Potential Energy and Spring Rate

Started by motomadness, September 02, 2004, 03:51:19 PM

0 Members and 1 Guest are viewing this topic.

Super Dave

This is fun.

More preload on a shock...

Straight rate springs still have a tiny bit of progressiveness to them...they are cut and worked at the ends so that they bottoms of the springs are flat.

Adding more preload will "push through" that little bit of progressiveness.

Some suspension guys will talk about how the actual coils can touch each other also.  That changes everything.  Basically, it will spring bind.

Super Dave

tzracer

Energy dissipation.

There is actually very little flow in a shock. The piston moves through the fluid which moves from one side of the piston to the other. If energy were dissipated into flow, the flow would have to continually increase.

Energy is dissipated as heat. As you start to ride, your shock heats up (my Husky 250CR had piggyback Ohlins, the reservoirs would get too hot to touch). It will eventualy reach an equilibrium temperature. Remember the rate of heat loss is related to temperature difference. As the temp of the shock increases, the rate of heat loss to the air increases. If the energy is not being dissipated as heat, where is it going?

The reason heat has the same units as energy is because heat is energy. What do you mean by KE and PE having length units? Remember, 1 Joule (J) = 1 N*m (Newton*meter), so 10 J  = 10 N*m. Heat dissipation works - think work and energy.

Ride height changing with preload is easy, as you add preload, the shock will compress less supporting the bike and rider, hence the bike will sit higher.

Free sag depends upon the bike, 125s and 250s run very little if any free sag. This is because the bike itself has very little weight, and when set up for the rider sag, the bike alone sags very little or nothing. My 250 generally had no free sag.

Two good books.

Motorcycle Handling and Chassis Design the art and science by Tony Foale (tonyfoale.com). Erik Buell even had a copy.

The Racing Motorcycle  A Technical Guide for Constructors volume 1
by John Bradley (I think I got my copy through Whitehorse Press)
Brian McLaughlin
http://www.redflagfund.org
Donate at http://www.donate.redflagfund.org
 
2 strokes smoke, 4 strokes choke

motomadness

Brian,

I think heat has a role in the energy equation, just not the main part.  You say there's not much flow, when there is not much movement either.  The smaller the reservoir, the more heat that's generated, but like you said the fluid will reach an equilibrium temperature, the damping variance reduces, and then the damping mechanism is mostly flow control.

motomadness

KE and PE have length units because PE = s.rate*displacement and KE = 0.5*mass*(time_derivative_of_displacement)^2.  I know these are Joules, but after an equilibrium temperature has been reached, changes in displacement define the damping effect.  

At this level, I think it's better to simplify the problem rather than wrap a complex operation such as heat dissipation into the equation.  Now the discussion should focus on hold a racer can control the movement of his/her suspension, not how they can control the heat generated from the movement.  Besides, none of us can do anything about the heat generated by making external adjustments.

You control movement by:
- adjust preloads (externally and internally)
- adjust fluid levels
- adjust fluid weights
- adjust spring rates and types
- adjust valve shim stacks
- adjust damping valve positions
- using of needle valves for rebound circuits
- changing fluids regularly
- changing damping catridges: 20mm to 25mm (more flow, less heat, more progressive damping)
- chassis geometry and linkages.

I'm sure there are more, but generally these probably represent the range of changes a racer will consider doing themselves, or have an expert perform for them.

tzracer

Spring potential energy = 1/2 * spring rate (k) * displacement (x)^2.

Heat, PE, KE are all different forms of energy, they all have the same units. PE and KE having length units does not make them fundamentally different than heat.

Heat is not really that important to the discussion of how to adjust suspension. Heat is the end result of energy dissipation, the energy absorbed by the suspension gets turned into heat by the friction of fluid flow. KE and PE in a damped oscillator system is not conserved.

What I suggest to racers is to learn what all the adjusters on their suspension do. Unfortunately it takes time, effort, and some money. I don't think there is a short cut.

Go to a practice day, a helper will come in very handy. Expect to spend the entire day learning about suspension, not trying to set a new track record. Take notes, don't try to rely on your memory.

Go out for a session and get down to a comfortable lap time, one you can easily do all day without having to push too hard. Now turn one of you suspension adjusters, such as shock compression damping, all the way up. Ride the bike a couple of laps. Stop on pitlane, write down what differences you felt. Turn the same adjuster all the way down. Ride the bike for a couple laps. Stop on pitlane, write down what differences you felt. Return the adjuster to the starting position. Repeat with all the suspension adjustments you care to learn about. this will give you first hand feel for what the suspension changes will do.

Sean, I am not quite sure in what direction you would like this discussion to go. What exactly are you after?
Brian McLaughlin
http://www.redflagfund.org
Donate at http://www.donate.redflagfund.org
 
2 strokes smoke, 4 strokes choke

motomadness

Brian,

I apologize for the arguement.  It's been a while since I have used these brain cells.  I see where you are coming from.  I think I was loosely using the idea of PE and KE to describe how energy is "conserved", or better transfer from potential energy of the spring to kinetic energy of the whole system in motion.  However, to describe the relationship of a mass/spring/damper system, I intended to really be talk about the equation of motion based on some forcing function, which really doesn't consider any heating effects.

Again this would be too complicated to discuss on this forum, but it helps to explain what the suspension is doing.

I also agree in your technique of trial and error at the track, however, before you go to the track it would be good if a rider knew a little more about what his/her suspension did.  That was my intention for this note.  To more or less convey the concept of suspension displacement and the rate of displacement, and that external and internal adjustments can be made to tune the response, but understand that once you've explored the range of potential responses of your suspension, other things can be done to change it further.  Kind of get people (newer riders) out of the mindset the suspension that Traxxion, Thermosman or Trackside installed for you is only the beginning.

I was using the idea of energy to create an image of rate of movement.  Do you think this was an appropriate comparison?


motomadness

Did some reading.

In The Racing Motorcycle: A technical guide for constructors,  Chapter 5, Section 6, pp329-330, it was clearly stated that the energy of the damper is dissipated by heat.  Furthermore they went on to discuss the type of dampers we have in our bikes.  Since we don't have friction dampers in our suspension components, the heat being generated is lower and excess energy is used to force oil through valves and orifices - flow control.

Thanks Brian and Zac for enlightening me.

Let's continue.  Care to start another thread of another interesting suspension subject?  If you don't I will pick something in a couple of days.

Super Dave

Heat...

Controling it can be an issue on how the shock works.

Specifically, I did testing of pre production shock warmers.

Post information that I got was that some shocks, to remain nameless, generated so much heat trying to operate in colder conditiions that they got out of the optimum range of temperature.  

When "pre heated" the shock had a more consistent feel and operated in the proper temperature range.

Butt feel, yeah, it was night and day...and, as a result, I use Hyperpro shocks as they don't exhibit the heat issue that the shock we were using had....especially in cooler conditions.
Super Dave


tzracer

QuoteThanks Brian and Zac for enlightening me.

Let's continue. ÊCare to start another thread of another interesting suspension subject? ÊIf you don't I will pick something in a couple of days.

No problem, it is a lot of fun.
Brian McLaughlin
http://www.redflagfund.org
Donate at http://www.donate.redflagfund.org
 
2 strokes smoke, 4 strokes choke