# of shocks?

[subrunner]

I'm setting up my straight axle Suburban as a prerunner/chase truck. My question is, would I be better off with 2-2.0" shocks per side or 1-2.5". The truck has a big block and front glass.

[Just4Fun]

1 2.5 shock will be perfect

[steveG]

I think one 2.5" would be great, butif you can come accross a couple used 2" for cheap, go with those....

[Josh_K]

I hate to get technical, but lets do the math.

Pie is 3.14 (rounded) so,

Take the radius of 2”, which is 1” and square it. Take this number (1) and multiply by pie (3.14) and it is 3.14 3.14 sq. in. is the amount of sq. in. of area that a 2” shock piston has. Now multiply 3.14 x 2, because you have 2 shocks and this give you a number of 6.28 sq. in. of piston shock area for two 2” shocks.

Take the radius of 2.5 which is 1.25 and square it (1.25 x 1.25 = 1.56) Now multiply 3.14 (pie) by 1.56 (3.14 x 1.56 = 4.9) 4.9 sq. in. is the amount of area that a single 2.5” shock has of piston area to absorb forces.

This means two 2” shocks have about 28% more area to absorb forces. If all the variables remain the same.

For simplicity I would opt to use a single 2.5 shock. It’s the way I went on my Bronco.

Josh

[matt_helton]

oh josh, reading your posts makes me soo happy. :)

why do you not respond to my PM's?? :(

[Josh_K]

I guess I havent figured this the new 3.0 version out. By the way, your claws are dismounted and sitting next to the house.

[FullsizeFun]

I hate to get technical, but lets do the math.

Pie is 3.14 (rounded) so,

Take the radius of 2”, which is 1” and square it. Take this number (1) and multiply by pie (3.14) and it is 3.14 3.14 sq. in. is the amount of sq. in. of area that a 2” shock piston has. Now multiply 3.14 x 2, because you have 2 shocks and this give you a number of 6.28 sq. in. of piston shock area for two 2” shocks.

Take the radius of 2.5 which is 1.25 and square it (1.25 x 1.25 = 1.56) Now multiply 3.14 (pie) by 1.56 (3.14 x 1.56 = 4.9) 4.9 sq. in. is the amount of area that a single 2.5” shock has of piston area to absorb forces.

This means two 2” shocks have about 28% more area to absorb forces. If all the variables remain the same.

For simplicity I would opt to use a single 2.5 shock. It’s the way I went on my Bronco.

Josh


After all that tech on how 2 2" shocks have more area than 1 2.5" shock, you say you would run a 2.5". hahahahaha

[Josh_K]

Hay helton, all I have form you is a picture of a 12yr old girl in my mail?

[matt_helton]

LOL.....man, i wanna come get those claws. ill find some time after the race. those things are gonna be awesome at the ror.

[DMSRACE]

Helton, you going to the tractor pulls with the claws????

[Greg]

Actually, the numbers are alot closer than what Josh has stated, 2 2.0 shocks have a piston area of 4.8"sq, 1 2.5" shock has 3.976"sq.

[subrunner]

With that said, which would you recommend Greg? My truck is setup to run either single or dual shocks, so simplicity's not an issue and price is pretty close.

[ChuckH]

2 shocks would run cooler temp wise (more suface area, and more oil) and might have a little more dampening, both factors woud help with the big block weight, i would go with 2 shocks.

[matt_helton]

hell yes Kevin, im gonna hang a few beer kegs off the front bumper, bolt on the claws and hit the tractor pull circuit. :)

[subrunner]

Just make sure you fill them with actual beer,lol.

[matt_helton]

oh yes they will be full, i need the weight to keep the front end on the ground, we can then empty them after the full pull!!

[Greg]

hmmm, id say 1 2.5", easier to mount, easier to valve, looks cooler. There you have it. Lets face it, its a suburban, I really hope your not tring to get 100% from those shocks all the time.

