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Spring center pins

miniwally

miniwally

St. Javier
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Okay.

If you can't post and have a discusion like big boys and girls don't post.

I was trying to read some of young Averys post about his axle swap. I stopped at the part where he used a carriage bolt for a spring center pin.

Where as I don't agree with using a carriage bolt I started thinking more and more on this and I wanted to get "into" it some more.

I myself have sheared my fair share of spring center pins. This was brought on by me not keeping the u bolts tight.

SO.
If you don't keep your U bolts tight you break center pins. Duh.

But if the u bolts are tight why do you need a center pin?
I don't have the strength numbers but I highly doubt that a 5/16 or 3/8 Grade 8 bolt has anywhere near the shear strength to locate an axle for more than line up puposes.

I say the center pin is in place for holding the spring pack together, locate the axle on the springs for assembly and then becomes "not needed" when the ubolts are tight. I think the axle is held in location by the clamping force of the U bolts only.
This would bring about the point that a center pin needs only to be a graded bolt. Ie. Grade 5 or better.

Discuss like grown ups please.
 
miniwally said:
Okay.

If you can't post and have a discusion like big boys and girls don't post.

I was trying to read some of young Averys post about his axle swap. I stopped at the part where he used a carriage bolt for a spring center pin.

Where as I don't agree with using a carriage bolt I started thinking more and more on this and I wanted to get "into" it some more.

I myself have sheared my fair share of spring center pins. This was brought on by me not keeping the u bolts tight.

SO.
If you don't keep your U bolts tight you break center pins. Duh.

But if the u bolts are tight why do you need a center pin?
I don't have the strength numbers but I highly doubt that a 5/16 or 3/8 Grade 8 bolt has anywhere near the shear strength to locate an axle for more than line up puposes.

I say the center pin is in place for holding the spring pack together, locate the axle on the springs for assembly and then becomes "not needed" when the ubolts are tight. I think the axle is held in location by the clamping force of the U bolts only.
This would bring about the point that a center pin needs only to be a graded bolt. Ie. Grade 5 or better.

Discuss like grown ups please.
Click to expand...

I am not sure why it is there. Later when Avery removed his carriage bolts they were warped and in danger of failing. I use 3/8-16 grade 8 allen bolts for my centerpins as I can get them really tight and I have had no issues with them at all. The threads are not even rolled where the leaves have contacted them through flexing. I have wheeled it with those pins and had no problems. I think even if you torque your ubolts there is not sufficient friction between the spring and axle to stop axle movement under heavy load. I feel it may be there for when the vehicle is under max load with max rotational force on the axle not for everyday cruising the street.

Just my .02

Ira
 
I don't have any true testing or technical information right off hand, but simply based on the design of u-bolt setup there is probably still some force being transferred to the center-pin. A little with axle location, and mostly with keeping the spring pack together (my speculation). With all the flexing and large amounts of force being sent through the springs I would guess that you could start spitting out leaf springs even if the u-bolts were tight.

I believe there was a paper published via SAE that talked about leaf spring suspension design in which I will try to review if I have time.
 
We need an enginerd to come up with some shear strengths and clamping forces.

I just can't see where any strength of 3/8" bolt is going to locate be able to take much load in shear.

:dunno:
 
A center pin pulls double duty keeping the pack together and the axle in the proper location. Without the center pin, the axle would be able to "walk" in all directions. Also, axle wrap would push the springs out and let the axle move front to rear.
 
Last edited:
I'm not going into storage to dig out my old M.E. textbooks, but I'll tell you this:
I bought an 02 F250 4x4 pickup which had been hit on the right front, not hard, just sheetmetal damage. Got all the repairs done, took it for a test drive, and it wandered all over the road. Take it in for alignment, and checks out fine. Test drive, still wanders like crazy. Finally found the culprit: broken center pin on driver side spring.
The center pin is not only a "holding" fastener, but also, and more importantly because we're talking about moving vehicles here with human occupants and innocent bystanders, a "locating" fastener. Use only center pins or grade 8 socket head capscrews for replacement. Anything less is dangerous.
Hope this helps.
Jimbo

edit; Leper beat me to it, I just expanded a bit
 
So why the discrepancy in grade 8 cap screws or centering pins?

My dad just went through this. Extremely scary death-wobblish symptoms just appeared in his truck. Felt like it had 4 wheel steering that was haywire. Apparently (and luckily) off-road in easy conditions, the leaf spring finally slipped on the axle perch, and he was able to stop without damage. The pin was sheared. (we're talking 1974 parts here)

He went to get another pin at the parts store, all they had were the universal long ones that he needed to trim. About 2 seconds with a hack saw and he was through the pin. Certainly doesn't sound like they are grade 8.

