Sprung / Unsprung weight ratio for going fast (Coilover tuning spin off)

[Bebop]

Bouncing back on this post form the coilover tuning thread:

it is not the setup up that is causing the buck, it is the fact that hes got 40s, attached to a 60, pushing against almost no weight above it. he needs more weight if he wants go fast.

What is the acceptable sprung / unsprung weight ratio in order to "go fast" but still retain some crawling capabilities ? Basically a fast/comfortable trail rig.
Does going on a trailing arm set up change this ratio (IE let's you go faster with a low sprung/unsprung ratio vs a 1:1 motion ratio car)?

I'll post here DesertCJ jeep (Hope he doesn't mind) since this is the object of the quote above :

Here is my personal rig, with a similar issue, but I have trailing arms and bypasses :

 

[mobil1syn]

Here is my personal rig, with a similar issue, but I have trailing arms and bypasses :

youre rig is a completely different animal

ooh just realized this was a different thread.

 

[Bebop]

youre rig is a completely different animal

ooh just realized this was a different thread.

Yes, different thread to keep 2 different problems somewhat separate.

Tell me more about my rig then.

Infos about it:

front:

150/250
7" up / 7" down
2" bumpstops
1" preload
No slide stop / single rate before coil bind up top

rear:

200/250
8" up / 8" down on the bypass shock
12" up / 12" down at the axle
No bumpstops for now, but will add them if needed
1" preload at the shock
No slide stop / single rate before coil bind up top

2.0 coilovers
2.5 Bypasses, 2 overlapping compression tubes and 1 rebound tube. Standard layout.

There is almost no weight in the back. 19 gal fuel cell right behind the seats and a 8274 all the way in the back. That's it.

When I hit a hard bump in the back, it bucks. I'm sure adding a spare in the back would do wonders, but that's not happening.

 

[DesertCJ]

It "Bucks" when you hit a hard bump going slow or going fast? Buck or Jar? I don't have any experience with trailing arms but I would think you'd need heavier valving than a shock mounted on the axle.

 

[Bebop]

What is the difference between bucking and jarring ?

I only feel that at speed on a succession of big bumps.

It does have pretty heavy comp valving in it (modified #100 stack)

 

[DesertCJ]

A buck would be the back getting thrown into the air by he springs from not enough rebound damping. A jar would be because either the springs are too stiff or too much compression damping.

"At speed, succession of big bumps" kinda tells me it is bucking? Which points to not enough rebound and or lower springs that are too stiff. Why aren't you running slide stops and letting the uppers coil bind? To move the cross over as high as possible? You don't have a single rate, you're bottom spring is just coming in later when the upper binds.

 

[Bebop]

I think it's a Jarring related problem. But it's very easy to confuse both so I might be wrong.

As soon as I enter the bump zone of the bypass, it jars / kicks up.

I think it's doing this because of the low sprung/unsprung weight ratio.

That's why I was wondering what was the acceptable ratio and if that's something I have to live with or if I'm doing it wrong in terms of suspension set up and I could get rid of this.

 

[AgitatedPancake]

This is kinda an obvious statement, but it helps me really visualize what's going on with the ratio of unsprung weight to sprung weight:

First scenario, a large amount of sprung mass and not much unsprung weight. You hit big bumps that shove the tires up toward the chassis. The chassis has enough mass that the force from the shocks is enough to stop the upward motion of the tire, and change its direction back down again rapidly via the springs without upsetting the chassis because the force required is minimal in relation to the momentum of that sprung weight, so it's ready for the next hit. (example: 500lbs unsprung weight, 1000lbs sprung weight, you can valve the shocks enough to control that unsprung weight without upsetting the chassis)

The next scenario, a car with light sprung mass but lots of unsprung mass via heavy tires and axles. You hit the same big bump as above at the same speed as above, shoving the axle up toward the chassis the same way. This time, the axle moving upwards towards the chassis carries a lot more energy due to the weight, and the light chassis has less ability to stop that upward motion without being upset. You can valve the shocks stiffer to attempt to slow the axle down, but that extra force into a lighter chassis is going to upset the car more, meaning more bucking without a real way to combat it. (example: 1000lbs unsprung weight, 500lbs sprung weight, there's nothing you can do to stop the bucking because enough force to change direction of the axle, is more than enough to want to shove the chassis instead...path of least resistance and all).

I think in this industry we get way too focused on 'holy grail" numbers that are set as goals, interpreted as some sort of standard. I think this (among other discussions) is something without a distinct cutoff. Just a...trend towards better. I don't think it would be right to say "this ratio is what you should shoot for" and put a specific number out there. Just that every little bit you improve the ratio, you improve the performance. I'm not going to say it's completely linear improvement, but I don't think it's some crazy exponential where as soon as you cross a specific threshold everything changes. A rockwell car is probably going to struggle, a 9" car on smaller tires is going to be better because it's half the unsprung weight. But an IFS car will be able to kick both of their asses, because it's got half again the unsprung weight of a 9" solid axle car. On a solid axle car we'll never have the sprung to unsprung ratio of an independent car, but we can still get pretty awesome results out of solid axles. If you want to set goalposts, they're really going to be way outside of what's even achievable with a solid axle haha.

