Optimum motion ratio

[Chuckster]

I'm thinking that the popular guideline of "try for 1:1 since it gets the shock moving fluid" is counter to contact patch compliance since it magnifies stiction.

I know of one successful current FF1600/F-1000 design that changed from 1:1 to 0.85 (shock displacement/tire displacement). The designer said he could not get proper shock tuning at 1:1.

Ideas?

[Drew Price]

Isn't the sensitivity of the damper to motion ratio dependent on the internal geometry of each individual damper??

Isn't the approach behind the Cane Creek and Ohlins double barrels to help resolution at low shock piston speed/displacements?

Lastly, how would you get the requisite 2" suspension travel with a motion ratio of 1:1 with the MTB dampers most teams use, which barely have 2" travel themselves, while staying away from the far side of the compression stroke when the internal spring rate of the nitrogen compressing changes the characteristics?

My motion ratios are around 0.73:1.

Best,
Drew

[flavorPacket]

I use motion ratios higher than 1:1. And we probably use about 1/2" of travel in bump, .4" in rebound.

[Chuckster]

Thanks guys-First I reckon I should have qualified better what is magnified (by large displacements of the shock vs tire travel).

It is shock seal friction, rod end friction, and bellcrank bearing friction. Those are all independedent of hydraulic action.

[jsmooz]

Drew you're kinda onto it.....The main thing to keep in mind with motion ratios is how the shock actually works. As the shock moves it's create a change in pressure between the area above the piston and an area below the piston. The key is to have a pressure that you can control. If you pressure is really high, ie low motion ratio, your blow offs or stages of damping become harder to control and you get higher damper force variation. If the pressure is lower you can fine tune the stages and better setup the car for certain driving conditions.

The gas charge helps to counter-act the fluid volume due to rod displacement, so there is no caviation and subsequent massive change in damper force characteristics.

With that the ratio it's always a balancing act between what you can buy, since most teams don't make there own dampers, and what you can package.

[Drew Price]

Jim,

Just from playing around with (but not having disassembled) a Fox or a Risse damper is feels like at the extreme end of compression travel the spring rate (resulting from the nitrogen charge) of the shock goes up sharply, and I am hypothesizing either:

a) The volume of the piston going into the oil chamber is enough to compress the N2 to the point that is provides (significant) bottoming resistance, or...

b) There is a snubber or bump protect spring internal to the shock which keeps the user from bottoming the piston against the back of the housing during "Wicked Sick Jumps, dude!" conditions to prevent damaging the shock.

In either case, unless we count on this internal snubbing to keep the shock from bottoming, we want to avoid this region of travel (and I should think what would stop the travel of the shock on a MTB would not stop an FSAE car from bottoming out).

I'm also confused that if this is in fact the case, why the shocks come with rubber snubbers on the outside of the piston to push against the free spring perch.

Comments from anyone who has taken one apart?

Best,
Drew

[B Hise]

dont rely on gas pressure for support. I wouldn't really use gas pressure for anything other than a little preload/ride-ht adjustment. That being said, you can calculate the shocks spring rate if you get the divider piston diameter from the manufacturer.

The rubber "snubbers" also called bump stops, give you a nice semi-soft surface to bottom on instead of coil biding the spring or bottoming the shock out. You can supplement these with "packers" or little plastic washers to adjust what level of compression the bump stop comes in.

Bump stops and packers work well and you should put them on the outside of the shock so you can tune the rate and loaded height without doing alot of rebuilds.

[Brian Evans]

I've never noticed an internal spring to do anything when the shock tops out in bump. I've used the shock to do droop limiting, both outside of normal travel so the springs don't rattle and as a zero-droop implementation. Bump rubbers are there for suspension tuning. If the bump rubber on your shock isn't the correct rate to work on your car, then you could change it.

Not saying anything about good or bad, just that these things are possible...

Brian

[Brian Evans]

Sorry, should have said tops out in droop, not bump.