[baja_driver]

have an 88 burban set up for pre-runner, dual fox all the way around with bumps...works real nice...national springs . I think they are the "pro series" and are 2.0

[Josh_K]

Well Greg (aka Archimedes), if you can, I would like for you to explain how I messed up the mathematical formulation of pi radius squared.

I am serious too. It’s been about 8 yrs since I used this and if I am doing something wrong I would like to be corrected. Could you inform me of your formula?

Ho, and b.t.w. I don’t mean to create a male “jump rope contest” I just want to know what I did wrong.

Thanks,

Josh

[ntsqd]

Area = Radius squared times PI aka: A = R^2*PI

[Josh_K]

Area = Radius squared times PI aka: A = R^2*PI

Isnt that what I did?

[BAPerf]

Pi = ~ 3.14159265358... or for the mathematically challenged, it's usually rounded off to 3.14159 or 3.14. (Not trying to be smart, just trying to answer the question "What is Pi?" before it comes up.

Maybe Greg used the piston diameter, as apposed to the ID of the shock body?

[Fourstroker]

It sounds like BAPerf hit it on the head. Using Greg's values and the stated formula. The diameters in question would be 2.25" for the 2.5" shock and 1.75" for the 2.0" shocks or exactly 1/4" smaller than the shock size for the piston size. Seams reasonable. If those values are correct it makes the case of the single 2.5 versus the dual 2.0 better. I'd go with the 2.5 myself

[BAPerf]

Okay, I'm an idiot. Josh already posted what Pi was. Oops!

Josh, Looks like Greg used 1.75" as the area for the 2" bodied shocks, and 2.25" as the area for the 2.5". Sounds like the walls of the can are ~ .125" thick according to Greg's numbers. I don't think he was trying to make anyone look bad, just using measurements a tad closer to actual sizes (I don't know if they are accurate or not, but 1/8" wall seems close).

Hope this doesn't come off bad.

[BAPerf]

Damn,

That was fast. I'm just too darn slow.

[klaus]

the diameter of the shaft(s) takes up useable piston surface area. That should be added into your formula....

[Fourstroker]

the diameter of the shaft(s) takes up useable piston surface area. That should be added into your formula....

Good catch Klaus. This brings the 2 alternatives even closer
So Here Goes:

2.5" Shock = 2.25" Piston = 3.97 Sq in Surface Area
less 7/8" Shaft = .60 Sq in Surface Area
Net Usable Surface Area = 3.37 Sq in

(2) 2.0" Shocks = 1.75" Pistons = 4.81 Sq in Surface Area
less (2) 7/8" Shafts = 1.20 Sq in Surface Area
Net Usable Surface Area = 3.60 Sq in

About 6% more surface area for the 2.0's
Good luck on your decision

[baja_driver]

are you sure both shocks have the same shaft size? thought the 2.0 was a bit smaller than the 2.5 and wouldn't the bodies be taken into consideration for heat ? I under stand that the smaller would heat faster 1 to 1 but now you have two bodies to get rid of heat. I prefer the one shock for race but use two on the big truck he is talking about....just curious.

[singlehanded]

7/8 should be the shaft size for 2.0 and 2.5's. I think most people prefer a 2.5 coilover and a 2.5 bypass for race on a mini(minimum). Well I think the resevoir is the most inportant factor, are the 2.0's going to have resevoirs? Also what it is going to be used for is key, a single 2.5 all around isnt going to hang under race conditions, it will get to hot and fade. But for just limited time useage either should be fine.

[subrunner]

Mainly just fooling around. I'd like it to work the best it can, though. One day(after it's caged) i may enter it in 1450 at an MDR race and just beat it up for 250 miles or so. Probably just be a big rolling roadblock,lol.