I'm not arguing or anything like that, trying to work through this. Kind of odd if they are that critical that a tougher metal wasn't used. I suppose shear strength grade 8 vs. 5 (for instance) is going to be more critical as under most conditions the bolt is doing very little clamping work?
 
I read a thread somewhere (pirate maybe?) about steering arms on a 60. It sounded like the consesus was this:

The friction between the steering arm and the knuckles takes up the majority of the shearing force. The bolts/studs do take up some of the shear force. From an engineering standpoint, this makes sense to me but it isn't how calculations are generally made regarding factor of safety, etc.

I imagine the center pins are much the same way and I bet Avery's center pin bent because the u-bolts were loose. Still, if you are doing any kind of serious wheeling I wouldn't run anything but a grade 8 center pin. My .02
 
So here is All Pro offroads Take on it

Spring Pins Leaf spring center pins are designed only to locate the spring on it's perch. Center pins are not designed to hold the spring in place. It is imperative that the U-bolts be properly installed, and tightened to prevent failure of the center pin. U-bolts should be checked for proper tightness before and after each off road trip and at 5,000 mile intervals. Damaged U-bolts should be replaced immediately.

Directly from their website.

I have been trying to find the grade of somebodies center pin but can't right now.

I think all major manufactures probably run a Grade 5. They cut way to easy like dyeager535 said.
 
Center pins aren't usually grade eight in automotive applications, only when you get into big trucks. We seem to be forgetting the clamping force/ability of properly installed u-bolts.
Again, the center pin is not a critical part, but in conjunction with other improperly adjusted/maintained parts, can become the straw that broke the camel's back.
Jimbo
 
I'd have to say that they are a critical part (I doubt that's quite what you meant by your statement, just clarifying:)) as even without immediate failure, the symptoms of the failed piece were evident in a setup that had otherwise been completely reliable for 30 years. So there is no question the u-bolts were tight, nor that there were problems elsewhere to cause *his* particular pin failure.

It was quite surprising to me that the leafs stayed in place as long as they did with the pin gone, especially given how many months it went with weird steering symptoms (not driven much of course over that period) before finally giving out to the point you could trace the problem.

I know GM likes to save cost, but you'd think something like this would be stronger given the criticality of it. At least IMO. After riding in his truck while it was doing it's thing, I can only imagine how bad things COULD get given slightly different circumstances.
 
Not an engineer and I didn't sleep at holiday inn last night but suppose the center pin is a failsafe in place to compliment the u bolt clamp force?

Would it be reasonable to think 95% of the locating force is provided by the u bolts and the remainder is from the center pin? Sure, friction can do alot but it doesn't hurt to have that little extra holding force.

Personally, they make grade 8 center pins so that's what I use. Most of CK5 tends to err on the side of overkill and I'm no exception. Better to do so than cut corners and risk catastrophe.

I for one am very glad the brotherhood takes safety so important that we are even having this discussion and that so many feel so strongly on the subject. Most modified cars these days don't have quite the destructive force our rigs typically have. High horsepower, heavy, with enough clearance to overrun a family sedan demands this level of attention to detail. I only wish all automotive mod communities followed our lead.

I think the ultimate answer to this question is this:

"Why run a grade 8 center pin? Because we can and it's the responsible thing to do."
 
Here is the most that I have been able to find.

A U-bolt serves in four critical related roles: • Primarily, the truck u-bolts provide the force required to clamp the leaf spring and related components firmly together. In addition to the leaf spring, these components include the top plate, axle seat, axle and bottom plate.
• The properly installed u-bolt eliminates any flexing of the leaf spring in the area between the u bolts. This is particularly critical since the hole for the centerbolt in each leaf acts as a stress concentration which would lead to rapid leaf breakage if spring flexing was not totally eliminated by the U bolt clamping force.
• By firmly clamping the spring to the axle seat the horizontal forces acting on the centerbolt are greatly reduced which in turn prevents shearing of the centerbolt.
• Proper clamping of the spring by the truck u-bolts provides the desired spring stiffness and contributes to maintaining the vehicle ride height and handling characteristics as originally specified for the vehicle.

So the real story probably is that the clamping force of PROPERLY torqued u bolts is what really hold the whole assemble together. The center pin is a fail safe type deal that takes some locating loads but not much.
 
Edit: math error

Without the center pin, you are relying strictly on friction between the leaves to keep them located (fore and aft). Lateral motion is (mostly) contained by the U-bolts.

