Shock mount length variation

Nice report Reggie:

"CHANGING PRE-LOAD DOES NOT CHANGE SPRING RATE. You don?t get a stiffer spring by adding pre-load. Rather you change the weight point at which the spring starts to move. Once the spring starts to compress it will move at its spring rating."

This is what I was trying to express.

 
As a rough estimate, the twist on the rear axle from a 10mm difference in length would be about 30Nm (which is about the same twist as the cylinder head nuts.) This assumes a spring rate of about 100N/cm and the width of the swing arm being about 30cm... All these are estimates but we are in the right ball park here. This means it's not as big as the twist required on the Jesus nut, nor is it as small as the twist required on a fairing screw but somewhere in between (like a cylinder head nut.) This also agrees with Simon's calculation of the force difference.

If the springs are constant rate, that twist won't change as the suspension moves, until one or other shock binds (but at that point you're almost certainly beyond the point where this matters!) IMHO any swing arm suspending 300kg of bike and rider ain't gonna worry about that amount of twist so while clearly undesirable, there are probably only one or two riders of Laverdae (Blair/Drmsby) who would possibly notice and even then not on the road.

--
Dick
 
Grant said:
Nice report Reggie:
"CHANGING PRE-LOAD DOES NOT CHANGE SPRING RATE. You don’t get a stiffer spring by adding pre-load. Rather you change the weight point at which the spring starts to move. Once the spring starts to compress it will move at its spring rating."

This is what I was trying to express.
Click to expand...

The thing is Grant, each spring has a different starting point, with the 25lb load differential maintained throughout the travel of the springs. Both springs still have the same rate but different loads.
 
But in mid range of their travel, which they will likely spend most of their time, it won't matter much to folk travelling at less than 10/10ths.
 
what a load pof old tosh and drivel,

put simply, on a twin shock bike they need to be the same left and right

and at any point in the suspension arc the dimension between the top amd bottom mount should always be the same, left and right

used shocks are cheapn
new shocks are not very much more
give up fags and booze for day and get a decent pair
CLEM
 
But in mid range of their travel, which they will likely spend most of their time, it won't matter much to folk travelling at less than 10/10ths.
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ignores the fact that there is still a twisting force on the swingingarm throughout it's travel.  Which can't be good for it.
 
I'd suggest on this 2900, which I reckon had previously been on a drag strip, odd mounts on the swingarm for what I reckon were for wheelie bars, some fancy frame bracing, and the fact it has an RGV front end.  :o

Bike belongs to a Lamborghini mechanic I know. I'm sort of thinking that this variation was some attempt to get the thing to run straight on the track We had a very interesting conversation, as had been had here, re the consequences.

Ian, here's the Laverda input.

My argument was based on mt 81 Jota, it had a variation of half a hole, 5mm or so. When the shocks were off that, I'd have the fit one shock and have someone lean on it so I could fit the other. That sucker, rode OK straight but cranked over, and if the corner had undulations, it would wallow through the corners. There was one time I was cranked over on a r/hander at 180klms and wallowed off the road and onto the grass  :o  Yeehaa! 

Left handers were more interesting, they'd shoot me onto the wrong side of the road, almost like the front was drifting, but it wasn't, it would happen suddenly, unlike a drift. Almost like it was tracking three apexes per corner. Rob Berryman can vouch for that, he watched me cross the road and near head on with a car, I seem to recall I went around the outside of the car!  :o :o :o

Two up and having a go was diabolical!

Yet, Eastern Creek Raceway, wasn't really an issue, it was pretty smooth, and bike tracked rather well.

I always put it down to that height variation.
 
Not nice at all from an engineering point of view, a condition like this should be avoided at all costs.  Probably quite common with some backyard bodgers though.

But, I'm with Rod here, will probably go mostly un-noticed in real life, especially on something as bendy as a Z900.  Put them on the right sides and it might even help soothe some of the more violent twists! :D :D

piet
 
Different centre distances on the shock mounts will mean that there's more pre-load on one spring than the other (as others have said). The net result is a twisting effect on the swingarm which will place a small load on the swingarm bearings. That's pretty much it - end of story.

The difference in pre-load is constant throughout the suspension travel so it won't induce more or less twist as the suspension moves. So it won't (on its own) induce wobbles or other heinous handling problems as long as your not hitting the stops on the suspension travel. It may marginally increase the friction in the swingarm bearings, but it would be negligible in comparison to the damping forces of the shocks.

It won't affect damping.

It won't induce a 30Nm twist on the rear axle.

It's unlikely (on its own) to make the bike steer differently in right and left turns. The preload imbalance would hardly be enough to bend the swingarm so the wheel is canted over off the central plane of the bike (I reckon there was something else wrong with your 81 Jota Chris - like worn swingarm bushes)

Anyway, you can easily compensate for it. To even it out, just crank up 10mm of pre-load on the adjuster for the slack spring or back off the loaded one.

Cheers,
Cam

 
LaverdaIan said:
Far as I'm concerned.. if a Kwaka 900 has a twist in the suspension because some D**head fits it up that way and the rider ends up spat down the road, then that's one less Jappo to give a f**k about.. so wot's your point on a Laverda forum CK.. ;)

You know how I feel about this sh*t.. :P

8)
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does that make crashing kawasakis an environmentally friendly activity?
 
Thanks CK, I feel much better now.. ;) Was a bit 'pishhhy' last night, apologies, although this discussion has been a good one. Think Cam has capped it well. Rears can be quite a task at times. I fondly remember getting a new set of 'somethings' to replace my worn out Marzos on my old 3CL Jota (black/gold) just the day before a run to Bathurst. Couldn't get the cheapo crappy things right at all, so ended up ditching them and fitting the old units at dawn just in time for Moffat and the lads to roll up.. your ride experience seems about right for those bikes at times as I recall.. :P

Henry... ha ha, except for the oil spill exposure risk you may be right.. ;)

8)
 
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