[Greg]

Josh, I just used the ID of the bodies to get those numbers. Im gonna figure if the shock is going on a suburban they wont be longer than 14" and therefore will probably 12". Most 12" 2.0 use a 5/8 shaft vs. 7/8. Would it make a difference in valving, probably not as the shaft will only affect the compression IF you need more port area (a really light vehicle, NOT a suburban) and the heavier vehicle will need more rebound valving (dare I use the term damping/dampening) so even if it were to ues a 7/8 shaft in a 2.0 it wouldnt make a difference in performance. I am assuming that all the shocks being considered would use external reservoirs, since its a sub., Im guessing you still run leaves. You thought about running a bypass at each corner? not because its "cooler" but more to help speed the rebound up so those leaves can work and because your gonna need all the comp. "damperening" you can get and with the res mounted on the rebound side of the piston you can reach much higher levels of damperening, easier, than using a convetional shock. Let the jousting begin!!!

[subrunner]

Hmm? Actually, I have been thinking of bypasses and yes it still has leaves.

[Greg]

Man I need to stop posting early in the morning, disreguard about 1/2 that stuff about port volume vs. shaft size, as I think (im not going in the garage just to pull shocks apart and look) but the pistons are the same in a 2.0 weather is has a 5/8 or 7/8 shaft. At this point it really doesnt matter. If tthe res line are the same size a 7/8 shaft will possibly has fewer issues with cavataion. NOW, tear into me like starving wolves to a fresh kill.

No matter what you end up with kirk, just valve it until it works.

[ntsqd]

Isnt that what I did?

Don't know, didn't check your numbers. For those whose calculators are PI challenged you can get reasonably close by using 22/7ths.

Good catch Klaus. This brings the 2 alternatives even closer
So Here Goes:

2.5" Shock = 2.25" Piston = 3.97 Sq in Surface Area
less 7/8" Shaft = .60 Sq in Surface Area
Net Usable Surface Area = 3.37 Sq in

(2) 2.0" Shocks = 1.75" Pistons = 4.81 Sq in Surface Area
less (2) 7/8" Shafts = 1.20 Sq in Surface Area
Net Usable Surface Area = 3.60 Sq in

About 6% more surface area for the 2.0's
Good luck on your decision

Shaft size only effects the piston area in Rebound, not in compression.

Damperening
That's the best yet. I vote we ALL use that term from now on.

[Greg]

NTSQD, please explain to me how the shaft size affects only rebound, and especially NOT compression.

[ntsqd]

It only affects the piston area on the bottom of the piston, not on the top. On the top there is still an End to the shaft, so it is not subtractive of area. In F1 the difference is enough that most damperens have a shaft that goes all the way thru the damperen's body. There the area of the shaft is subtracted from both sides of the piston.

So area of the top would be PI*R^2, but area of the bottom would be PI*R(piston)^2-PI*R(shaft)^2

The Volume displaced by the shaft could play a role in Comp damperening depending on where the Res is ported in. That is another offset dialed out by the F1 design, no gain or loss of fluid volume.

[ChuckH]

Their called "Through shaft shocks" ( ntsqd knows that i bet ) another advantage with them is that you dont need a reservoir to handle the changing shock oil volume because its always the same no matter where the piston is, gas pressurization would still be needed with high shock temps to control cavitation though.

[ntsqd]

I did at one time, but had forgotten. Don't get old.

[Greg]

Ntsqd, actually the port volume is all that makes a difference reguarding the shaft being on one side of the piston. That being the case, unless you actually had to make the rebound ports smaller to allow for the shaft itself, wouldnt it make a difference. Since the mount for the piston (on 2" foxes for example) is 1/2" on all varieties of 2" be it 5/8" shaft, 7/8 shaft, etc. In other words, atleast for our application (f1 and motorcycle steering dampers dont apply here), the shaft size has no effect on damping. Your second statment that shaft diameter will affect rebound also needs correction (although in a later post you say it can affect compression). All things being equal, especially the hose diameter to the res., the larger shaft will have to flow more volume to dipalce the shaft. This being the case, a shock with the larger shaft is less likely to cavitate, during high speed compression, than the same shock with a small dia shaft. Many of the top teams use orifices in the res line, or a smaller line itself, to reduce cavitation. Where the res port is mounted is not significant to this topic. But , yes, place ment of the res port to the slow speed side of the piston eliminates this whole cavation battle.

