Coil-Over tuning, springs and valving

[Provience]

you have to decide if comfort or speed matters. from there you spend a lot of time chasing the other side of the coin.

yeah unsprung weight = slower in every situation excepting only the moment that you go past COG and roll over, so certainly speed is the first and primary victim. once i'm actually together i'll chime back in with real world numbers and setting it up for itself.

 

[DesertCJ]

i want to introduce the least amount of spring rate. you have enough to control with just the terrain, no need to increase the complexity with a bunch of spring rate.

This right here explains why they would be leaning towards a mostly single rate setup.

 

[Bebop]

i want to introduce the least amount of spring rate. you have enough to control with just the terrain, no need to increase the complexity with a bunch of spring rate.

That's where I'm lost. I understand what you're saying, but since all solutions shown on the graph have a similar combined rate and a similar full bump force, I don't see what the impact will be on the tuning/behavior.

 

[Bebop]

I'm not the gospel by any stretch of the imagination. But it's to prevent harshness at the cross over from a huge step up in spring rates. It's not a rule, it's a guideline One guy's "Harsh" is nice for another guy because he isn't bottoming and can handle a few bumps.

Edit: In my case, I don't even feel the cross over between 94lb and 250lb.....BUT the huge step up seems to be causing rebound problems. It's not even really a problem. I can go about as fast as I care to or should be going safely. The bucking and stepping out of the rear is happening on the biggest whoops I was driving at like 60mph. It could be better...

Thanks, I feel that this helps me understand better.

I think a factor would be when at that transition point (especially if it’s near ride height or there when weight transfer happens on a climb). If the jump is larger, there can be a tendency to bounce between the rates. Compress > Rate Ramps High > Pushes back past the transition > Rate Drops A Lot > Falls Back across the Transition. Combine this with setting valving, a larger rate transition could be more likely to fall out of balance with the valving.

Ahhhhhhh !

Making progress now. I don't think I've ever felt that, but I totally understand what you're saying.

 

[DesertCJ]

Your yellow graph line is way off. Who runs with 10" of 200lb spring rate? And it still has the lowest total force

 

[Bebop]

Your yellow graph line is way off. Who runs with 10" of 200lb spring rate? And it still has the lowest total force

Why would it be wrong?

The difference in total force at full bump between the yellow and brown line is less than 10%. Not very huge of a difference.

 

[DesertCJ]

Why would it be wrong?

The difference in total force at full bump between the yellow and brown line is less than 10%. Not very huge of a difference.

Because I doubt the dual rate stop would even go that low on the shock. You'd be sitting on the 200lb lower with 10" of shock shaft showing at ride height on a 16" shock.

 

[mobil1syn]

This right here explains why they would be leaning towards a mostly single rate setup.

but they are giving up the reduced spring rate in the ride zone

 

[DesertCJ]

but they are giving up the reduced spring rate in the ride zone

Preach brother!

 

[mobil1syn]

so let me make something clear before we get too carried here.

- trail rig setup is a very different animal compared to an u4
- the fast u4 guys tune is significantly different than the mid pack guys, which is different than the participants aka trail rig guys
- every driver has a preference, in my experience shannon never wants to bottom out, so he is willing to put up with a miserable ride. while others will trade comfort for checking up occasionally.

the formula for a trail SA has been proven. if you want to look at the fast u4 guys, great but know that formula is for going FAST and does not translate well.

 

[Provience]

but they are giving up the reduced spring rate in the ride zone

I was talking yesterday with a guy looking to go to a coilover on his daily truck and that's one of the points I brought up as his chief complaint is overly stiff ride for normal driving. keeping 3 or 4" of crossover makes a big difference

edit: and he had somebody trying to upsell him to bypass shocks as well another thing that just isn't needed for somebody who isn't bombing around the desert regularly.