[B Hise]

noticed an internal spring to do anything when the shock tops out in bump. I've used the shock to do droop limiting, both outside of normal travel so the springs don't rattle and as a zero-droop implementation. Bump rubbers are there for suspension tuning. If the bump rubber on your shock isn't the correct rate to work on your car, then you could change it.

i have seen bumps stops used on the inside of a damper as a "soft" droop limiter. I don't have much experience tuning with them however.

Droop limit is a good tuning tool and an easy way around some of this "understeer" problem people tend to have.

[BenB]

I would think your motion ratio should be based on the amount of wheel travel that you want. Say you have +/-1" of suspension travel and your damper has =/-2" of travel then you would want a motion ratio of 2:1.

You should use the full range of travel of your damper so you move as much fluid as possible. The reason for this is that by moving more fluid you can use larger valves on the damper because you have more fluid doing the damping work. This lowers the pressure you need to run the damper at which is better for consistency, wear and hysteresis.

[jsmooz]

Drew - The gas pressure has little to do with that end of stroke stiffening. Are you watching to see if you're hitting the compression bumper? Usually there is a compression bumper on the rod, external of the shock, like you've described. There shouldn't be anything internal above the piston to cushion topping out, at least not normally, because the force would then go through the piston, which is just a bad idea.

I've never played much with the MTB shocks so I don't know how there setup internally. If they're using a bladder to separate the gas and oil you might be compressing the bladder some which is causing the increase, but that shouldn't happen.

As Brian has done, rebound bumper are common to the internals. The impact from rebound is waaaay less then compression bottoming.

Another thing not mentioned yet about motion ratio is that you want enough lenght to allow "constant" rod speeds. I say this meaning that you want the piston to be moving over a large enough distance so that it always isn't accel or decel, as it would in small MRs for the same wheel travel. Steady state conditions are much easier to tune and understand. Yes I understand the "constant" isn't going to happen because the vechile rolls and hits bumps at various rates and durations, but I'm just trying make it a little simpler.

[Kyle Roggenkamp]

Originally posted by jsmooz:
Drew - The gas pressure has little to do with that end of stroke stiffening. Are you watching to see if you're hitting the compression bumper? Usually there is a compression bumper on the rod, external of the shock, like you've described. There shouldn't be anything internal above the piston to cushion topping out, at least not normally, because the force would then go through the piston, which is just a bad idea.

I've never played much with the MTB shocks so I don't know how there setup internally. If they're using a bladder to separate the gas and oil you might be compressing the bladder some which is causing the increase, but that shouldn't happen.

As Brian has done, rebound bumper are common to the internals. The impact from rebound is waaaay less then compression bottoming.

Another thing not mentioned yet about motion ratio is that you want enough lenght to allow "constant" rod speeds. I say this meaning that you want the piston to be moving over a large enough distance so that it always isn't accel or decel, as it would in small MRs for the same wheel travel. Steady state conditions are much easier to tune and understand. Yes I understand the "constant" isn't going to happen because the vechile rolls and hits bumps at various rates and durations, but I'm just trying make it a little simpler.

The gas pressure will increase the end of stroke dampening slightly in a properly set up shock. A good way to see this is to run a shock on a dyno with pressure in it, and run it without pressure in it. That's also the best way I've found to tune a shock with a dyno-remove the pressure and run it. That way you don't have any dynamic dampening forces acting on the shock. This helps you understand what exactly is going on with your valving, especially with progressive valve stacks. You do tend to run into cavitation issues if you run it fast with no pressure though.

If it gets drastically stiff towards the end of it's travel, your IFP is probably not set correctly and you are compressing the nitrogen too much. If it's too far off, you won't even be able to compress the shock the whole way- the rod will bend before you bottom the shock.

[Kyle Roggenkamp]

Related question to the topic:

It's easier to tune the valving of the shock if it is moving more fluid as it provides more "resolution", but has anyone measured the temperature of the shock when using a higher ratio, say 1:1 rather than .75:1? I wonder if the more damper movement creates more heat, bringing the fluid closer to fading. I really don't think it is one of our major concerns in a competition like this but I've never measured temperatures on a shock for FSAE use.