[subrunner]

Ok, this is really informative. Now, I have a simple question. Bear with me cause I'm new to this.
Do bypass shocks have valving or is it all done with the bypasses?

[ZTFab]

Damperening....


That's the best yet. I vote we ALL use that term from now on.[/QUOTE]


I Like it too....I'm in....

[FABRICATOR]

Some external by-pass shocks have flow through the piston and some do not. A solid piston requires by-pass circuits that cover every bit of piston movement.

Wouldn’t the flow be the same in both directions on a standard shock with a top ported reservoir? Both directions should be 100% minus the shaft volume.

A shock with a reservoir ported to the bottom does flow differently. Compression would be 100% through the piston. Rebound would be 100% minus the shaft volume.

It is doubtful any on-road shocks would flow anywhere near enough fluid. A shock on a fast dezert vehicle needs to be able to move around 75 to 90 feet per second.

[subrunner]

I'm referring to a 2 or 3 tube bypass.

[ntsqd]

Greg, I'm still suffering from jet lag but the pressure that the piston develops is directly related to the piston area and the force on the shaft. The size of the ports also come into play since they are bleeding off pressure. From a pure hydraulics point of view (no ports in the piston) the shaft area is subtractive. I hadn't considered the role that the ports would play. You could probably size them to offset the shaft area, but I suspect that would only hold true for one piston speed and applied force. Have to think on that for a while.

Sub, for the most part by-passes have ports and valving on the pistons. Usually they are set so stiff that they do not have any effect until things are really bad.

[Stephen]

We raced a class 3 K5 with a single 2.5 per wheel and in the whoops it would fade after just a few minutes of running hard. They were smooth bodies. I would think a single 2.5 would work on your 'burb pretty well, given that you shouldn't ever pound it in the whoops like a racer.
I think a bypass shock would be ideal for your situation, the bottom out section would work as a good bumpstop that's mounted near the end of the axle tube so it takes force off the bumpstops which are typically mounted farther inboard. This saves stress on your axle housings.
They'll also run a little cooler and be easier to valve on a vehicle that you're not going to be motivated to mess with the internal valving too much. The could also let you change settings a little for different conditions if you like.
I guess a lot of it comes down to how hard you want to drive and when you let up on the truck. There's lots of prerunners that are built to handle full race conditions and for good reason, when the race truck is waiting on a part 50 miles from you, the chase truck needs to be fast and durable.
Some advice on the length of the shocks, if you think you're going to have 14" of wheel travel (which is very likely in the front), get a 16" shock. Leaves are really hard to nail down exact limits on and are typically mounted narrower than the shocks by a good bit. Also, the bumpstops are usually mounted narrower on a leaf front and on a single wheel hit will move the shock farther than the spring and bumpstop. Basically leaf springs are really imprecise so give yourself a little leeway on the expensive shock to keep from topping or bottoming.

[subrunner]

Wow! That was some good info. Thanks. I eventually want to drive the truck as hard as it'll take. lol!!

[baja_driver]

Mainly just fooling around. I'd like it to work the best it can, though. One day(after it's caged) i may enter it in 1450 at an MDR race and just beat it up for 250 miles or so. Probably just be a big rolling roadblock,lol.


see if this is the kind of cage you want...if the pic's work, 6 beard seats with 3" four point all the way around, will try and find suspension pic's.

[subrunner]

Exactly, I've got that 1st pic saved on the comp for ideas. Just a matter of time and money, lol.:)

[ChuckH]

When i see a roll bar right next to peoples heads without padding it makes me cringe, more pre runners need window nets too, a friend of mine messed his arm up pretty good when he rolled his.