 

[Provience]

for me, for a primarily trail rig axle mounted 14" shock, my goal is 7" of shaft at ride height, 4" before the slider stop, not great difference between the springs, no sway bar. copy/paste from my build thread. whenever it rolls, tune from this baseline. i'll be able to get real corner weights once it's all done. easy enough to use the springs to measure what the sprung weight is, then use my crane mounted scale to lift the tire, subtract the difference, and have my unsprung weights per corner

Rear axle weight with links and small tires: 482 lbs
Front axle with some stuff: 294 lbs
Tires and wheels: 161lbs ea


puts me around 1420 lbs unsprung, leaves me about 2980 lbs for sprung weight. figure prolly 60/40 weight split would be 1788 front and 1192 rear.

calcumates to 87 lb/in for the rear so 150/200
and 131 lb/in for the front so 250/300

i need to order springs, and that is rather convinient because i'll have pretty much every combination i'd need to dial in corners when all said and done that way. right or wrong, i should only need to swap springs one time if i'm guessing wrong when it is all done

edit: after talking to 4wheelunderground briefly through emails, he thinks those numbers are a bit high, especially in the front. i've reproposed 150/200 (86 lb/in) for the front and 125/175 (73 lb/in) for the rear. changed my weight bias guess to 55/45 and took an average using 10" and 9" compression for a 'goal rate' of 87.25 in the front and 71.25 rear.

math follows 3000lbs * 0.55 = 1650 lbs/2 = 825lbs sprung per corner front
3000lbs * 0.45 = 1350lbs/2 = 675lbs sprung per corner rear

825lbs / 10" = 82.5 lbs/in for 3" preload and 7" shaft exposed, centered 14" shock
675lbs / 10" = 67.5 lbs/in """"

825/9 = 92 lbs/in for 2" preload
675/9 = 75 lbs/in ""

(82.5+92)/2 = 87.25 for about 2.5" preload front
(67.5+75)/2 = 71.25 ""

 

[DesertCJ]

Why do you need to know your unsprung corner weight? I understand wanting to know but? I'm not the best with excel but here are some weak graphs showing a straight 100lb rate 16" travel vs a 94lb combined going to 175lb at 11" on the left and to show the difference between going to 175lb at 6" of compression (I don't know why you'd want to) vs at 11". You can see the total force is about 500lbs different. Hopefully that shows why the graph jumping to the higher rate at 6" is off IMO? That's way too soon to be jumping to the higher rate.

BTW, great discussion guys. Exactly what I was hoping for!

 

[Bebop]

What is interesting IMO is comparing different set of springs with the same combined spring rate and understanding why you'd want one pair over the other ones.

For our 94ish combined spring rate, we could use:

125/350 --> 92 lbs/in (in blue)
150/250 --> 94 lbs/in (in brown)
175/200 --> 93 lbs/in (in green)

Which option do you choose, and why?
How does the slide stop position impact this choice?

We are talking about a SA car with 1:1 motion ratio.

I agree that putting the slide stop at 6" was a dumb idea on the previous graph (yellow line). I put it that for the sake of argumentation.
In this new graph I have put the slide stop at 10", 12" and 14" on each combination.
I didn't show 1" to 8' since the behaviors are the same.

 

[DesertCJ]

Green line, because it bucks at speed in the whoops the other ways

Seriously though, because with the heavier lowers you need more rebound valving to control the high spring force which could then start packing in theory because it's not letting the shock extend as fast for the next bump. You want as much of that shock travel as possible to slow the next hit with compression valving.

 

[Bebop]

Green line, because it bucks at speed in the whoops the other ways

hahahahahaha

Seriously though, because with the heavier lowers you need more rebound valving to control the high spring force which could then start packing in theory because it's not letting the shock extend as fast for the next bump. You want as much of that shock travel as possible to slow the next hit with compression valving.

That's where I disagree.

If you look at the blue with slide stop at 14" and the green line with slide stop at 12" they will produce the same amount of force at full bump, which in turn should require the same amount of rebound to control.
Now if you compare the same lines at, let's say the 13" mark, the blue line is under the green one, requiring less compression or rebound to control because less forces are involved.