[HenningO]

Kyle, at first I agreed with your statement, but after I thought about if for a second I realized it's false.

Think about it, with a higher ratio you won't need as high damping forces to achieve your desired relative damping at the wheel (which is what we actually care about). Or in other terms, you still have the same amount of "mechancial" energy that you have convert into heat.

Only reason I can see why you would see higher temperatures is because you'd have more internal frictional losses (seal drag).

[Drew Price]

I am not coming anywhere near the compression bump stops. I should have been more clear about the specific application though, and what I mean about the shock stiffening up at full compression.

The Risse Jupiter 3's we use are an emulsion nitrogen charged damper (the left one on the drawing, with the middle being 'Nitrogen Charged', and the right having an external reservoir), where the nitrogen is not separated from the oil by a piston, or diaphragm.
http://www.calsci.com/motorcycleinfo/Images/SHOCKS5.gif


The emulsion construction eliminates the nitrogen/oil sealing interface, making it simpler, BUT it limits the installed angle to be closer to vertical. If the shock inverts during travel the nitrogen gets pulled through the valves, and makes an audible noise, and the oil gets gas whipped into it. My J3's have to be mounted schraeder side up, and not invert. They are around 15* from the horizontal.

On the matter of compression spring rate increasing, it will be entirely dependent on how much air space there is for the nitrogen, and at what pressure it is at.

I have a feeling that there is too much pressure in my shocks, possibly coupled with not sufficient volume of N2, which would make the internal spring rate from the nitrogen compressing (from the decrease in volume of the oil chamber as the shock piston displaces the oil) increase pretty quickly. Less N2 pressure and/or more N2 volume would help decrease this effect, BUT less N2 pressure lowers the effective boiling point of the oil, which does us no good.

The Vanilla RC I played with behaved very similarly, even though it had a built-in reservoir, and I imagine that this effect is a result of the small oil chamber volume (compared to a automotive application) compared with a fairly robust piston. That is part of the reason small(ish) dampers come with reservoirs, it gives a larger volume of N2, so you don't get into the non-linear region as soon, and so that the N2/oil interface seal can be made separately, simplifying the design/construction/assembly/whatever of the main tube.

On a bike, the additional stiffening can be a good thing, as seen in the new 'Bottom-Out-Resistance' features on the current MTB shocks.

Anyways, all I was saying was that in MY experience, the shocks go non-linear and stiffen up at the extreme compression end of the travel (even before they hit the external rubber bump stops). I will be checking the pressure in my shocks.

Best,
Drew

[Kyle Roggenkamp]

^^Yeah, that's a good point. The shock is doing exactly the same thing regardless of the motion ratio used, and the same amount of heat should be generated whether a larger valve with higher flow is used or a more restrictive valve with less flow is used. Friction from the seals is relatively small, especially in a quality shock.

It would still be interesting to see any real world results to see just how warm these shocks are getting.

Didn't mean to jack the thread...

[Kyle Roggenkamp]

Originally posted by Drew Price:
I am not coming anywhere near the compression bump stops. I should have been more clear about the specific application though, and what I mean about the shock stiffening up at full compression.

The Risse Jupiter 3's we use are an emulsion nitrogen charged damper (the left one on the drawing, with the middle being 'Nitrogen Charged', and the right having an external reservoir), where the nitrogen is not separated from the oil by a piston, or diaphragm.
http://www.calsci.com/motorcycleinfo/Images/SHOCKS5.gif


The emulsion construction eliminates the nitrogen/oil sealing interface, making it simpler, BUT it limits the installed angle to be closer to vertical. If the shock inverts during travel the nitrogen gets pulled through the valves, and makes an audible noise, and the oil gets gas whipped into it. My J3's have to be mounted schraeder side up, and not invert. They are around 15* from the horizontal.

On the matter of compression spring rate increasing, it will be entirely dependent on how much air space there is for the nitrogen, and at what pressure it is at.

