wymple

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Re: This might come as a shock.
« Reply #25 on: September 22, 2021, 08:37:05 AM »
Nobody has yet brought up the touchy subject of the flux capacitor's part in all this. I do know this much. I am short, at 5'8 on a good day. Cranking up the preload for me to carry a passenger has always made it harder for me to swing a leg over the bike & reach the ground. Regardless of the science involved.
No trees were harmed by the conveyance of this message, but a lot of electrons were seriously disturbed.

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Re: This might come as a shock.
« Reply #26 on: September 22, 2021, 09:00:21 AM »
Well...?
The thread hasnít been nuked and a fight hasnít broken out yet. Thatís a bit of a shock isnít it ?
I have learned a lot from this thread and there is nothing wrong with a respectable debate, regardless of the differences in oneís opinion. Just wish I was smart enough to offer any reliable information 🤔

Online 80CX100

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Re: This might come as a shock.
« Reply #27 on: September 22, 2021, 09:02:04 AM »
I read somewhere that Henry Ford grabbed the first model T off the line and took it for a spin across a lumpy field.  His debrief note read "Needs shock absorbers."  Some say he specified hydraulic, some say he was not specific.  It doesn't matter.  The point is that springs and shocks are two different components of a suspension system, and they have been forever.  With that said, I'd like to offer how I understand them.  Maybe it will help.  Keep in mind that I'm speaking in generalities.,,,,,,,,,,

Hey RK,     Tks very much for adding to this topic, as always, extremely well written  :bow:

ps glad to see you're doing well and back in the fold

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Offline tris

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Re: This might come as a shock.
« Reply #28 on: September 22, 2021, 09:30:09 AM »
When are we going to get to the "shocking" part?

I'll have a stab for sh*ts and giggles

Compression damping
Controls the rate at which the spring compresses when an upward force is applied to the wheel - say a bump in the road

Rebound damping
Controls the rate at which the spring expands when the wheel comes across (say) - a hole in the road


I'm happy to delete this reply in it's entirety if I have that wrong!  :grin: :grin:
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Online rodekyll

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Re: This might come as a shock.
« Reply #29 on: September 22, 2021, 12:54:39 PM »
Nobody has yet brought up the touchy subject of the flux capacitor's part in all this. I do know this much. I am short, at 5'8 on a good day. Cranking up the preload for me to carry a passenger has always made it harder for me to swing a leg over the bike & reach the ground. Regardless of the science involved.

You are seeing the science involved in action. 

The shock has a sag of a given amount (we use 1" as an example, because 1 is an easy number to work with.  Yours may be different) when the bike is at curb weight.  We'll call the the static "zero" point.  When you sit on it, that spring has more sag because of your weight.  That puts the seat closer to the ground, and you can flatfoot when you're sitting on it. 

But the additional static sag reduced the shock travel, because the shock and the spring both settled the same amount under the additional weight.   To regain the static zero point, the spring needs to be adjusted (preloaded) to regain that 1" of static sag while you're sitting on it.  So the spring is set to an additional weight factor -- say 175#.  That means the curb weight of the bike is no longer sufficient to compress the spring -- it needs your additional weight to arrive at 1" sag.  So with no load on the bike, it now sits 1" or more higher than it used to.

So the relationship of the seat to the ground changes with preload adjustments.  The relationship between the seat and the pegs does not change.  This is important with shaft drive bikes because one of the physical limits of our suspension is the ujoint/swingarm angle.  We want to attempt as neutral a static swingarm angle as possible.  Keeping the spring properly tuned preserves that angle.  Note that longer or shorter shocks will change swingarm angle, possibly past the limits of longevity for the ujoint, or to the point that oil migrates from the rear drive.

 

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