Wanted: Book about motorcycle suspension

[Maha]

Suspension is a really fucking complex dynamic system, if you want to a (very basic) understanding of how to set up a bike read the link gwigs posted. If you want a thorough understanding of how it works you'll need a PhD in Physics (or Mechanical engineering)

That is a good read indeed, and for the average rider, probably all that is required. Very easy to understand from the first sentence. I will take the time and print it out.

''I tried suspension setting from motorcycle newspapers, sports bike magazine, top racing websites and forums and they were all crap. In fact two sources never recommended the same setting for my bike and no one adequately explained how they got those settings. I felt I had to find my own suspension settings for my bike. In order to do so, I had to understand how it worked and this article will attempt to explain how I set up my bike and how you can too.''

[Fergus]

Yes, 'spring constant' is what I was after. I just say 'weight' because everyone sorta understands it. eg, "You need a heavier/lighter spring".

Spring displacement could have been worked out, if I'd given a shock and a stroke length...(I think).

Nope..

Stroke length only tells us the maximum displacement for a given spring, it gives no information about how hard or soft the spring may be...

If the shock uses fluid friction (as 99.9% of them do) it has no effect on static equilibrium...


To calculate the spring constant 'k', you need a spring displacement for a given force:

k = -F/x (hookes law) where F is force (in newtons) and x is the displacement in metres (the change in spring length that 'F' caused)


So if you had said "the spring is compressed 20mm in this position" or whatever then a 'k' value could be calculated.

[Drew]

Nope..

Stroke length only tells us the maximum displacement for a given spring, it gives no information about how hard or soft the spring may be...

If the shock uses fluid friction (as 99.9% of them do) it has no effect on static equilibrium...


To calculate the spring constant 'k', you need a spring displacement for a given force:

k = -F/x (hookes law) where F is force (in newtons) and x is the displacement in metres (the change in spring length that 'F' caused)


So if you had said "the spring is compressed 20mm in this position" or whatever then a 'k' value could be calculated.

I'm pretty sure, that all of that can he worked out from the information I supplied.

I don't see how giving you nothing but a sag figure, is the way to determine what spring I need. There is no sag figure to give, because the bike is custom made and has nothing in place where the shock will be.

[nosebleed]

I've been avoiding learning about suspension because it all seems like a black art to me. Some kind of magic, something that you go to a magician to have adjusted.

But I feel like it's time I started trying to understand it. To that end, I wouldn't mind getting a book (as opposed to an online resource) about motorcycle suspension.

Does anyone have a book they would care to recommend?

Try "Sportbike Suspension Tuning" by Andrew Trevitt
ISBN-13: 978 1 893618 45 9
ISBN-10: 1 893618 45 5

I've got a copy here and it's an "easy" read. Still takes a few passes to get thing to sink in tho'

[Fergus]

I'm pretty sure, that all of that can he worked out from the information I supplied.

I don't see how giving you nothing but a sag figure, is the way to determine what spring I need. There is no sag figure to give, because the bike is custom made and has nothing in place where the shock will be.

Ok, here we go..

If it's so simple, can you give me a spring weight requirement for a direct mount shock, 25% of the way down a 570mm swingarm. 15 degrees off vertical, and 25 degrees relative to swingarm at static. Combined bike and rider weigh 180kgs.

That'd be super.

Assumption: weight distribution is 50:50 (ie 90kg/wheel)


Static Equilibrium => summing moments about any point in space will equal zero. I will choose the swing arm hinge as my point of reference.


Moment due to weight = opposing moment due to spring

cos(50)(.570m)(90kg) = sin(25)(0.1425m)(spring weight force)

Spring weight force = (cos(50)(.570m)(90kg)) / (sin(25)(0.1425m)) = 547.5kg


So the spring must withstand 547.5kg (or 5371N)

Hookes law:
F = kx

We have F and we have two unknowns; k and x, we have used all the given information, we are stuck.. without knowing either k or x we cannot calculate the other and we have no information at all about k or x...


