low and high speed damping

[mrspeedy]

Definately interesting stuff .....

I under stand the low speed damping thing, but what exactly is the difference between "pack" and high speed damping ? or is it the same thing ?

[elvo]

Bump stops are a no-no. The chassis has got to hit the ground before anything in the suspension starts to bind. More so, even, even with the chassis flat on the ground you need ~5mm of extra uptravel. It's easy to see why when looking at some of Jimmy's fabulous action shots: our cars roll hard and far, and tracks are not flat.
(The 1:1 dune buggy / sand rail / desert racer crew will disagree though: they'll consider it a sin if the chassis hits the ground before the shocks hit the rubber bump stops.)
Even similar things to bump stops, like increasing the spring and/or damper rates near full compression: doesn't really work. It's been tried a number of times, results have never been 100% satisfying. Often it's very nice on bumps and rough stuff, but there's always a downside when it comes to control, and 'centering up', like when lining up for jumps and flying through chicanes. It's similar to what happens if rebound and compression damping rates are too far apart: roll and pitch movements become asymmetrical and a bit weird.

This may be far out, but who says R/C cars need 'ideal' damping rates? R/C drivers' bums and spines are safely on the rostrum, they don't really care about jolts as long as the car stays in control.

[Apricot Slice]

Originally Posted by mrspeedy

Definately interesting stuff .....

I under stand the low speed damping thing, but what exactly is the difference between "pack" and high speed damping ? or is it the same thing ?

As the piston moves through the shock body the oil passes through holes in the piston. this gives a damping effect.
The effect is smooth and progressive for the most part.
When a certain speed is reached the flow characteristics of the oil passing through the holes change and a much higher rate of damping is the result.
This is called 'Pack'

High speed damping is damping specifically aimed at handling the effects of high speed suspension movement independently from low speed suspension movement.

(or at least that's my understanding of it. please correct me if i am wrong)

[baw888]

This is the thread I've been waiting for.

After working on numerous Mounain bike suspension systems and creating some great acting forks in the process. I though shim stack pistons in an RC car would be a breeze.
I did make some great looking pistons using Stainless steel shim material. placed above and below the piston. Covering some of the rebound holes and most of the compression holes.
After trial end error and months of rebuilding with various shims I came to the following conclusion:

It would stiffen the slow speed compression, so the car would behave like the oil was too stiff. Felt like the suspension was binding. Equals terrible handling on the bumps.
On jump landings if would let the oil flow, and the car would slap the ground.

It seemed like I had made a shock with the worst of both worlds.

So if we(you) are to ceate a damper of the next level. We need to establish what we need the shock to do, eg, what is the graph of piston speed/resistance of the ideal damper. If we can work this out, then we can engineer the rest.
Maybe we will come the the conclusion that the damper needs a linear resistance up to a point, then a higher resistance to help in landings, ie exactly what we already have.

I have had a different line of thought, Rock Shox in the Moutain bike world created a fork that basically disconnected the dampers from the wheel, and inbetween had a very stiff, short spring, that would smooth out the transition between compression and rebound. It was like riding on a very large balloon tire, But still retained the same levels of damping throughout the rest of the stroke.

Could we use this theory to let the wheel quickly react to bumps on the ground, and then be able to increase the overall stiffness of the shock.
In other words, stiffer suspension to stabalise the car and make it more drivable, but still allow the tyre to follow the ground and keep traction. Much like a Yokomo Bmax and Kyosho FS. (By using flex)

[The Pookster]

So based on Ghea's info that their measurements showed a progressive damper characteristic (getting stiffer, providing progressively more and more force) at high damper velocities this is essentially what 'pack' is.

When landing from a jump or on larger bumps the damper will be travelling at high velocities so pack in compression is useful. But because an RC damper is simple you'll also get 'pack' in rebound which is not desired as it unsettles the car on bumps or a rough track. I assume this rebound 'pack' is what the Ghea pistons take away.

I will be measuring AE, Ghea and Losi pistons on a full size cutting edge damper characterisation rig soon so will find out.

And to answer some other trial and error experiments that have not been successful to date, you are setting an impossible task unless you have the damper measurement data to know exactly how it's behaving.