So just strive for the best ratio you can, and tune around what ya got but don't expect the world

 

[Bebop]

Very cool approach AgitatedPancake


If that helps pin point, my car is an east coast rig and rarely leaves low range. I would like to be able to hold it in 3rd low (about 50mph) over rough terrain without experiencing this problem and am wondering if it's a feasible goal or if I'm dreaming.

If that helps visualize, imagine Dirty Turtle or AOP U4 races as the terrain.

I know that lowering unsprung weight would help (lighter tires vs the heavy ass 43s and lighter axle vs the steering 14 bolt), but that's choices I'm not ready to make.

 

[DesertCJ]

I think it's a Jarring related problem. But it's very easy to confuse both so I might be wrong.

As soon as I enter the bump zone of the bypass, it jars / kicks up.

I think it's doing this because of the low sprung/unsprung weight ratio.

That's why I was wondering what was the acceptable ratio and if that's something I have to live with or if I'm doing it wrong in terms of suspension set up and I could get rid of this.

Open the bypass tube until it starts bottoming and then add a turn. Maybe it's more complicated than that but?

 

[DesertCJ]

This is kinda an obvious statement, but it helps me really visualize what's going on with the ratio of unsprung weight to sprung weight:

First scenario, a large amount of sprung mass and not much unsprung weight. You hit big bumps that shove the tires up toward the chassis. The chassis has enough mass that the force from the shocks is enough to stop the upward motion of the tire, and change its direction back down again rapidly via the springs without upsetting the chassis because the force required is minimal in relation to the momentum of that sprung weight, so it's ready for the next hit. (example: 500lbs unsprung weight, 1000lbs sprung weight, you can valve the shocks enough to control that unsprung weight without upsetting the chassis)

The next scenario, a car with light sprung mass but lots of unsprung mass via heavy tires and axles. You hit the same big bump as above at the same speed as above, shoving the axle up toward the chassis the same way. This time, the axle moving upwards towards the chassis carries a lot more energy due to the weight, and the light chassis has less ability to stop that upward motion without being upset. You can valve the shocks stiffer to attempt to slow the axle down, but that extra force into a lighter chassis is going to upset the car more, meaning more bucking without a real way to combat it. (example: 1000lbs unsprung weight, 500lbs sprung weight, there's nothing you can do to stop the bucking because enough force to change direction of the axle, is more than enough to want to shove the chassis instead...path of least resistance and all).

I think in this industry we get way too focused on 'holy grail" numbers that are set as goals, interpreted as some sort of standard. I think this (among other discussions) is something without a distinct cutoff. Just a...trend towards better. I don't think it would be right to say "this ratio is what you should shoot for" and put a specific number out there. Just that every little bit you improve the ratio, you improve the performance. I'm not going to say it's completely linear improvement, but I don't think it's some crazy exponential where as soon as you cross a specific threshold everything changes. A rockwell car is probably going to struggle, a 9" car on smaller tires is going to be better because it's half the unsprung weight. But an IFS car will be able to kick both of their asses, because it's got half again the unsprung weight of a 9" solid axle car. On a solid axle car we'll never have the sprung to unsprung ratio of an independent car, but we can still get pretty awesome results out of solid axles. If you want to set goalposts, they're really going to be way outside of what's even achievable with a solid axle haha.

So just strive for the best ratio you can, and tune around what ya got but don't expect the world

All of that is why you want the lightest springs possible to hold the rig up. The axles and tires aren't pushing against the chassis, the springs are.

 

[Bebop]

Open the bypass tube until it starts bottoming and then add a turn. Maybe it's more complicated than that but?

Short tube is 2 turns in
Long tube is 1 turn in

I'm happy with it's performance now.

The problem I'm trying to describe rarely happens, but when it does it's after I blew through all the travel and hitting the bump zone of the bypass. It does way harsher all of a sudden and jars.

 

[DesertCJ]

Short tube is 2 turns in
Long tube is 1 turn in

I'm happy with it's performance now.

The problem I'm trying to describe rarely happens, but when it does it's after I blew through all the travel and hitting the bump zone of the bypass. It does way harsher all of a sudden and jars.

How's the rebound setup? Is the shock packing?

 

[AgitatedPancake]

All of that is why you want the lightest springs possible to hold the rig up. The axles and tires aren't pushing against the chassis, the springs are.

I'm not sure I fully agree with the "why" of what you're saying here. The shocks are doing 90% of the work translating force into the chassis when going fast. Shocks can make thousands of pounds of resistance/force (depending on shaft speed) while springs only do a fraction of that, and in a constant linear manner independent of shaft speed. But shocks dissipate energy, and springs store and rebound that energy, so I agree it seems like the best path for the shocks to do all the work instead of storing and rebounding it with springs

 

[DesertCJ]

The springs are pushing on the chassis with thousands of pounds of force. Something like 1500lbs-3000lbs on each corner at full compression depending on the spring. That's not a fraction of the shock piston force. It's very significant...

