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Suspension Compliance


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Here's one for Balistic and other mechanically knowledgeable persons:

 

When I did Level 1, our classroom seminarist (Andy Ibbot) argued that we should try to keep the suspension in the mid-third stroke.

 

Being an engineer, I asked him why? - Andy argued that the suspension was more compliant there there and I asked again why?, because as long as we don't hit the limits of the shock (so that the spring/dampers are still working) I cannot see a reason that the shock should be less compliant in the outer parts of the stroke, than the mid-stroke.

I could see he struggled to find an explanation that would satisfy the engineer in me after the second why, so for the sake of the overall training I let it pass. I think understand what we are trying to achieve by being in the mid-stroke, but I didn't find the explanation to be well founded.

 

So, can anyone explain why the suspension should work better in the mid-third stroke than the outer part of the stroke (still assuming that we don't hit mechanical stops like top/bottoming out)?

 

Thanks,

 

Kai

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I don't know what Code have in mind, but there are situations when at least some systems are more compliant/working better near mid-stroke than near the end of stroke:

 

1: Progressive fork springs get stiffer as they compress, meaning it takes more and more effort to move the suspension as it compresses

2: Most street bikes have progressive rate rear suspensions that again makes them pretty firm near the end of its suspension travel

3: The closer you are to either end of travel, the greater the risk of topping out or bottoming out, situations where the suspension stop working alltogether

 

Of course, a progressive system will be most compliant at the beginning of its stroke, but then there is the risk of topping out, as mentioned. It can also be too soft for the load, increasing the chance of instability.

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Hi Kai,

 

The big reason that the we want to do this, (as much as possible), is because the fork (lets take for example), same gos for rear but we also have linkage rations which also complicate this further), rises in it's ressistance the further you go through the fork. This is not because the spring rate changes, as it doesn't, but becuase the air gap between the oil and the forks gets compressed greatly and ramps greatly it's ressitance towards the end of the stroke (hence why changing oil weight and oil gap are fine tuning options on the bike.

 

 

We obviously neeed oil in the forks and gas in the rear to control the stroke and it's rate of progress through the stroke and I understand this is a by product of the compromise of suspension. We need the suspension to be compliant to allow the wheel to soak up bumps, and also to track the road surface and not transfering those large bumps/ripples or whatever to the bike.

 

 

I hope this simply helps explain, I don't have the very detailed in depth explanation of it, but it's enough for me to know, appreicate and feel the difference in it when i set my bikes up for it.

 

 

Bullet

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Hi both,

 

My counter-argument about keeping the suspension in the mid-third is that if the suspension is so useless in the outer two thirds, why didn't the manufacturer then just make a component with the 1/3rd stroke? surely that would be much less costly, have less weight, etc etc. Surely there must be a reason for the full stroke!

 

But let's analyze the problem and see if we can figure out together what the physical reality is behind the "friendly advice".

 

Let's first agree about what's involved in a suspension unit: the spring, the suspension oil/valves, and the air volume above the oil.

 

1) The oil is passed through 2 (different) valves to control the speed in the two directions (ie compression and rebound damping). As the oil only provides dampening when it is forced through the valve, the actual position of the fork does not matter to the oil dampening.

 

2) Spring. Assuming we don't hit 'coil bind' (ie the coil is compressed so it touches itself and thereby stops being a spring), the spring will exert a force that is linear to the distance/displacement from the 'free length' and the spring constant (stiffness) - displacement velocity does not matter here.

 

3) Air volume above oil. Works like an extra (very) soft spring, until the volume disappears - here we get "hydraulic lock" since the shock cannot collapse anymore, and the suspension does not work anymore. For the discussion, let's assume that it doesn't happen.

 

The combination of the 'real' spring and the air volume gives a set of progressive stroke/Force curves like the ones in the attachment (which I blatantly stole from the Öhlins Front fork kit FGK137 Mounting Instructions, since I have such a thing on my trackbike).

For convenience, I added two lines to distinguish between the three regions and labeled them according to whether it's in the top/mid/bottom of the stroke.

 

For reference: the recommended oil level is 160mm, which is the line that ends around 330 N. As I recall, I used the 160mm oil level (I don't have my notebook handy so I'm not sure), while a national-level racer with the same suspension used 130 or 140mm oil level, ie a more progressive spring.