[The Pookster]

Originally Posted by elvo

Bump stops are a no-no. The chassis has got to hit the ground before anything in the suspension starts to bind. More so, even, even with the chassis flat on the ground you need ~5mm of extra uptravel. It's easy to see why when looking at some of Jimmy's fabulous action shots: our cars roll hard and far, and tracks are not flat.
(The 1:1 dune buggy / sand rail / desert racer crew will disagree though: they'll consider it a sin if the chassis hits the ground before the shocks hit the rubber bump stops.)
Even similar things to bump stops, like increasing the spring and/or damper rates near full compression: doesn't really work. It's been tried a number of times, results have never been 100% satisfying. Often it's very nice on bumps and rough stuff, but there's always a downside when it comes to control, and 'centering up', like when lining up for jumps and flying through chicanes. It's similar to what happens if rebound and compression damping rates are too far apart: roll and pitch movements become asymmetrical and a bit weird.

This may be far out, but who says R/C cars need 'ideal' damping rates? R/C drivers' bums and spines are safely on the rostrum, they don't really care about jolts as long as the car stays in control.

This is a somewhat negative view of the possibilities. You need to understand what bumpstops could offer and how damper performance is not about ride it is about maximising the contact load between the tyre and track for more of the time and therefore going faster. F1 invests a lot in damper technologies for the same reason and I don't think they,re interested in smoothness of ride.

[baw888]

Does anyone make a data logger for shocks this small? If we put this on the track, then we might have an idea of what speeds various bumps are producing.

[Chris Doughty]

Originally Posted by elvo

Bump stops are a no-no. The chassis has got to hit the ground before anything in the suspension starts to bind. More so, even, even with the chassis flat on the ground you need ~5mm of extra uptravel. It's easy to see why when looking at some of Jimmy's fabulous action shots: our cars roll hard and far, and tracks are not flat.
(The 1:1 dune buggy / sand rail / desert racer crew will disagree though: they'll consider it a sin if the chassis hits the ground before the shocks hit the rubber bump stops.)
Even similar things to bump stops, like increasing the spring and/or damper rates near full compression: doesn't really work. It's been tried a number of times, results have never been 100% satisfying. Often it's very nice on bumps and rough stuff, but there's always a downside when it comes to control, and 'centering up', like when lining up for jumps and flying through chicanes. It's similar to what happens if rebound and compression damping rates are too far apart: roll and pitch movements become asymmetrical and a bit weird.

This may be far out, but who says R/C cars need 'ideal' damping rates? R/C drivers' bums and spines are safely on the rostrum, they don't really care about jolts as long as the car stays in control.

100% agree with this.

having our chassis scrape the floor on the face of the jumps is pretty much how our cars jump nicely on the big hit jumps. especially so in 8th scale, if you have one wheel hit its bump stop it will throw the car to one side.

in an ideal world without the laws of physics I'd like the following

low speed damping - slow (for body roll and cornering)
high speed damping - fast (to deal with stutter bumps)
(now here is the catch)
high speed damping - slow (to land the big jumps)

unless there is a way to differentiate between high speed 'bump' and high speed 'landing jump' then I will stick with what I've got and put up its limitations.

adding in a curve ball, can a cars suspension geometry help to overcome what we are lacking in our current shocks and a 'magic' set of shocks not work until the geometry changes to catch up with magic shock technology?

[Chris Doughty]

Originally Posted by The Pookster

This is a somewhat negative view of the possibilities. You need to understand what bumpstops could offer and how damper performance is not about ride it is about maximising the contact load between the tyre and track for more of the time and therefore going faster. F1 invests a lot in damper technologies for the same reason and I don't think they,re interested in smoothness of ride.

I don't think Elvo is being negative, I'd call is realistic, I know Elvo and I know that all of his opinions come from real-world test experiences, I've also experienced similar effects to what Elvo describes.

if you have ever driven a car where the suspension travel does not allow the wheel past the chassis when the chassis is flat on the floor you will know how garbage it really is, especiallyf if it happens to be the rear wheels!

[c0sie]

Originally Posted by Apricot Slice

the top speed of a humming bird is equal to that of a goose.

Best sentence ever!

[MrWolf808]

i dont think its a case of being right or being wrong, realistically we are not trying to stop the chassis from hitting the floor, considering the speeds and flight times especially at high speeds and not taking into consideration driver error (a high factor in my driving), we have to expect the chassis to hit floor, its just a case of making the shift from big landing to control again that matters. surely the same theory applies there the slower the transition at full compression at high speed the more effective the "landing" its the speed of the piston at that point that you need to control. surely its the engagement of a second piston or secondary spring, how about a smaller spring at the very top of the stroke internally on the damper.

