Next dumb suspension question

[adouglas]

I like to ask at least one stupid question every day. It keeps me honest.

Now that we've covered preload in a theoretical sense, here's the next one:

Stiffer springs and less sag (i.e. higher preload) are often recommended for race/track bikes. Why?

The usual caveats apply... depends on rider preference, preload is not a substitute for the correct spring, preload does not stiffen the suspension, compression damping does play a role, etc.

I think that it's because at track speeds the suspension loading is higher, so you may need stiffer springs and higher preload (to maximize the amount of travel available for compression) to prevent bottoming.

The higher suspension loading comes from far greater cornering forces, plus (for the front) more weight transfer under braking. Suspension loading on the straights (prior to braking) and under acceleration aren't all that different from street pace.

Discusserate.

[alex.s]

more Gs

smoother road

[csmith12]

Originally Posted by alex.s

smoother road

I wish that was always true. lol Some tracks are pretty crappy.

[adouglas]

Well, there is this BRAND NEW, beautiful facility that's about to open up here...

(evil grin)

[Motofool]

Originally Posted by adouglas

I like to ask at least one stupid question every day. It keeps me honest.

Now that we've covered preload in a theoretical sense, here's the next one:

Stiffer springs and less sag (i.e. higher preload) are often recommended for race/track bikes. Why?............

Discusserate.

Let's examine what the racer needs from his suspension while rolling on a straight trajectory first:

Imagine that the greater mass of the bike and driver remain at the same level while the smaller masses of the swingarm-wheels-tires copy a regularly undulated pavement surface.

Those not only follow the shape of the surface but apply the necessary normal force that good traction demands:
Friction force (traction) = Coefficient of friction x Force normal to the surface

What does the work better for faster speeds, a soft spring or a hard spring?
Why do you think that way?

[alex.s]

this has me thinking about suspension setup and one detail in particular, where you start from. your explanation, hernan, made me think that you would want to start with maximum compression damping, and minimum rebound damping, then work your way into a correct setting with tuning... but i guess it's probably more sensible to just start right in the middle and go up or down from there based on the results you're seeing

[Motofool]

Originally Posted by alex.s

this has me thinking about suspension setup and one detail in particular, where you start from. your explanation, hernan, made me think that you would want to start with maximum compression damping, and minimum rebound damping, then work your way into a correct setting with tuning... but i guess it's probably more sensible to just start right in the middle and go up or down from there based on the results you're seeing

Experimentation is the only way because many factors are playing in this game of keeping those contact patches (which have mass and vertical accelerations) planted for as different as possible conditions of speed, accelerations and forces (both horizontal and vertical).

Take a look at this general concept:
http://en.wikipedia.org/wiki/Unsprung_mass

[Somchai]

This one reminds me of a saying:
Theory is when one knows everything and nothing works -
Practice is when everything works and nobody knows why.

For me it is so that I test it as is affects the bike or/and my riding, like with Preload, turn it off completely and then excessively back on and you feel what it's doing when you ride the bike with those setups...

[adouglas]

Originally Posted by Motofool

What does the work better for faster speeds, a soft spring or a hard spring?
Why do you think that way?

Insufficient specificity.

The answer to the question as posed is "soft/hard enough to keep the tire in contact with the ground, and the chassis as undisturbed as possible."

Too soft and the wheels will unload through inertia every time they hit a bump. I can also envision a scenario in which the bumps are severe enough and frequent enough for the wheels to not return to the road surface before the next bump.

Too hard and the chassis gets upset at a minimum. WAY too hard and the bike could launch off the bump.

[csmith12]

Originally Posted by adouglas

I can also envision a scenario in which the bumps are severe enough and frequent enough for the wheels to not return to the road surface before the next bump.

Actually, it doesn't have to be as severe as one might think. Telemetry of rear wheel vs front wheel speed proves this to be the case on what some riders consider relatively smooth surfaces.

[Worldtraveller]

Originally Posted by adouglas

Insufficient specificity.

The answer to the question as posed is "soft/hard enough to keep the tire in contact with the ground, and the chassis as undisturbed as possible."

Too soft and the wheels will unload through inertia every time they hit a bump. I can also envision a scenario in which the bumps are severe enough and frequent enough for the wheels to not return to the road surface before the next bump.

Too hard and the chassis gets upset at a minimum. WAY too hard and the bike could launch off the bump.

