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Kansas T

Spring rate and/or shock valving question

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My buggy rides rough in rough sand, and bottoms out easier than I think it should when jumping.  Our local dunes in Oklahoma get pretty tore up over a weekend, so I'm dealing with a lot of buggy and sxs ruts/tracks/chop.  I can watch the front suspension move and it seems to absorb the rough stuff, but watching my side mirror, I can't see the rear suspension moving to absorb the roughness.  When the back tires hit something, it shakes the whole car.  I'll put the GoPro on it next trip to really see whats happening.

Four corner scales say I've got 750lbs on each rear tire.  I used the FOA spring rate calculator and came up with a recommend rate of 200lb/in.  I used a 350/500 spring combo to get close to the recommended rate.  The car set high in the rear with no preload and road really rough.  I've swapped springs several times and am currently running a 300/300 spring combo.  The ride height is at 33% droop with no preload.  I calculated the effective coilover load (https://www.crawlpedia.com/coilover_springs.htm) today and came up with 637lbs on each rear spring.  Based on that calculation I'm undersprung, but no preload to get the desired ride height means I'm oversprung, according to what I've read here in other threads.

Based on what I've been reading, people recommend getting the correct springs first, then adjusting the valving.  How do I know if my springs are correct?

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Are your rear springs both the same length?  not very progressive if both springs are the same rate, one should be at least 50 lbs heavier so it gets more stiff as the springs compress... this will help resist  bottoming out and still give a smooth ride over small chatter. Is the secondary spring collar coming into contact with the spring divider? Need some pics

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The springs are 16 and 14 inches in length.  As I understand it, two 300lb/in springs work out to 150lb/in when combined.  Then when the crossover ring hits (its 3" away at rid height) it switches to the lower spring and back to 300lb/in.  I can take some pics tonight, what do you want pics of?  Its a single 2.5" Fox coilover per rear corner.  They have external reservoirs.

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May help to see how much preload you have and where the collar is at. And  in the angle of the shock as well to help us help you out. I remember you building and showing the thing on a motocross track in action, but it's been a few years.

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Ok, I'll get some pics tonight.    It wasn't mine on a MX track, though I'd love to try that. 

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Angles and mounting points are critical on the spring calcs. If you are not needing 2+” of preload then you are over sprung.

That said, harshness is mostly in the valving. Need a lot of free bleed on lighter cars and heavy chop.

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I ride in same place and deal with same issues. I am sprung close with 300/500 in rear both 14" length springs. Utilizing the crossover sooner helped the bottoming out in the rear and also in the front. My front spring rates are 100/150 and the front is working really well.

In the light chop nothing will help with such a light car other than alot of free bleed in the valving like what goatpoker said. That will be the next step i take to get the ride smooth for the rediculous chop we deal with in Oklahoma. It was absolutely brutal this past weekend.

I finally got mine to handle the big stuff really well but the light chop is where i need alot of work.  Next step is to mess with valving in the front and rear.

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2 hours ago, GoatPoker said:

Angles and mounting points are critical on the spring calcs. If you are not needing 2+” of preload then you are over sprung.

That said, harshness is mostly in the valving. Need a lot of free bleed on lighter cars and heavy chop.

I've measured several times, so I know they're all correct.  If the calculators cant get me close, is it just trial and error to get where I need to be with the springs?  That gets real expensive.  Please explain "free bleed".  I've got the valving written down at home, I can post that later.

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Eibach will let you send springs back.

You have to order a certain color, but you can try them and return them.

I am doing that next season.

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Do you guys mean lighter rebound damping when you say more free bleed?  He has a single 2.5 fox. 

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Here's some pics.  The front of the buggy is torn apart, so I can't get it out of the trailer to get current side shots.  It sets pretty close to the same as this picture.   The last pic is what the current valving in the rear is.  For a little background on the valving.  I tried multiple different settings shortly after the car was assembled and wasn't getting the desired results.  After chatting with Ed (Pound Sand), he sent me some suggestions and this is where we ended up.  The car drove 100% better than before, but tends to bottom easier on jumps than before.  It really wasn't been an issue, but now I'm driving the car harder than I used to, so I need to address it.  These are Fox 2.5 x 14 coilovers.

IMG_1801.thumb.jpg.c93db4820f6f2bca253ffae9395854f7.jpg

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IMG_1805.thumb.jpg.ac804b62deb97dd2b469a74204ed3ab6.jpg

 

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Posted (edited)
20 hours ago, Kansas T said:

My buggy rides rough in rough sand, and bottoms out easier than I think it should when jumping.  Our local dunes in Oklahoma get pretty tore up over a weekend, so I'm dealing with a lot of buggy and sxs ruts/tracks/chop.  I can watch the front suspension move and it seems to absorb the rough stuff, but watching my side mirror, I can't see the rear suspension moving to absorb the roughness.  When the back tires hit something, it shakes the whole car.  I'll put the GoPro on it next trip to really see whats happening.