We can plug some random figures in:
if we say k is 15kg/mm:
x = F/k = (547.5kg)/ (15kg/mm) = 36.5mm (of spring compression)


If we say x is 20mm
k = F/x k = (547.5kg)/(20mm) = 27.4kg/mm (spring constant)


Your spring rate is the least of your problems though, your swing arm is closer to being vertical than it is to horizontal (50 degrees from horizontal!!)

[Drew]

Ok, here we go..



Assumption: weight distribution is 50:50 (ie 90kg/wheel)


Static Equilibrium => summing moments about any point in space will equal zero. I will choose the swing arm hinge as my point of reference.


Moment due to weight = opposing moment due to spring

cos(50)(.570m)(90kg) = sin(25)(0.1425m)(spring weight force)

Spring weight force = (cos(50)(.570m)(90kg)) / (sin(25)(0.1425m)) = 547.5kg


So the spring must withstand 547.5kg (or 5371N)

Hookes law:
F = kx

We have F and we have two unknowns; k and x, we have used all the given information, we are stuck.. without knowing either k or x we cannot calculate the other and we have no information at all about k or x...


We can plug some random figures in:
if we say k is 15kg/mm:
x = F/k = (547.5kg)/ (15kg/mm) = 36.5mm (of spring compression)


If we say x is 20mm
k = F/x k = (547.5kg)/(20mm) = 27.4kg/mm (spring constant)


Your spring rate is the least of your problems though, your swing arm is closer to being vertical than it is to horizontal (50 degrees from horizontal!!)

Swing arm is at an angle of 10° from horizontal. Where did 50° come from?

The random figure you put in there, should be the average optimum sag a suspension tuner uses as a starting point.

Sooooo, from the information I supplied it is possible to give me a starting point with a spring that will be pretty bloody close to right...Assuming 50/50 weight distribution.

Now that we can work that out, it's time to dream up the shim stacks. The rate of change in piston speed relative to a constant swingarm movement is pretty minimal because of the shock geometry, so unfortunately it needs to start quite aggressively or have so much travel the bike would hold it's own on a motoX track...

Discuss.

[p.dath]

http://www.gostar-racing.com/informa...ion_set-up.htm

Some good info on how suspension works and how to set up..

You can download a pdf from the site.

I guess this is why I want a book. I downloaded the PDF and printed it out, got to the bit about static and rider sag, and though that just doesn't sound correct. I then when looked at the web page version, and it defines it differently from the PDF. So now I have one site with two different definitions for the same thing.

[Maha]

I guess this is why I want a book. I downloaded the PDF and printed it out, got to the bit about static and rider sag, and though that just doesn't sound correct. I then when looked at the web page version, and it defines it differently from the PDF. So now I have one site with two different definitions for the same thing.

I printed out the web page version, and will go by that...like I said earlier, it needn't be technically confusing...right down to ''that funny looking spanner thing''.
The longer this thread goes, the theories and the rights/wrongs about the subject matter, will become even more so.
What works for you ...is right.

[Robert Taylor]

Swing arm is at an angle of 10° from horizontal. Where did 50° come from?

The random figure you put in there, should be the average optimum sag a suspension tuner uses as a starting point.

Sooooo, from the information I supplied it is possible to give me a starting point with a spring that will be pretty bloody close to right...Assuming 50/50 weight distribution.

Now that we can work that out, it's time to dream up the shim stacks. The rate of change in piston speed relative to a constant swingarm movement is pretty minimal because of the shock geometry, so unfortunately it needs to start quite aggressively or have so much travel the bike would hold it's own on a motoX track...

Discuss.

Id beg to differ that the spring rate may be close. There are LOTS of other dynamic factors happening. The maths only gets you so far, the rest is real world testing. Especially with damping changes

[Fergus]

Swing arm is at an angle of 10° from horizontal. Where did 50° come from?

a direct mount shock, 25% of the way down a 570mm swingarm. 15 degrees off vertical, and 25 degrees relative to swingarm at static. .

shock is 15 degrees from vertical, swing arm is 25 degrees from shock. so swing arm is (15 + 25 =) 40 degrees from vertical =50 degrees from horizontal.