[baw888]

Quote
low speed damping - slow (for body roll and cornering)
high speed damping - fast (to deal with stutter bumps)
(now here is the catch)
high speed damping - slow (to land the big jumps)

We can make this happen.
What about bump stops that move, or more like a seesaw with the bump stops on each end. If the car is flat then both bump stops engage and stop the car hitting the floor. (In turn leaving the car closer to its natural state ready for th next turn or whatever)
But when cornering, ie, when one arm is raised and the other in droop, the bump stops can move to allow this to happen.

[The Pookster]

Originally Posted by DoughtyUK.net

low speed damping - slow (for body roll and cornering)
high speed damping - fast (to deal with stutter bumps)
(now here is the catch)
high speed damping - slow (to land the big jumps)

Not sure what you mean?

Low speed is slow damper velocity, measured in metres per second say. Damper force is measured in Newtons.

When you say slow, fast, slow do you mean damper velocity or force?

[Chris Doughty]

I should have been clearer with my descriptions

low speed damping(body-roll) - equivilant of thick oil - slow reaction
high speed damping(stutter bumps) - equivilant of thin oil - fast reaction
high speed damping(jump landing) - equivilant of thick oil - slow reaction

[ghea]

This is a test of a r/c piston with 3 different high speed characteristics.
The one with the most linear curve seams to work best on the track and we believe we can make them even more linear soon.

I am not sure if landings cause the fastest shock speed though?
Maybe its the rough conditions on the fast parts of the track?

To use log equipment on car seams a bit complicated to me but i am sure
some of you will get it to work

I think it would be easier with a high speed camera and watch the cars movement in slow motion to see whats really happen and also measure shock speed.

/Ronny

[Chris Doughty]

not the best camera work, but take a look at this
http://www.facebook.com/#!/video/vid...5433783&ref=mf

FYI - my car was too soft in this clip

[mrspeedy]

Originally Posted by Apricot Slice

As the piston moves through the shock body the oil passes through holes in the piston. this gives a damping effect.
The effect is smooth and progressive for the most part.
When a certain speed is reached the flow characteristics of the oil passing through the holes change and a much higher rate of damping is the result.
This is called 'Pack'

High speed damping is damping specifically aimed at handling the effects of high speed suspension movement independently from low speed suspension movement.

(or at least that's my understanding of it. please correct me if i am wrong)

So hole size and "pack" is a way of increasing the higher speed damping rates then ?

I like things in simple terms

Just out of interest can anyone tell me why Associated pistons work differently to the Losi pistons ?

[Chris Doughty]

Originally Posted by mrspeedy

So hole size and "pack" is a way of increasing the higher speed damping rates then ?

I like things in simple terms

Just out of interest can anyone tell me why Associated pistons work differently to the Losi pistons ?

using thinner oil and smaller holes does make the flow of oil more turbulant at high speeds so yes.

light oil + small holes = more pack therefore more high speed damping rate
thick oil + large holes = less pack therefore less high speed damping rate

with the right combitnation of oil and piston both could feel very similar on the bench.

AE pistons are 2 holes, each hole is bigger than the Losi 3 hole pistons. Losi pistons also have rounded edge vs the square edged AE pistons, I'm sure this also has some effect, some oil will always get around the outside of the piston so with that in mind I'd say the Losi piston is more prone to pack

[The Pookster]

Lots of opinion on this thread, everyone's entitled to those.

Very little fact or data though.

The shock speed is not too hard to calculate. Landing from a large jump, i.e full rebound to full compression, will be about the fastest shock speed the car sees. Piston has to travel the full distance, time for this is in the region to 0.05 to 0.08 seconds by my observations and timings. As a sanity check this equates to the region of 3 to 5 Hz, which seems about right (I will be taking some measurements of my B4 to work out its natural suspension frequency soon).

On a B4 the rear shock travel is 26mm, so speed based on the above timings is 0.32 to 0.52 m/s.

If you look at the Ghea data the damper is already starting to be progresive at 0.5 m/s and the difference in the damper force, from highest to lowest, is 50N down to 40N. This is a 20% drop so a significant change and you would expect a real difference (not saying whether it is better or worse) on the car, especially as the change in force at the lower velocities is much smaller.

[MrWolf808]

a very good point but one thing i dont think we are taking into account is the force applied, you would have to have a universal spring rate to be able to calculate the force applied too the damper under each load setting. i am now trying too look up the formula to calculate the above.