Rebuilt shocks

[Tractors are go]

K tech advertised an air suspension for the gsa, just after I ordered the hyperpro shocks.

I've used ktech for years, Aprillia and Ducati's, and found them friendly and helpful.

I look forward to the comparison of different shocks.

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[stanley]

Lots of problems with air springs (shocks): they ramp up in a very nonlinear way, and the air pistons create lots of stiction. Adding a second air chamber on the negative side of the air piston reduces this, but worsens the ramp up.
Give me a well matched steel spring every time: nice and progressive and consistent. Maybe titanium if money is no problem

BTW, my touratech /tractives are manually adjusted.... I couldn't bring myself to pay more for much less with the electronic jobs. Construction is very different to the Wilbers.... Proper separation of nitrogen and oil in even their cheapest shock for starters... None of this cheap emulsification rubbish!

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[Tractors are go]

No, assuming that temperature stays constant; half the area doubles the pressure. It is linear.

Fournales (?) Make air shockes for the dgt and airbus from memory and they don't appear to have any major problems.

AA VW transporters had rear air suspension, which are much sought after by vw T4 owners.

The main problem, is leaks but that is also true of oil filed or those with separate nitrogen chambers too.

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[stanley]

Oh dear. I dispair.........
Tractors aren't go...

You have quoted something a bit like Boyle's law. In its rawest form, it produces a curved graph (hyperbola? I can't remember exactly)

For examples (plucking some vague spring rates)....
With a coil shock-
One kg applied will result in 1mm of compression. 2kg = 2mm.10kg applied will result in 10mm of compression. Plot that on a graph.


With an air shock- (and using your/Boyle's formula)... one kg applied will result in 1mm compression. 2kg applied = less than 2mm. 10kg much less than 10mm. I really can't be bothered to work out the amount, but you plot that on a graph.

One line straight and one line curved maybe?

To make it easier for you... Get a bicycle pump and put your finger over the outlet. Press the handle in a bit.... Easy? Press it further.... It gets harder?

Not ideal for motorcycle suspension applications then? Unless of course you want the ultimate in "bottoming out protection" or are using a complex linkage driven system.

The rest of your post...


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[Santa-2512]

Lots of problems with air springs (shocks): they ramp up in a very nonlinear way, and the air pistons create lots of stiction. Adding a second air chamber on the negative side of the air piston reduces this, but worsens the ramp up.
Give me a well matched steel spring every time: nice and progressive and consistent. Maybe titanium if money is no problem

BTW, my touratech /tractives are manually adjusted.... I couldn't bring myself to pay more for much less with the electronic jobs. Construction is very different to the Wilbers.... Proper separation of nitrogen and oil in even their cheapest shock for starters... None of this cheap emulsification rubbish!

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Yes i agree with the stiction part but only very loosley,

Stiction normally only occurs when the piston remains static for an amount of time ( say parked overnight) and is then is asked to

perform at the limits of its operating parameters.

Given that in this case the non load condition would be an (expansion) aspect. When the bike is parked. any stiction at this

"expansion" end would be un-noticable, because the riders weight when sitting on the bike the next moirning, would manually contract

the piston releasing any stiction.

If however the normal operating parameter was a compressed aspect, then under certain conditions (time & low temp) asking the

piston to initially expand could demonstrate stiction,

However convention would dictate that the piston is in a nominal position which would allow compression & expansion from rest.

 

[Tractors are go]

Technically it is an inverse non linear graph. All I can remember of Boyles Law is that temp is supposed to stay constant or it buggers up the experiment.

However, most understand the concept, half the volume double the pressure.

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[Bendy toy]

I did a bit of digging about over air springs. They will work fine on a bike BUT a normal coil spring diameter unit is likely to stiffen to quickly or be too stiff by the time its got a high enough spring rate. Lorry axle air bags are like donuts with a small diameter piston pushing a large air bag. They don't ramp up the pressure like a piston in cylinder. They are also pretty fat compared to metal coil springs.

It looks like an sir spring could easily work but would need to a be a larger diameter than the steel coil springs used on bikes. The GS does not have enough space for larger diameter springs so air bags would most likely not work properly.

 

[stanley]

Technically it is an inverse non linear graph. All I can remember of Boyles Law is that temp is supposed to stay constant or it buggers up the experiment.

However, most understand the concept, half the volume double the pressure.

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I'm glad that we now agree it's non-linear

BTW, non-linear is probably good for some applications.

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[Bendy toy]

Air spring characteristics depend on the air bag shape. As said a relatively small piston pushing an air bag into a donut shape does not have the pressure ramp up issues.