 

If it is correct that the advice comes from the progressiveness of the air-spring, then we can ignore the oil damping and valves.

 

If we look at the 3 regions, then we have about 30-50N change in the top end, 50-125N in the mid-stroke, and 100-600N change in the bottom end. OK, so why is the progressiveness of the spring a problem? - having a progressive spring help you to avoid bottoming out the fork, after all :)

 

The real problem (I guess) is that the damping requires movement, since it is the forcing of the suspension oil through the valves that generates the damping. But then again, if the fork doesn't move as much (due to the higher effective spring constant), who needs damping?

 

What if we could engineer a fork/suspension that had a truly linear relationship between stroke and Force? - would that change the arguments for keeping the suspension in the mid-third of the stroke?

 

It's past midnight, so I'll leave it here for others to laugh/comment on until I have more energy to think this through.

 

 

post-15296-0-33137800-1344337744_thumb.png

 

Kai

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i think you can see from the Ducati with rossi on, it's in it's Mid third, and he's still on the brake, turning and loading the front. This isn't be accident, clearly. I do know when Keith wrote the text originally, the kit was pretty poor, and today, the suspension is massively, massively better, which does allow more tolerance to bid technique. I do believe that it still applies though in every sence.

 

I think it's also impossible for the vendors to make a linear stroke fork, it is of course a sealed unit, and when you compress sealed things, it's not possible to keep the pressure inside constant, hence the stroke pressure differences. road bikes, do on the whole have progressive springs, race bikes do not, they're linear. also, you must remember Kai, that the springs are normally around 10nm, so the air spring at 100 nm (as an example) is a much bigger force than the spring at the end of the stroke.

 

Bullet

post-15526-0-93625300-1344356692_thumb.jpg

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Something to consider is that the bike's geometry is affected, too - compressing or extending the forks or rear shock towards the ends of their travel can change the handling characteristics of the bike. Presumably (and this is an assumption on my part) the bike is initially set up so that the best handling DURING a corner is with the suspension in the middle, and the farther away from that you get the less compliant the bike will feel.

 

Also there is the obvious fact that if the suspension is in the middle of its stroke you have the most available travel in both directions to absorb bumps in the road.

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The real problem (I guess) is that the damping requires movement, since it is the forcing of the suspension oil through the valves that generates the damping. But then again, if the fork doesn't move as much (due to the higher effective spring constant), who needs damping?

OK, since no one picked on this one, I'm going to answer myself: The reason we want the suspension to move is to

 

i think you can see from the Ducati with rossi on, it's in it's Mid third, and he's still on the brake, turning and loading the front. This isn't be accident, clearly.

Let me be clear here: I'm not arguing about whether keeping the suspension in the mid-third stroke is good or not - I'm grinding my Axe over the arguments provided.

 

I think it's also impossible for the vendors to make a linear stroke fork, it is of course a sealed unit, and when you compress sealed things, it's not possible to keep the pressure inside constant, hence the stroke pressure differences.

I'm sorry, but that is not correct: you can make a linear stroke front suspension. Just think of your steering damper; it's a sealed unit, but it still provides the same damping regardless of where you are in the stroke. The trick is to isolate the oil damping system from the collapse of the fork. If you've noticed the external "cartridges" on the motoGP forks, I guess they could be doing already (or have tried it and gone back to a progressive system).

 

Something to consider is that the bike's geometry is affected, too - compressing or extending the forks or rear shock towards the ends of their travel can change the handling characteristics of the bike. Presumably (and this is an assumption on my part) the bike is initially set up so that the best handling DURING a corner is with the suspension in the middle, and the farther away from that you get the less compliant the bike will feel.

Hotfoot, my gut feeling is that it's the geometry argument that is the real reason - that the changes in trail and rake over the stroke is the key problem. Question is, how do we figure out if this is 'it'?

 

Kai

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Steering dampers don't have springs in them...

That doesn't matter, my dear Bullet :)

 

Hint: how is the oil damping system and the spring of the rear shock attached to each other?

Assuming a linear spring, Is the spring constant of a rear shock linear or progressive?

Why is it different from a front fork?

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I'm thinking we should set you a little task of going to ask mr Ohlins some of these questions and come back and report your findings?