 

[AgitatedPancake]

The springs are pushing on the chassis with thousands of pounds of force. Something like 1500lbs-3000lbs on each corner at full compression depending on the spring. That's not a fraction of the shock piston force. It's very significant...

Hmm. Admittedly wether it's right or wrong, I'm looking at it as how much force is changed from ride height toward full compression, not counting the static weight on the springs. So if you have 6" uptravel from ride height, with a 100lb primary rate that steps up to a 150lb secondary rate after 1" of compression, the difference in spring force between ride height and full compression is only 850lbs. So if you're at ride height and you hit an obstacle that sends the axle to full bump, that's the maximum effort the springs can offer to compensate. But in that same range, the shock can be generating multiple thousands of pounds of force to help with the same cause.

 

[DesertCJ]

You're math is on but if it sends the axle to full bump its double the 850. 1700lbs acting on one end of the chassis. The compression valving is adjustable obviously and if you go to stiff, yeah it's going to start acting on the chassis. You will probably notice when you cross that balance...

 

[mobil1syn]

It does have pretty heavy comp valving in it (modified #100 stack)

Short tube is 2 turns in
Long tube is 1 turn in

I'm happy with it's performance now.

The problem I'm trying to describe rarely happens, but when it does it's after I blew through all the travel and hitting the bump zone of the bypass. It does way harsher all of a sudden and jars.

toss a ziptie on the shaft next time out and see if you are using all the travel

radflos?

 

[AgitatedPancake]

You're math is on but if it sends the axle to full bump its double the 850. 1700lbs acting on one end of the chassis. The compression valving is adjustable obviously and if you go to stiff, yeah it's going to start acting on the chassis. You will probably notice when you cross that balance...

Hmm 1" @ 100lb/in spring rate, plus 5in at 150lb/in spring rate is 850 per corner. 1700 for the total weight of that end of the vehicle, but if observing it that way you're also observing the force from both shocks at the same time so I figured single corners were easier to analyze. I don't have shock dyno charts handy, but I feel like I've seen some pretty steep force curves dependent on shat speeds. Anyhow this is just how I observe things, not trying to say anyone is right or wrong, just observing the relationship between the force generated by springs vs shocks

 

[Bebop]

toss a ziptie on the shaft next time out and see if you are using all the travel

radflos?

Yes, radflos. Valved, but completely unmodified. I've heard about soft poppet springs/no spring, poppet bleed etc but that's above my pay grade.

I am using all the travel.

 

[mobil1syn]

Yes, radflos. Valved, but completely unmodified. I've heard about soft poppet springs/no spring, poppet bleed etc but that's above my pay grade.

I am using all the travel.

im willing to bet your transition to the bump zone is causing the 'buck'

 

[Bebop]

im willing to bet your transition to the bump zone is causing the 'buck'

Yes. I wrote in an earlier post that this is what it feels. And it sucks.

I think it's a Jarring related problem. But it's very easy to confuse both so I might be wrong.

As soon as I enter the bump zone of the bypass, it jars / kicks up.

I think it's doing this because of the low sprung/unsprung weight ratio.

That's why I was wondering what was the acceptable ratio and if that's something I have to live with or if I'm doing it wrong in terms of suspension set up and I could get rid of this.

I'd like to know what to do to mitigate that, if you have some pointers. Thanks

 

[thedirtman]

Not only are you getting into the bump zone of the bypass but you should also be engaging the air bumps at the same time, maybe back the pressure off the air bumps and play around with valving in that zone.

 

[DesertCJ]

Short tube is 2 turns in
Long tube is 1 turn in

I'm happy with it's performance now.

The problem I'm trying to describe rarely happens, but when it does it's after I blew through all the travel and hitting the bump zone of the bypass. It does way harsher all of a sudden and jars.

He said he doesn't have air bumps in the rear. Sounds like his bypass tubes are mostly open so the bypasses are barely using any of the valving in the shock until the piston gets past the compression tubes into the bump zone and then makes a huge transition to the piston valving.

I would probably try lighter compression valving in the bypasses and also less bypassing(Close the compression bypass tubes more to make the valving do more work). That should give a smoother transition into the bump zone.

 

[Bebop]

He said he doesn't have air bumps in the rear. Sounds like his bypass tubes are mostly open so the bypasses are barely using any of the valving in the shock until the piston gets past the compression tubes into the bump zone and then makes a huge transition to the piston valving.

I would probably try lighter compression valving in the bypasses and also less bypassing(Close the compression bypass tubes more to make the valving do more work). That should give a smoother transition into the bump zone.

Makes sense. I'll try to lighten up the stacks and report.

 

[mobil1syn]

Makes sense. I'll try to lighten up the stacks and report.

close the tubes all the way, then open them to 4 turns on both short and long first.

 

[Bebop]

close the tubes all the way, then open them to 4 turns on both short and long first.

Ok, I'll do that.
Then do I try to play with tubes first or just valving for a while ?