I have a feeling that there is too much pressure in my shocks, possibly coupled with not sufficient volume of N2, which would make the internal spring rate from the nitrogen compressing (from the decrease in volume of the oil chamber as the shock piston displaces the oil) increase pretty quickly. Less N2 pressure and/or more N2 volume would help decrease this effect, BUT less N2 pressure lowers the effective boiling point of the oil, which does us no good.

The Vanilla RC I played with behaved very similarly, even though it had a built-in reservoir, and I imagine that this effect is a result of the small oil chamber volume (compared to a automotive application) compared with a fairly robust piston. That is part of the reason small(ish) dampers come with reservoirs, it gives a larger volume of N2, so you don't get into the non-linear region as soon, and so that the N2/oil interface seal can be made separately, simplifying the design/construction/assembly/whatever of the main tube.

On a bike, the additional stiffening can be a good thing, as seen in the new 'Bottom-Out-Resistance' features on the current MTB shocks.

Anyways, all I was saying was that in MY experience, the shocks go non-linear and stiffen up at the extreme compression end of the travel (even before they hit the external rubber bump stops). I will be checking the pressure in my shocks.

Best,
Drew

I've never been a fan of emulsion shocks. They are more simple, but your typical IFP shock is nothing complex either, and with IFP's you don't have to worry about mounting positions. How much pressure are you running in your shock? I don't know a lot about the temperatures that these shocks reach, but I would be very surprised if ou were approaching temperatures near boiling points. What suspension fluid are you using?

[Drew Price]

I have not had a chance to check the pressure that Risse charged them to, more important things going on right now, and I will save most of that till when we start tuning, but I can almost guarantee it's too much.

I too would be very curious to see what kind of temps we are seeing, but as far as boiling the fluid, doesn't a large part of aerating the fluid come from 'cavitation,' (it's not really cavitation, but I don't have a better word on hand) on the trailing low pressure side. If there is insufficient pressure from the nitrogen charge, the pressure can be low enough to 'boil' the fluid, even at room temp, and aerate the oil by agitation, them damping goes to S***.

Kyle, I don't know what the fluid is that Risse uses, will have to find out though.

Best,
Drew

[Kyle Roggenkamp]

I guess I've never thought that the pressure would drop that low on the back side to boil. Maybe if the shock is at atmospheric pressure- I just don't know.

I don't know what everyone else uses for fluids, but I've always seen really good results with Torco and also Amsoil.

[Chuckster]

I originally asked the question that started this thread and while boiling of fluid is certainly interesting.....

-it isn't the gist of what I was after. With this low track speed regime we are in, doesn't anything that slows the actual shock velocity down help in that difficult to valve 0 to 1 inch/sec shock velocity range?


In other words, if the wheel velocity over bumps didn't translate to as high a rate at the shock-would not that be an advantage to achieving an underdamped system involving the shock ?

Ignoring range of wheel or shock travel and shock heating-is there any system or shock valving reason to go the other way and make the shock travel as fast or faster than the wheel velocity ?

[B Hise]

any ISO 0 or ISO 1 hydraulic fluid will work just fine. Spectro fork oil works well and is easy to get from most bike shops and is priced reasonably. You can buy fluid from Ohlins, Penske, JRZ etc but you're just going to pay alot of money for a label.

[Kyle Roggenkamp]

Originally posted by Chuckster:
I originally asked the question that started this thread and while boiling of fluid is certainly interesting.....

-it isn't the gist of what I was after. With this low track speed regime we are in, doesn't anything that slows the actual shock velocity down help in that difficult to valve 0 to 1 inch/sec shock velocity range?


In other words, if the wheel velocity over bumps didn't translate to as high a rate at the shock-would not that be an advantage to achieving an underdamped system involving the shock ?

Ignoring range of wheel or shock travel and shock heating-is there any system or shock valving reason to go the other way and make the shock travel as fast or faster than the wheel velocity ?

Sorry for veering off topic back there...

In my own experience, it is easier to fine-tune a shock with higher piston velocities, as the shock seems to respond to valving changes much better with more fluid flowing through it.

A good book you might want to pick up is The Shock Absorber Handbook by John Dixon. There is an entire section on Motion ratio's and rocker design.