[blind655]

Here are a few pics of my cage.. I agree tho on the last cage, look like someone could slam their head or somthing else on tubes.
http://mikesoffroad.phpwebhosting.com/cage/P1010002.JPG
http://mikesoffroad.phpwebhosting.com/cage/P1010004.JPG
http://mikesoffroad.phpwebhosting.com/cage/P1010091.JPG

[subrunner]

Very nice! Now I just have to figure out how to fit 7 seats,lol.

[baja_driver]

its only looks close because of the angle of the pic..they are padded and you don't get anywhere near them when strapped in. ..

[SurferChris1]

I made a post about more 2.0 shocks vs. single 2.5 or 3.0 shocks a few months back on dezertrangers. I got very little good information. but it might be worth a quick read.

http://www.dezertrangers.com/cgi-bin/ib/ikonboard.cgi?;act=ST;f=12;t=11670;hl=bangedyoursi ster

[SurferChris1]

Alright I was thinking about if I was really gonna be happy with my 2.0 bilstein 1700's in the long run as far as dampening and cooling and whatnot, and I was seriously considering looking for a larger shock for the rear then i started thinking about just adding another 7100 so i did some quick calculations, for things like surface area and fluid capacity. and here is what i found. you multiply either the area or circumfrence by the height of the shock can and you get the volume and the surface area so these percentages should remain constant regardless of shock length, and I'm not taking into acount a lot of things like the thickness of the shock can and thickness (displacement) if the shaft, or the presence or lack of reservoirs, I was just trying to get some quick numbers.

I know most all you know this I'm just writing it all out so you can follow my thought process. or maybe its been a while since high school geometry...

a 2.0 has an area of 3.14 in^2
so 2 2.0's would have an area of 6.28 in^2
a 2.5 has an area of 4.91 in^2
and a 3.0 has an area of 7.065 in^2

A 3.0 shock only has 12.5% more volume to hold more oil than 2 2.0 shocks


a 2.0 has a circumfrence of 6.48"
so 2 2.0's effectively have a circumfrence of 12.96"
a 2.5 has a circumfrence of 7.85"
a 3.0 has a circumfrence of 9.42"

so 2 2.0's have 37.6% more surface area than 1 3.0 shock this is important to dissapate heat.



So anyways It looks to me based on this that a pair of nice 2.0's is pretty comprable to a 3.0 race shock, and 2.0's would be a lot cheaper, especially for me because I already have 2 of them and with how offen I see cheap 2.0's on race-dez calssifieds but I don't see anyone running this setup on trucks, I do see lots of buggies with multiple 2.0's in the back however. I would think in the desert too that if you blow a seal or mess one up you would still have some dampening on that corner, and you could afford to carry a spare where as a spare 3.0 is a lot of money to have in a spare part. I know pete albano's truck had 2 (very modified) 5100's on his front end before he recently went with coilovers and that truck was amazing.

the downsides I can think of are the cost/work of having to valve 2 more shocks when trying to dial them in, and having to fabricate more shock mounts, and I guess having to cut a bigger hole in the bed (or floor if your building an explorer like me)

So am I missing something, Is a double shock setup not as good as other set ups?

sorry for the long post...

[subrunner]

Comments?

[FABRICATOR]

a 2.0 has an area of 3.14 in^2
so 2 2.0's would have an area of 6.28 in^2
a 2.5 has an area of 4.91 in^2
and a 3.0 has an area of 7.065 in^2

A 3.0 shock only has 12.5% more volume to hold more oil than 2 2.0 shocks


a 2.0 has a circumfrence of 6.48"
so 2 2.0's effectively have a circumfrence of 12.96"
a 2.5 has a circumfrence of 7.85"
a 3.0 has a circumfrence of 9.42"

so 2 2.0's have 37.6% more surface area than 1 3.0 shock this is important to dissapate heat.