Non linear is good for many applications. Sports bikes have expensive rising rate linkages to do the same job.

 

[stanley]

Bendy, the linkages also compensate for the inherent limitations of a single pivot design: a problem we don't have with our bikes' complex axle paths.

Have you read Tony Foales work? I think you would like it

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[Bendy toy]

Tony Foales was looking to avoid the suspension/drive jack up on shaft drive bikes. Good stuff.

The Yamaha Fazer 600 Mk1 used the same rising rate linkage and rocker unit and the same shock as the Diversion 900. The earlier XJ900 had twin shocks and no rising rate stuff.

The shaft drive Diversion 900 had a solid swing arm but suffered very little rear axle power jack-up probably due to the rising rate linkage. The older XJ900 had normal twin shocks so the back end jacked up like an airhead BMW.

However the 600 Fazer had a chain drive so the rising rate linkage was there to give a progressive stiffening so the springs did not need a bump stop. The linkage gave more leverage onto the shock at low compression (= more compliance to bumps).

I think rising rate suspension is great on a bike. You get the benefits of a soft spring over small oscillations but a stiff spring for bigger bumps.

 

[stanley]

Tony Foale explains all elements of motorcycle chassis design! He wrote the book! It's a good starting point to further understanding. There's so much to it.
Understanding the modified axle paths of tele and para-lever helps to understand the requirements' of our shocks, and how we could, maybe, consider modifying our riding style to benefit.
Probably only of interest to a small minority, but absorbing to a born fiddler

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[Bendy toy]

Tony Foale explains all elements of motorcycle chassis design! He wrote the book! It's a good starting point to further understanding. There's so much to it.
Understanding the modified axle paths of tele and para-lever helps to understand the requirements' of our shocks, and how we could, maybe, consider modifying our riding style to benefit.
Probably only of interest to a small minority, but absorbing to a born fiddler

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BMW's Paralever separates suspension forces from drive forces so they don't interfere especially during acceleration. I suspect they set it make the back end squat under power rather than rise.

The TeleLever front end is to separate braking forces and suspension forces. It feels like they dialled in some brake dive to make the bike feel more normal to riders brought up up on normal telescopic forks. I believe the dialled in too much and use the rear brake to mask the issue. The forks dive a lot more when the ABS is off (linked brake system is not working the back brake)

The TeleLever looks pretty normal but I think its a shame they did not use the Hossack design as intended by Norman Hossack. His front fork is about as simple as it gets for bike front suspension and can be made with low precision parts. Rose (ball) joints are used to adjust out any manufacturing tolerances or change the geometry slightly on such as race bikes. The downside is a trellis fork leg looks odd to the average conservative minded bike magazine writer so BMW went with the much heavier (and a lot more costly) Saxon fork they renamed Telelever.

 

[stanley]

Tele lever feeds braking forces to the engine block: therefore, a heavy/strong head tube is not needed. It uses a slightly forward arc as it compresses; it is severely restricted from compressing unless the front wheel can travel away from the engine. Heavy use of front brake restricts this; so braking has some impact on suspension. It also makes it poor at dealing with small, or square-edged, bumps.
It does, however, separate braking/suspension forces from steering. I like it!

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[Bendy toy]

The front wheels on cars move more or less vertically. On "normal" bikes the front wheel moves back 10mm for every 50mm of fork compression so heavy braking causes fork dive. So cars don't dive on the brakes while most bikes do. Cars do dip (as does the GS) but that's weight transfer.

Norman Hossack uses two lever arms. One above the wheel and another about 6" higher. This gives the same vertical wheel travel with less weigh and complication. His forks do not telescope like the GS.

However, tele forks are better off-road. The suspension can be bounced over obstacles using the brakes in ways the GS can't do. BMW have dialled in so power squat to help but it's not the same. Road racers would be better with Hossack/Saxon front ends but tradition goes for tele forks.

This one has what amounts to a Hossack front end. They could brake to a bend apex then get straight onto the power. No waiting for suspension to settle.

 

[Bendy toy]

This gives more idea. I'm not sure about the rear shock linkage but at least its not cooked by the engine. The Hossack front suspension is actually no more complex than a twin wishbone car system though this bloke keeps on about how complex it is.

I love how the low pressure area around the back wheel is used to pull air thought the flat mounted radiator. Perhaps not so "cool" on a hot commute along M25.

 

[stanley]

The best motorcycle engineer ever? Probably! Pity he died so young.

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[Bendy toy]

He used many of Norman Hossack's ideas but added even more cool stuff. The journalists all say it was complicated but it really wasn't. They've obviously not looked inside a half decent set of forks or considered the engineering needed to make high tolerance sliding tubes.