A shock is connected to a motorbike by a pivot and linkage, a linkage which in most cases is not linear I.e. 1to1 ratio of shock movement to Swingarm movement. (I do have a linear racing linkage in my Ducati), but most bikes cannot use these because of the multiple usages of a bike.

I know the external canisters you see on the MotoGP forks/WSB forks contains the oil and air gap levels, you remove the top and you easily adjust them. I don't have detailed knowledge though, but have seen competitors messing/experimenting with them in racing. Is very high end stuff though. I just use cartridge kits, which are exceptional and reasonably good value for money.

I do agree entirely that's linear spring doesn't change it's resistance during any of it's stroke, but the compressed air or nitrogen in a rear shock, must affect the end of the stroke. The chart you provided (which I have seen before too), shows exactly this.

I'm afraid I don't have a mechanical engineering background, and thus don't have the in depth technical explanations on the specific details. Perhaps someone in our midst does? I do think the Ohlins route is good one though? Fancy the challenge of finding out?

An interesting thread indeed this is,.,

Bullet

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There are several questions in this, so let's first break them apart so it's easier to understand and answer them.

 

First, there's the original question: Why only use the mid-stroke?

So far, we have two possible answers: A/ the air-spring gives a progressive spring rate close to bottoming out, and B/ geometry preservation (having topped/bottomed out the fork changes the rake/trail significantly).

 

What I have claimed that if it's really the progressiveness of the air-spring, then we could go and create a front fork without the air-spring, ie: a fully linear front fork. One problem solved, but it may bring up other problems.

 

During this discussion the rear linkage is brought up. As I understand Andrew Trevitt's book "Sportsbike Suspension Tuning", the linkage is there to deliberately make the rear shock action more progressive - to avoid bottoming out the rear shock under severe conditions.

 

The air spring featured in the front fork may actually serve exactly the same purpose, so removing it may create more problems than it solves.

Also, if we make the front fork spring truly linear, but still need to support the same force/weight, we would need a harder spring overall - which could be detrimental to the feedback from the front tyre.

 

I don't have particular exclusive access to Kenth Öhlin nor his engineers, but maybe less will do.Time to go the source, I think.

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For those who may be interested, here is a nice article on basics of how front forks work - it talks about progressive springs and different types of damping (high/low speed, shim stacks, etc.):

http://www.motorcyclecruiser.com/tech/fork_suspension_technology/index.html

 

And a very interesting read from some people who were toying around with suspension on a cruiser, with some data collection. It's a lot different from a sport bike but does cover some cool info:

http://www.motorcyclecruiser.com/tech/improve_ride_suspension_performance/index.html

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Someone recently suggested I should Google "Dave Moss" and then start reading, watching, learning... I finally did and I think it's been well worthwhile. If you're curious then try for yourself; if not, well then there's nothing to see here ;)

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Someone recently suggested I should Google "Dave Moss" and then start reading, watching, learning... I finally did and I think it's been well worthwhile. If you're curious then try for yourself; if not, well then there's nothing to see here ;)

 

Could have been me. I'm a big fan of Dave Moss.

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Someone recently suggested I should Google "Dave Moss" and then start reading, watching, learning... I finally did and I think it's been well worthwhile. If you're curious then try for yourself; if not, well then there's nothing to see here ;)

 

Could have been me. I'm a big fan of Dave Moss.

 

Brad;

 

Thanks for that tip; I stayed up late last night watching Dave Moss videos with CSS Coach and WERA racer Kristie Martel.

 

Rain

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Brad;

 

Thanks for that tip; I stayed up late last night watching Dave Moss videos with CSS Coach and WERA racer Kristie Martel.

 

Rain

Last week I watched those as well. It's almost an addiction, once you watch one video then you want to watch the next one and then the next one... :)

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Brad;

 

Thanks for that tip; I stayed up late last night watching Dave Moss videos with CSS Coach and WERA racer Kristie Martel.

 

Rain

Last week I watched those as well. It's almost an addiction, once you watch one video then you want to watch the next one and then the next one... :)

Kristie was my coach at Laguna (in the rain) and at NJMP and over time has become a friend. Watching her riding improve after each session with Dave Moss was really impressive; especially how nonchalant she was after getting taken out in the race. She got right back after it once she lined up another bike. Beyond that, watching what Dave Moss did in each video was very informative because the results of his work was immediately available in Kristie's racing and that alone made me a believer.

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