Four corner scales say I've got 750lbs on each rear tire.  I used the FOA spring rate calculator and came up with a recommend rate of 200lb/in.  I used a 350/500 spring combo to get close to the recommended rate.  The car set high in the rear with no preload and road really rough.  I've swapped springs several times and am currently running a 300/300 spring combo.  The ride height is at 33% droop with no preload.  I calculated the effective coilover load (https://www.crawlpedia.com/coilover_springs.htm) today and came up with 637lbs on each rear spring.  Based on that calculation I'm undersprung, but no preload to get the desired ride height means I'm oversprung, according to what I've read here in other threads.

Based on what I've been reading, people recommend getting the correct springs first, then adjusting the valving.  How do I know if my springs are correct?

When you say 750lbs on each rear tire, did you subtract unsprung weight from that?  Should be somewhere around 125lbs/corner with a sand setup and 150lbs/corner with the steelie/33x12.50R15 ATs pictured. 

Chances are your springs are too long, not too stiff.

17 hours ago, 2turbofords said:

Are your rear springs both the same length?  not very progressive if both springs are the same rate, one should be at least 50 lbs heavier so it gets more stiff as the springs compress... this will help resist  bottoming out and still give a smooth ride over small chatter. Is the secondary spring collar coming into contact with the spring divider? Need some pics

Combined springs have less rate than individual, so 300 over 300 is actually 150 in-lb until it hits the crossover.

Edited by Rockwood

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42 minutes ago, Rockwood said:

When you say 750lbs on each rear tire, did you subtract unsprung weight from that?  Should be somewhere around 125lbs/corner with a sand setup and 150lbs/corner with the steelie/33x12.50R15 ATs pictured. 

Chances are your springs are too long, not too stiff.

Combined springs have less rate than individual, so 300 over 300 is actually 150 in-lb until it hits the crossover.

I made that mistake originally.  I have since recalculated, subtracting the unsprung weight, and the springs still appear to stiff.  The steel wheels actually have 35s on them, but I only use them for running around on country roads.  The sand tire setup is my only concern.

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I’d need a bit more detail on the spring preload. (Free Length, preload, length at ride height) but given a few things I’d say you have:

1. Too much rear spring. I run 350/450 in a rear engine V8. 1,050 lbs per tire. 

2. You do have a pretty serous flutter in the rear. 1.10x 0.015 should make her breathe pretty well.   You could go 1.0 x 0.015 in the flutter (maybe even 0.020 if you want it to really move well)

3. After the flutter you go right to a 1.60 so you leave the ports open A LOT. This will essentially kill your damping and lead to the bottoming on jumps.  Maybe go with a full stack of 0.012 after the flutter.  1.75, 1.60, 1.45, 1.30, 1.15, 1.0 all 0.012.

4. Rebound is a little wonky. Do you not have a 1.75 shim in there? If not your shock is totally not doing anything. That progressive stack is of no use in a coil sprung car. Straight 12’s or 10’s if you like it springy.

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22 minutes ago, GoatPoker said:

I’d need a bit more detail on the spring preload. (Free Length, preload, length at ride height) but given a few things I’d say you have:

1. Too much rear spring. I run 350/450 in a rear engine V8. 1,050 lbs per tire. 

2. You do have a pretty serous flutter in the rear. 1.10x 0.015 should make her breathe pretty well.   You could go 1.0 x 0.015 in the flutter (maybe even 0.020 if you want it to really move well)

3. After the flutter you go right to a 1.60 so you leave the ports open A LOT. This will essentially kill your damping and lead to the bottoming on jumps.  Maybe go with a full stack of 0.012 after the flutter.  1.75, 1.60, 1.45, 1.30, 1.15, 1.0 all 0.012.

4. Rebound is a little wonky. Do you not have a 1.75 shim in there? If not your shock is totally not doing anything. That progressive stack is of no use in a coil sprung car. Straight 12’s or 10’s if you like it springy.

Free length, the lower spring is 16" and the upper is 14".  Length at ride height, the lower spring is 13.75 and the upper spring is 11.5.  There is no preloading of the springs.  I installed them and ran the collars down until they touched the spring.