Yeah, and think about the difference in setup at the same track for different conditions (the most obvious being wet vs. dry). The bumps are the same, the straights are more or less the same, especially for the little bikes, but the setups for wet weather are almost universally much softer (both tires and suspension).

[Motofool]

Originally Posted by adouglas

.......... I can also envision a scenario in which the bumps are severe enough and frequent enough for the wheels to not return to the road surface before the next bump...........

Perfect!

Why?
What would be useful to counteract the effect of the crest or valley on the wheels: less or more force?, less or more damping?, less o more suspension's available travel?

Keeping everything equal, changing only the speed of the bike: what changes relative to those wheels?

Hints:
* Vertical acceleration = Force / Unsprung mass
* Spring force is proportional to vertical deformation
* Damping effect is proportional to vertical speed

[cbinker]

smoother and tackier surfaces at tracks typically. stiffer springs allow you to get the power to the ground. I am tired, this is all i got right now.

[Finesse]

ummmm too much math....... next question please

[adouglas]

Originally Posted by Motofool

Perfect!

Why?
What would be useful to counteract the effect of the crest or valley on the wheels: less or more force?, less or more damping?, less o more suspension's available travel?


Keeping everything equal, changing only the speed of the bike: what changes relative to those wheels?

If we change only the speed, then more energy will go into the suspension, trying to launch the bike over a bump. So more spring force to counteract that energy input and more compliance to keep the wheels planted.

But again, it depends... "more" and "less" are relative.

More force will absorb the greater energy of hitting a bump at a higher speed, and return the wheels to the ground faster.

Less damping will allow the wheels to move more easily in both directions... in the case of launching off a bump, less rebound damping will help because the suspension will extend more quickly.

More travel will accommodate bigger bumps and also allow for higher G loads due to cornering and braking.

BUT... TOO much force and/or damping and the chassis will be upset. Too little damping and the wheels could leave the ground at the crest of the bump and may bounce when they land again.

So it seems to me that simplifying it to "more" and "less" still leaves out some important information.

Same answer... enough to keep the wheels on the ground.

[adouglas]

Follow-up....

Higher preload must also play a role here, right? As speed (and therefore the energy going into the suspension) increases, I imagine that it would be desirable to require more energy to get the spring to start deforming. Preload does that, without making the spring stiffer.

Put another way, say you crank in a lot of preload, so much that hitting a bump at low speed would produce no result. Hitting the same bump at a higher speed would; more energy.

Am I nuts?

[cbinker]

preload!! - NOOO!!!

[Worldtraveller]

Originally Posted by adouglas

Follow-up....

Higher preload must also play a role here, right? As speed (and therefore the energy going into the suspension) increases, I imagine that it would be desirable to require more energy to get the spring to start deforming. Preload does that, without making the spring stiffer.

Put another way, say you crank in a lot of preload, so much that hitting a bump at low speed would produce no result. Hitting the same bump at a higher speed would; more energy.

Am I nuts?

From a physics standpoint, you're not nuts, but the terms you are using are slightly incorrect.

From a rider's perspective, you've got it about right, though.

There is always going to be a balance between pre-load and damping. I've read lots of articles, and I understand the physics, so I get what the descriptions are telling me are happening. But I've only been to a couple of suspension seminars (Dave Moss is da man!), and it still seems like voodoo at the bike level.

I know I've finally got to the point where I'm not the limiting factor so much anymore, and I've wrung almost as much out of my 250 race bike as I can (I'm only a few seconds off the leaders now, and keeping pace with the fast guys on stock bikes). I also know that I'm starting to feel the bike a lot more, it's less settled in the corners, when it's really going hard on the braking, and when the rear is starting to slide, so I'm at the point where I really need to start looking at doing something about the suspension for next season.

[cuong-nutz]

You're nuts. But not Cuong-Nutz.

You want to start off with the correct spring rate. More preload decreases the fork travel as the spring bottoms out sooner. How well suspension follows the ground goes into the dynamics of compression and rebound.

Spring rate is constant and is based on weight. How fast or slow the spring compresses is controlled by the dampening and compression controls.

All I got since my pho e is about to die.

[cuong-nutz]

Racing or at speed, the bike stays at plane and becomes stationary due to higher forces or Gs seen. Slower speeds and hitting a bump, the bump is abrupt because the energy? Is greater than the speed/force overcoming the bump because the suspension (spring rate,comp, rebound) is tuned for speed (more Gs)

This phone is too slow.