 

[HYDRODYNAMIC]

I am a total newb in shock tuning, but here is my understanding and reasoning.
75% rule is for people using the dual rate stop on a solid axle car. The spring rate change from dual to single will still be manageable for the rebound dampening. If the upper spring is too light it will not have enough force to kick out the axle back to the ground because the rebound is set too heavy. If the rebound is set too light for the light upper spring then when you G out and fully compress the lower spring it will be shooting out too fast and be topping out too often. Both dual and single rate should be manageable for the rebound valving.

The 1-2" preload for front and 2-3" preload for rear axle mounted comes from having enough down force in the suspension to put weight on the tire to actually do some good as well as have force to kick the axle out back to the ground when going faster. If you have a lighter spring rate then you will probably need more preload as you can compress it much easier and the force change is not as much. If you have a heavier rate on trailing arms then you will be closer to 1" of preload as the motion ratio magnifies what is happening at the axle.

 

[Provience]

Why do you need to know your unsprung corner weight? I understand wanting to know but?

1) i'd like to know for the sake of knowing, the shock needs to be tuned to the car, so it doesn't "matter" in that sense.

2) if all the weight is on the top vs half being on the bottom, the shock still has to be able to handle all that force. it's easy to see the sprung weight and how it acts, but you need to be able to control the extra force from the unsprung weight getting shoved up as well. if you were to "tune" the exact same shock setup for two cars with the same corner sprung weight, and one was IFS (light) and the other SA (heavy) the car will perform very differently due to the mass/force of the unsprung getting slammed around. i.e. it will need more damping to slow it coming up and need more (or less because gravity ) damping so that the spring doesn't launch it down uncontrolled, in my head anyways

 

[DesertCJ]

You could never really have the transition at 13" or 14" because the upper coil would probably bind before collapsing to 2" or even 3". You're technically correct though. You'd have to look at the block height of the upper spring to see. What I also suspect is that the steeper ramp rate of the heavier spring will produce higher shaft velocity than the lighter springs despite the lower overall force requiring just as much if not more rebound valving to control. That's out of my league, I'm not an engineer but I do work for one...

 

[Provience]

What is interesting IMO is comparing different set of springs with the same combined spring rate and understanding why you'd want one pair over the other ones.

For our 94ish combined spring rate, we could use:

125/350 --> 92 lbs/in (in blue)
150/250 --> 94 lbs/in (in brown)
175/200 --> 93 lbs/in (in green)

Which option do you choose, and why?
How does the slide stop position impact this choice?

We are talking about a SA car with 1:1 motion ratio.

I agree that putting the slide stop at 6" was a dumb idea on the previous graph (yellow line). I put it that for the sake of argumentation.
In this new graph I have put the slide stop at 10", 12" and 14" on each combination.
I didn't show 1" to 8' since the behaviors are the same.

i would say green line with a "single zone" shock because then you can set the valving for the combined and be closer to the single rate, rather than having a larger jump in the spring rate. for a multi-bypass shock, you would be able to set the shock at a different damp level based on travel and would be able to use the larger jump in spring rate and still control it in similar fashion.

 

[DesertCJ]

1) i'd like to know for the sake of knowing, the shock needs to be tuned to the car, so it doesn't "matter" in that sense.

2) if all the weight is on the top vs half being on the bottom, the shock still has to be able to handle all that force. it's easy to see the sprung weight and how it acts, but you need to be able to control the extra force from the unsprung weight getting shoved up as well. if you were to "tune" the exact same shock setup for two cars with the same corner sprung weight, and one was IFS (light) and the other SA (heavy) the car will perform very differently due to the mass/force of the unsprung getting slammed around. i.e. it will need more damping to slow it coming up and need more (or less because gravity ) damping so that the spring doesn't launch it down uncontrolled, in my head anyways

I got you. I just don't know how to apply those numbers to what rebound valve shims to use. Maybe we'll get there? Right now I'm definitely still at trial and error.