Not to nit pick...but something might be wrong with that assumption of 12.96” circumference for 2 – 2.0’s. 12.57” would be the circumference for a 4" piston. 2 - 2.0’s don’t equal a 4.0:)

[hoeker]

OK. time to skip all the geometry lesson, areas, volumes, etc. people on here know how mulitple shocks behave relative to bigger single shocks and can give you real world advice, ask your buddies. now let me give you mine. 2.0 shocks are fine for coil overs but unless your rules mandate the use of them they ARE a sacrifice for performance! i KNOW this because i raced a CORR stock truck that required 2" shocks with out resevoirs and the shock setup was ALWAYS a compromise and usually a very poor compromise! i ran as many as 4 shocks per corner and after 8 minutes they were faded to the point of almost no damping. my new truck has a 2" coil over and 3" bypas on each corner and the setup has been great. for a pre-runner i'd recommend a 2.5" shock per corner and add your 2 incher's to the RDC classifieds!

good luck

[WannaB-class5]

I will add to the last post a bit...
Its not a heavy burb but I used to have 2-2.0 on the rear of my bug and it did well. I recently went to a 2.5 and couldn't be happier. Both work but for the money and performance it was nicer to have one 2.5 (and its eaisier to mount)

[singlehanded]

I would go with whatever fits your application best and which ever shocks you can get parts for easily. I am going with king on all my new shocks and bumps and replacing my old shocks with kings eventually because I found it was very hard getting parts to rebuild my saw's and with king it is only a 15 min drive from my house. Just a lot easier for me.

[Stephen]

On the surface the numbers seem to indicate an advantage to multiple small shocks compared to on bigger one, but I'm sure there are lots of internal factors to look at that I don't know anything about. Shaft areas, port sizes, shapes and locations, margin areas on the pistons and probably even flow characteristics of oil in the tubes should make a difference. I know only enough to guess at some of the factors but I'm sure these all figure into the choice of shock sizes.

One guaranteed fact, if running a herd of 2.0's was better in any way, you'd see 10 per wheel on the trophy trucks if that's what they needed to go faster.

[tinman]

Two reasons a 2.5 is better 1.better piston flow(more oil flowing thru it) and 2.more oil in the shock itself.And remember its the i.d. of the body that counts and not the o.d..

[subrunner]

One guaranteed fact, if running a herd of 2.0's was better in any way, you'd see 10 per wheel on the trophy trucks if that's what they needed to go faster.
Good point!

[subrunner]

I guess I'll be going with 2.5"s, bypasses if I can find a cheap used set, smoothies if not.

[ntsqd]

Two reasons a 2.5 is better 1.better piston flow(more oil flowing thru it) and 2.more oil in the shock itself.And remember its the i.d. of the body that counts and not the o.d..

If the area of the 2x 2.0 pistons is greater than the area of a a single 2.5 piston then assuming the same everything else the 2.5's total oil volume is less, not more.

So if a 2.5 is truly better than a pair of 2.0's, and not just easier to fit in the space available, then it comes down to flow thru the oil ports in the piston.

[SurferChris1]

I will add to the last post a bit...
Its not a heavy burb but I used to have 2-2.0 on the rear of my bug and it did well. I recently went to a 2.5 and couldn't be happier. Both work but for the money and performance it was nicer to have one 2.5 (and its eaisier to mount)

I agree the bug handles great with that SAW on the back, but be fair man the "2-2.0's" that used to be on there were pro-comps. (Sorry man, I had to call you on that) But like I said. It's perfect with that 2.5" SAW.

I am planning on running 2 14" Bilstien 7100's per corner on my explorer. I'll let you know how I like it if this thread is still going in the next several months.

[WannaB-class5]

Well it was one Rancho and one Pro-comp! :D

[subrunner]

Ahh! So you let the secret out of the bag. Now I know what to do, I'm going to 4 wheel parts and buy 16 pro comps/ranchos(whichever's on the shelf) and go to town,lol. :D

[steveG]

Well it was one Rancho and one Pro-comp! :D

LOL, are either of those actually considered shocks?

[ntsqd]

Yea, it's what they do to your spine.