You are going to get about 24,000 pounds of clamping force by tightening one 3/4" U-bolt to 200 ft-lbs. This is an estimate, but should be close enough. Fc = T/(k*D) where Fc is clamping force, k is the thread friction coefficient (0.2 for a dry steel bolt), and D is the bolt nominal diameter (.0625 ft). 16,000 pounds is the clamping force for one leg of the U-bolt, times 1.5 for two legs as it's not quite additive.

We'll say that the spring steel has a break-free (not sliding) friction coefficient of about 0.5 (this would be common for a rough-yet-hard surface). Then we'll go ahead and say that the two U-bolts are close enough that their combined clamping force would be around 1.5x a single U-bolt. So now we've got ~36,000 pounds of clamping force. That means there's about 18,000 pounds of friction force between leaves (Ff = 0.5*36000). For normal use, they're probably not going anywhere without the centering pin.

When you introduce axle wrap, however, the whole spring pack wants to form an "S" shape. This is where things get funky and you start breaking stuff. The leaves will want to translate against eachother and introduce shear loads into the center pin. Even if they don't translate relative to eachother, they will certainly want to translate against the spring perch. A Grade 8 center pin will fail in shear at around 93 ksi, which translates to about 10,270 pounds being applied at a right angle to the bolt axis. The big load comes not from torque being applied via the gas pedal, but when things snap back to their normal shape (the "rebound" impact load).

A carriage bolt would offer essentially no extra shear loading ability as far as keeping the springs where they should be. A grade 8 would offer about a 57% increase in loading ability (an extra 10300 pounds over and above the friction force of 18000 pounds due to clamping).

Note that this is a pretty simplistic look at the problem, the real dynamic system is much more complex. Among other things, I'm not considering malleability, just ultimate shear strength.
 
All I know is that the center bolt is there for a reason and I believe that its important to locating the spring on the axel. Now for my personal experience:

I was driving behind my friend last year and back then my truck was was lifted with 4" blocksback then. And my friend stopped short at a stop light and I had to slam on my brakes, and i felt the rear brakes lockup, then as I have almost come to a stop I hear a bunch of pops and then a clunk. I get out to see what had happened and it turns out that my driveshaft pulled out of the slipyoke on the t-case. So in the middle of traffic I have to unbolt the driveshaft from the diff and I had to drive home in 4wd. So, after I got home I found out that both my rear leaf spring broke at the front eye, which caused the axel to wrap backwards and pull the driveshaft out. It wasn't until I replaced the springs that I found out that the pins on the lift blocks also sheared and the whole axel appeared to have moved backward along the springs by about a cm or so.

So I replaced the original stock rear springs ontop of blocks with brand new 4" lift springs. I keep the u-bolts tight too.
 
I have spent a fair amount of time searching on websites today and have not found a single one that provides the grade for their center pins.
I think grade 8 is overkill. JMO.

I would just go to the hardware store and get grade 8 allen head screws and use them.
I bet that nobody sells a Grade 8 Spring center pin.
 
One other thing to think about: what the engineers came up with on leaf spring design has been around for generations; the leaf spring goes way back to whatever year they started them under cars and trucks.
What I'm getting at is the pins (whatever material they used back then and today) were designed for the springs to the vehicle in which they were mated to. Until such said vehicles were in wrecks, etc., then pins may or may not have been sheared, depending on if such wreck occurred in the front, etc.
Now you fast forward to today, to the off road world: rock crawlers, mudders, sand runners, etc., what kind of forces are applied to the pins on this type of driving? Simple. A lot more than what the daily driving cars and trucks were succumbed to. All these off road duties we throw at our beloved trucks probably were not designed in mind by the engineers back then, nor today either.
I would say a lot of center pin failure is to blame solely on the owner/driver. Failure to use the correct material (hence Avery's problem), failure to torque u-bolts to the recommended specs, failure to ____________ (fill in your answer), etc., etc.

Me, personally, would rather employ grade 8 pins in leaf springs. If any other grade were more readily available to the general, buying public, I'd ask for aircraft grade 12 bolts to use. Even though they may have a higher tensile rating, their shear strength is going to be a lot more than grade 8's. I know the grade 5 will flex more before it shears where the grade 8 will shear before it flexes more, the grade 12 should be able to outdo either the 5 or 8 in both flex and shear strengths.

The big question now is, where does one obtain or buy grade 12 aircraft fasteners?
 
i just make my own get a regular grade 8 bolt and put it in a lathe and turn the head round until it will fit in the hole corectly
 
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