 

[Provience]

I got you. I just don't know how to apply those numbers to what rebound valve shims to use. Maybe we'll get there? Right now I'm definitely still at trial and error.

i would think "more damping" would be the answer. if we were to say the total force of the corner is what the springs needed to be set to, you'd have very different ride heights between the lighter and heavier unsprung weight rigs. So to get the desired ride height/free shaft, the combined rate would need to be similar, assuming the sprung weight is the same. maybe the answer would be in running a wider spread between upper and lower, so that on crossover the extra weight could be accounted for?

the compression damping would need to account for the total corner weight, so that would need to be set obviously higher as the extra unsprung and similar sprung would mean greater total corner weight.

maybe rebound could be similar based on the sprung weight to help avoid packing, let the weight pull it back down and the heavier compression damping "deal" with the extra mass when it comes up, but similar rebound so that the sprung weight can still get away from the axle quickly.

obviously this would lead to more likely packing of the spring, but unsprung weight=seconds in all regards and all sports, so obviously you would be giving up speed to account for that. good ride can certainly be attained, it will never be as fast though. hence aluminum parts, open cromo structures, lightweight rotors, cut down tires and IFS.

 

[AgitatedPancake]

so let me make something clear before we get too carried here.

- trail rig setup is a very different animal compared to an u4
- the fast u4 guys tune is significantly different than the mid pack guys, which is different than the participants aka trail rig guys
- every driver has a preference, in my experience shannon never wants to bottom out, so he is willing to put up with a miserable ride. while others will trade comfort for checking up occasionally.

the formula for a trail SA has been proven. if you want to look at the fast u4 guys, great but know that formula is for going FAST and does not translate well.

For the time being, I don't have the experience to offer any solid input on the pros and cons in this discussion, but have a decent (and growing) understanding of the theory, so it's fun to follow along.

This last point about the cambells never wanting to bottom out, compromising in the ride department. I've totally seen this exact phenomenon, in fact brought it up with a friend in a separate discussion. Cody Waggoner's car (Cameron Steele driving), versus I believe Wayland Campbell rolling up behind him. The Lasernut car definitely looked like riding on a cloud of titties through the nuisance stuff versus Wayland being beat up with how violent the car bounces around. But it looked like it was using a lot more travel at low speeds to create that effect. Interesting compromises by both, but the video is pretty cool for the sake of analysis

It's on the book of faces so I'll link it, don't know if I can embed it.
https://www.facebook.com/cody.waggoner.58/videos/10224008176302325

 

[HYDRODYNAMIC]

For discussion Randy Slawson is running Kings on his Trail Bomber not his RACE car. Although I would assume he is still going to be faster than some of the Ultra4 racers. Single shock no external bypass. No air bumps. Looks like 16" coilovers with 8" uptravel and 8" down. The rear is on trailing arms with shocks close to the axle. His dual rate stop or crossover is set maybe 1" above ride height with the vehicle unloaded so by the time people are in it, it should be on the stop and all uptravel will be single rate and down travel will be dual rate. Again no air bumps.

Mike Johnson #43 Ultra4 also has a single fox shock solid axle front end. With air bumps x 4 corners. His crossover is set maybe 1-2" with a very light and short upper spring and heavy and longer lower spring. On the rear trailing arms again a light and short upper and heavy and longer lower spring. It is hard to tell if this follows the 75% as the short spring might have a high rate since it is short and only has a few coils and the long spring might be lighter since it has all the extra coils.

Erik Miller Ultra4 solid axle with external bypass's and air bumps. Has what looks like the 75% rule with the crossover set 1-2" from ride height. Looks to be 8" uptravel and 8 down.

All three are using the dual rate within 1-2". Both Miller and Slawson are near 75% rule on the front ends.

This is all based off of pictures so nothing is for sure.