I'll keep the flutter info in mind, after I get the springs right.  With as tore up as our dunes get, I probably want as much movement as I can get.  I have ran a full stack of .012 in the rear before, but that was with no flutter and the original 350/500 springs.  I'll have to try it again and see what happens.

Before I put this rebound valving in the car, it had a really bad buck in the rear.   Small jumps or even cresting a dune would send the rear high nearly endo the car.  This valving mostly cured that problem, so I haven't questioned it until now.  I'm not questioning your statement ("That progressive stack is of no use in a coil sprung car"), but I would like to understand why?  I really don't was the rear rebound "springy" again.

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Posted (edited)

Yo Kansas you are getting very smart on spring rates and some good learning going on here. You said 750 per rear tire? That seems like a strech to me. What were the fronts? And total? If that weigh was inaccurate it could be throwing your calculations off 

Edited by Jetjock15

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The front weights were 400 per tire.  Theses weights were with two passengers.  Both weighed about 210-220.  Total weight is 2300, give or take a few lbs.  What makes you think 750 is a stretch?  High or low?

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Some quick math. With 350/500  and 4.75” of compression, your shock is seeing 977lbs of sprung weight. So your 750 per tire seems reasonable. Need to take the tire and brake weigh off as unsprung and redo your spring calculator.  You are likely a bit over sprung. 

Mid’s tend to buck so you’ll need more rebound damping when compared to a rear engine.  You never confirmed if you are actually missing the 1.75” shim in the rebound stack?  That’s a big deal.

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Posted (edited)

With passengers makes more sense. I’ve just weighed a lot of cars and 750 per rear normally gets you around 2500 lb car which yours is not. The passengers answers my question. If you base all your calcs  off “with passenger” than you’d need do have that weight on your testing sessions to get it dialed. Your on the right track. Just like you suggested I’ve put go pro on suspension and can watch replay frame by frame, need to see the tire and the ground, make your adjustments off that. Does the tire Bounce or hang (pack). It’s a lot of work but worth it in the end.  Make sure your off the collars for all cruise situations but on the collars for last 1/3-1/4 of shaft travel 

Edited by Jetjock15

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3 hours ago, GoatPoker said:

Some quick math. With 350/500  and 4.75” of compression, your shock is seeing 977lbs of sprung weight. So your 750 per tire seems reasonable. Need to take the tire and brake weigh off as unsprung and redo your spring calculator.  You are likely a bit over sprung. 

Mid’s tend to buck so you’ll need more rebound damping when compared to a rear engine.  You never confirmed if you are actually missing the 1.75” shim in the rebound stack?  That’s a big deal.

All the measurements I've given have all been with the 300/300 springs.  I ran the heavier springs several years ago when I originally assembled the buggy, but they are no longer installed.

Yes, the 1.75 shim is missing.  I'll be sure to get it back in there.

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Sorry, i was thinking 350/500 for some reason. 300/300 gives you 712lbs sprung. 

You could go 250/350. Gives you less spring rate and more step up. 

Get that 1.75’ back in and try this compression

1.75 - 10

1.0 - 12

1.75 - 15

1.6 - 15

1.5 - 12

1.3 - 12

1.15 - 12

1.0 - 12

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On 4/29/2019 at 1:43 PM, stumpalump said:

Do you guys mean lighter rebound damping when you say more free bleed?  He has a single 2.5 fox. 

free blead holes in piston. lets oil bypass both shim stacks. ive drilled out 2 more holes in mine and it made a huge difference in chop.

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24 minutes ago, GoatPoker said:

Sorry, i was thinking 350/500 for some reason. 300/300 gives you 712lbs sprung. 

You could go 250/350. Gives you less spring rate and more step up. 

Get that 1.75’ back in and try this compression

1.75 - 10

1.0 - 12

1.75 - 15

1.6 - 15

1.5 - 12

1.3 - 12

1.15 - 12

1.0 - 12

Thanks for your suggestion.  I ordered a pair of 200 springs for the top, so we'll see what that does for the ride height and preload.  I've gotten several suggestion for rebound shimming very close to that, so I'll get it in there and see how it reacts.

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 are you happy with the ride height, and the amount of preload I wouldn't start swapping springs quite yet? How much air pressure do you run in the bump stops? from the dust on the shaft looks like you're using quite a bit of them as part of the last half of the travel? 250 to 300 pounds is nothing for those bump stops, and it may help slow the bottoming out as the bump stiffens as it compresses.

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Is there a way to put bipasses on the rear of your car?  They seem to be a lot easier to dial in with the external tubes once you get the valving close.

 

I don't know much about valving....good read.  Subscribed.  :moof:

 

~jw

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