The random figure you put in there, should be the average optimum sag a suspension tuner uses as a starting point.

Sooooo, from the information I supplied it is possible to give me a starting point with a spring that will be pretty bloody close to right...Assuming 50/50 weight distribution.

So from the information you supplied.....and knowledge of 'optimum sag' from a suspension tuner

I'd guess 'optimum sag' is dependent on not just the swingarm geometry you've given, the tuner would probably want details like whether you intend to ride on sand dunes or drag strips or to the dairy and back...More information is required.

Either way, it's a highschool physics problem and doesn't really belong in this thread, I don't know anything about suspension, I just don't like it when people say something like:

If it's so simple, can you give me a spring weight requirement for a direct mount shock, 25% of the way down....

You gave a problem with no solution and it's clear now you have no idea how to do the problem yourself

[Smifffy]

I'm looking forward to the complete suspension primer that P.Dath will post within 6 months of reading his book. Then we'll get the answer to this puzzle.

[BuzzardNZ]

Id beg to differ that the spring rate may be close. There are LOTS of other dynamic factors happening. The maths only gets you so far, the rest is real world testing. Especially with damping changes

Robert, It's too bad that you don't do the occasional tour around the country sorting out suspenders for those interested in getting their bikes setup by someone who really knows what they are doing. I don't have time to come up to New Plymouth to get mine done and I realize tools and other gear would be a pain to transport. I've had a well known bike shop in Wgtn try and set it up, but I've never been happy with the results.
I'm happy with everything about the SV , except the bloody suspension.

If you ever toured the country ( kinda like a traveling peddler/gypsy ) I'd buy your wares!

[Drew]

shock is 15 degrees from vertical, swing arm is 25 degrees from shock. so swing arm is (15 + 25 =) 40 degrees from vertical =50 degrees from horizontal.

The shock leans forward 15° from vertical. The swingarm tilts down 10° from horizontal. 25° angle between shock and swing arm.

So from the information you supplied.....and knowledge of 'optimum sag' from a suspension tuner

I'd guess 'optimum sag' is dependent on not just the swingarm geometry you've given, the tuner would probably want details like whether you intend to ride on sand dunes or drag strips or to the dairy and back...More information is required.

Most people know I'm a road racer, but I should have pointed out the shock is for a track bike.

Swingarm angle was given to indicate the rartes of change. The sag will be inside a certain small window almost always, untill track testing starts.

Either way, it's a highschool physics problem and doesn't really belong in this thread, I don't know anything about suspension, I just don't like it when people say something like:

It was an example, of how technical suspension is. A poor one perhaps, but it clearly illustrates that there is no non technical way to work on it.

You gave a problem with no solution and it's clear now you have no idea how to do the problem yourself

Of course I don't know holw to get the answer myself. Why the fuck do you think I asked? I can make small sety up changes on a bike after Robert sets it up, and usually get positive results. Building the shit, is WAY above my head!

[Drew]

Robert, It's too bad that you don't do the occasional tour around the country sorting out suspenders for those interested in getting their bikes setup by someone who really knows what they are doing. I don't have time to come up to New Plymouth to get mine done and I realize tools and other gear would be a pain to transport. I've had a well known bike shop in Wgtn try and set it up, but I've never been happy with the results.
I'm happy with everything about the SV , except the bloody suspension.

If you ever toured the country ( kinda like a traveling peddler/gypsy ) I'd buy your wares!

He's done at least one day in Wellington, doing exactly what you're talking about.

Think he's done a couple of 'workshops' too.

[Drew]

Id beg to differ that the spring rate may be close. There are LOTS of other dynamic factors happening. The maths only gets you so far, the rest is real world testing. Especially with damping changes

Gets the bike rolling though. That's the goal to start with, and a known set of variables to get that starting point will be invaluable when the rubber hits the tarmac, no?