[alex.s]

depends on the valving. if its a two speed valve or a simple valve. two speed valves (like emulators) will flow more when the force reaches a certain level. this means you can set the normal damping higher but it reacts well to hitting bumps.

lets not forget about how tire pressure plays into all this because at the end of the day, its a spring and shock sitting ontop of basically a balloon of air. that balloon of air plays a big role in how the shock and spring act.

[JeffroJ]

Go from WOT in 6th gear to braking as hard as you can while throwing the bike into a hairpin turn, trail braking to the apex, then back to full throttle til the next turn.

Imagine this with soft springs, the suspension would be all over the place and most likely put you on your butt. Stiffer springs equals less dive, getting the chassis settled quicker giving you control of the bike through the turn.

[Motofool]

Originally Posted by adouglas

If we change only the speed, then more energy will go into the suspension ...........

So it seems to me that simplifying it to "more" and "less" still leaves out some important information.

Same answer... enough to keep the wheels on the ground.

Exactly; and energy is work and work is the product of force times vertical distance.

Same bump at higher speed leaves us with the same distance; then force is the only thing that can change by increasing.
That is a reactive force that grows more as the coil compresses.

Let's say that the reactive force for slower speed was 10 units and 1 unit of deformation of the coil.
Let's say that the reactive force for faster speed is 20; then, we have 2 units of deformation of the coil.

That would be non-transcendental if we had unlimited real state regarding deformation of the coil, but we don't, due to geometry, steering, chain, stability, balance, etc. associated to sport bikes.

The only way to reduce those 2 units of deformation is, as @cuong-nutz very well put above, by changing the rate of the spring.
A spring that develops higher reactive forces (20 units) with less deformation (1 unit).



This is so complex that we have to make some things fixed or unrealistically extreme in order to get it.
The bike has inertia and limited mass, the heavier the wheels and tires respect to the bike, the more their inertia will resist the up and down movements and the more these changes will upset stability and traction.

The spring is very willing to get rid of the energy transferred by the combination of bump and speed of the bike; the problem is that it overshoots.
When the spring pushes back to the original point to which the suspension should peacefully return, that energy becomes mass (of wheel+tire) times maximum vertical speed.

That is what damping is for: to reduce the natural oscillations created by a simple spring to one.
It does it by converting that energy that you correctly mentioned into heat, via viscous resistance to movement.

For that reason, a higher rate spring needs more damping.
We could keep the same damping (for slower speeds) if we could drastically reduce the unsprung mass (of wheel+tire).



But damping becomes a problem itself, because the faster the spring tries to move, the resistance of the damper increases by the square.

Then, we have a straight line in the graph posted by @csmith12 (linear reaction of the spring) trying to coordinate efforts with a parabola (quadratic reaction of the damper).

Here is where the multi-valves explained by @alex.s come to help.
For high speeds of the spring, they automatically and gradually reduce viscous resistance.



Returning to preload: that is always a terrible thing.
The reason is that we are pre-deforming one end of a spring that has limited range.
Therefore, we are reducing the range of movement of the spring getting nothing in return, dynamically speaking.

Pre-load is the brutal way that we have to prepare an initially incorrect suspension to be at the middle of its reduced range(same up and down displacement) for maximum load and lean angle.
While turning at 45 degrees of lean angle, your springs feel 1.4 times the static weight of bike+rider while fight crests and valleys of the track that are 1.4 bigger.
That is why we can afford adjusting that middle point of the suspension or static sag at the top 25~33% of the full range.

Some pre-load is necessary, however, in order to overcome damping at full extension.
Zero pre-deformation means insufficient reactive force to fully extend the hydraulic damper quickly enough to have full range for the next bump.

[alex.s]

[adouglas]

Originally Posted by Motofool

That would be non-transcendental if we had unlimited real estate regarding deformation of the coil

T-shirt.

[Somchai]

The answer is:

[Motofool]

Originally Posted by adouglas

T-shirt.

Link to original page on YouTube.

[Rifleman]

glad I read this post... and kept my mouth shut.

The only part that I can see that wasn't touched on was the weight shift during braking and acceleration.

It would be nice (but impossible) to keep the center of grav in the same location during braking and acceleration, the suspension and tires would work as designed...

but the bike geometry changes, thus handling changes...

stiffer suspension limits this change as well...

the more important points have been touched on ( and a number of things that were simply ? in my book were answered)

well done gents... a round of apple sauce for ya'll