 

[mobil1syn]

This last point about the cambells never wanting to bottom out, compromising in the ride department. I've totally seen this exact phenomenon, in fact brought it up with a friend in a separate discussion. Cody Waggoner's car (Cameron Steele driving), versus I believe Wayland Campbell rolling up behind him. The Lasernut car definitely looked like riding on a cloud of titties through the nuisance stuff versus Wayland being beat up with how violent the car bounces around. But it looked like it was using a lot more travel at low speeds to create that effect. Interesting compromises by both, but the video is pretty cool for the sake of analysis

i am biased, as thewayne is a friend and i worked for him for a while. i also worked for cody for a little bit prepping that car.

comparing apples and oranges. having spent a decent amount of time in codys car before cam drove it, it was crazy how it rode. its super power is eating up nuisance rocks. then thewayne started codriving for cam and spent significant amount of time tuning plus some special doodads. the portals are a game changer in that sense.

campbell chassis keep the nose high (again to keep up with shannons style of driving), i have not done any measurements but from the looks of it, the travel bias on campbell is 75up vs 25 down.

also if you watch the video, you see that the rear axle is doing most of the 'input to the chassis' for wayland.

 

[Treefrog]

I'm not trying to compare anything I promise :)

I'm just trying to understand why the 75% rule exists.

Forget there are trailing arms or high motion ratio.
For the sake of the argument, let's imagine a shock on axle buggy with 1:1 motion ratio.
Let's say this buggy needs a 90ish lbs/in combined spring rate (so the graph applies).

Why is any of the combinations in the graph above better than the other one ?
Why would a upper spring rate at 75% of the lower one be the ideal combination ?



This is a different problem and I will absolutely graph that as soon as we have some progress on the other topics. We're already spread pretty thin here between short course, desert/U4, trail riding and high/low motion ratio.

I'm just trying to understand where does the 75% rule come from.

Upper spring should be around 75% of lower is another way of saying that the step up ratio should be a bit over 200%, 233% to be accurate. Its easier to convince people to run 75% difference between the springs than to convince them to run all the calculations to figure out the spring rates if you have X ride rate and want 233% step up rate, only to get the same result.

 

[Bebop]

Upper spring should be around 75% of lower is another way of saying that the step up ratio should be a bit over 200%, 233% to be accurate. Its easier to convince people to run 75% difference between the springs than to convince them to run all the calculations to figure out the spring rates if you have X ride rate and want 233% step up rate, only to get the same result.

Ok.
Fine by me, but why ? Why would you want this magic step up rate of 233% ?

 

[Treefrog]

Ok.
Fine by me, but why ? Why would you want this magic step up rate of 233% ?

The recommended step up ratio is 200% to 300%, and that puts you comfortably in it, but still towards the softer side.

 

[Bebop]

The recommended step up ratio is 200% to 300%, and that puts you comfortably in it, but still towards the softer side.

Who recommends that ratio ? Why is this number recommended ?

edit : just for fun, I did the step up ratio calc with the 75% upper/lower spring ratio. It does equate to 233% as Treefrog mentionned.

With a = upper spring rate, b = lower spring rate, c = combined spring rate, if we assume that a = 0.75b,

c = (ab) / (a+b)
c = 0.75b2 / (0.75b+b) = 0.75b2 / 1.75b
1.75bc = 0.75b2
1.75c = 0.75b
2.33 c = b

 

[Treefrog]

Who recommends that ratio ? Why is this number recommended ?

edit : just for fun, I did the step up ratio calc with the 75% upper/lower spring ratio. It does equate to 233% as Treefrog mentionned.

With a = upper spring rate, b = lower spring rate, c = combined spring rate, if we assume that a = 0.75b,

c = (ab) / (a+b)
c = 0.75b2 / (0.75b+b) = 0.75b2 / 1.75b
1.75bc = 0.75b2
1.75c = 0.75b
2.33 c = b

Too low of ratio and the ratio on the stop is too light to be useful in absorbing bigger hits. To high of ratio and the transition will be very noticeable and will be rough on the medium hits.