Shock absorbers are overrated. So are springs for that matter.

So 3 miles into Nawahtzel the lower shock mount breaks off the trailing arm/knuckle on Dave Hintz's BMW. That not being quite enough to make him stop he keeps going until the coilover gets torn off and parts are strewn down the stage and there is all kinds of smoke from the tire rubbing. He calls me on the cellphone and tells me to bring the trailer to the start of the stage. There's a real danger that if we don't get the car out in time it will be stuck in there after they shut the forest down at 1 pm. By the time I get there Rick has hiked a quarter mile up the stage and retrieved most of the pieces of the shock. Not all, though, the spring top hat and the tender spring separator are still out there some where. He calls me back and says they found a chunk of wood to wedge in there and are driving it out! They end up driving 17 miles of Nawahtzel this way and finish the rally.



Shock parts. Most of them, anyway.



Broken shock mount.



Broken knuckle.

proof that the Hintz Brothers are awesome.

---

-Tom
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Tom B
proof that the Hintz Brothers are awesome.

And that cast iron in single shear isn't.

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

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john vanlandingham

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Tom B
proof that the Hintz Brothers are awesome.

And that cast iron in single shear isn't.

Yup, this is the number one part of the car that has given me the most concern and it finally did (perhaps predictably) fail. Derik Nelson is already in the process of making custom chrome moly tubular trailing arms with double shear shock mounts. The stock cast iron arms are heavy as shit anyway. In the meantime we'll go to the separate shock/spring in the stock location setup which should put less stress on the lower mount.

Even with the car on custom Douglas Fir suspension it took us a while to pass them! You guys rule!

---

Robert.

"You are way too normal to be on Rally Anarchy." Eddie Fiorelli.

New from Blammo, Log Rally Suspension!

---

Andrew Steere
Lyndeborough, NH
KB1PJY

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Doivi Clarkinen

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john vanlandingham

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Tom B
proof that the Hintz Brothers are awesome.

And that cast iron in single shear isn't.

Yup, this is the number one part of the car that has given me the most concern and it finally did (perhaps predictably) fail. Derik Nelson is already in the process of making custom chrome moly tubular trailing arms with double shear shock mounts. The stock cast iron arms are heavy as shit anyway. In the meantime we'll go to the separate shock/spring in the stock location setup which should put less stress on the lower mount.

All the stress comes from the damper.
Spring only transmit the force it sees, the damper charges it---and the only change any damper makes is to slow things down,
and that is where the stress comes from..

So if the spring is here or there , a coil spring a torsion rod or an old wet noodle, it is the degree---how much it slows things down that stresses mounts.

And usually its compression damping, specifically high shaft speed compression damping that if wrong, is hardest on mounts.

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

This post was so entertaining, I shared it elsewhere. Amongst the oh wows and awesomes were two I felt I should share back

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cjonesy22002;153633403
never can tell what you wood have to do if you were in the same situation

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Trouser Trout;153633589
They should branch out and create their own suspension division.

That would give other guys that offroad their BMWs treemendous support

John,

I've seen you explain this before, but something is amiss...

I'm no engineer... but if a spring has a rate of 100 lbs/inch, then to compress it we add weight. So when then car goes up in the air (off a jump), and falls back down, it compresses the spring beyond static ride height (lets say 4 inches of compression for 400lbs) correct? so to do that the "falling car" is compressing the spring lets say another 4 inches so an extra 400 lbs have been transferred to spring perch, meaning the load on the perch has been doubled.

I understand that the damper is dealing with a lot of that load, but it is just slowing the time over which that load is applied. There is still increased load being applied to the spring perch, because otherwise, the spring would not compress.

Correct?

Matt

Edit...

Oh so my point is when you add a coil over to what was once just a damper mount, the mount is seeing more loading... substantially more loading...



Edited 1 time(s). Last edit at 09/25/2012 10:21AM by Mad Matt F.

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Mad Matt F
John,

I've seen you explain this before, but something is amiss...

I'm no engineer... but if a spring has a rate of 100 lbs/inch, then to compress it we add weight. So when then car goes up in the air (off a jump), and falls back down, it compresses the spring beyond static ride height (lets say 4 inches of compression for 400lbs) correct? so to do that the "falling car" is compressing the spring lets say another 4 inches so an extra 400 lbs have been transferred to spring perch, meaning the load on the perch has been doubled.

I understand that the damper is dealing with a lot of that load, but it is just slowing the time over which that load is applied. There is still increased load being applied to the spring perch, because otherwise, the spring would not compress.

Correct?

Matt

Edit...

Oh so my point is when you add a coil over to what was once just a damper mount, the mount is seeing more loading... substantially more loading...

No the SPRING itself does not "add' and force. The weight x speed x time factor in the shape of the bump create the stress. (ie a tall but round and looooooong bump is one thing, a short pot hole is another thing, time element in the pothole is "100% force instantaneously" )
If it "this much" stress. the spring just compresses "that much"..

Example:---bear with me--- you weigh XXkg and I'm going to punch you.....
My fist has M mass, swung at V speed making "bazillion newtons"..
Now as long as there is instant movement, its not going to hurt your chin..

Let's say you're sitting on a office chair with Sooooooper Bitchin Casters.
As first begings to contact chin, you simply roll away---more or less.
If your head is against the wall---and thus cannot move---the force applied to chin is going to be applied to chin, and it will hurt.

The shocks change energy into heat, thus it is them, the shocks, that
generate the stresses because it is the shocks which resist movement, the springs don't resist , they collapse (to whatever degree they need to)

Carry the spring rates to absurd degees---aburity as usual being a great tool---
you have a 300 pound/inch spring. Place 3 lbs on it---it delflects 1/100 of an inch

Take a 2000 lb/in spring (you have to imagine a fixed thing like the bodyshell, and a moveable part like a swing arm)on a 2000lb car and push with 4000 lbs. it just moves 2"...

Now think about a damper with ranicid fish oil in them that's watered down with used Asahai beer, ya know like the things on the mighty Justy.
They don't stress anything cause they totally limp..

Now thing of the auto-crosser's wet dream with so much compression damping that it does not move a 1/10mm even when you drive the car off a freeway ramp like the Nazis did at the end of the Blues Brothers, an epically great film.
Regardless of the spring, that shock is going to stress the hell out of the mounts

Aagain I emphasize it is the high shaft speed part of compression damping that is the big booogy man.

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

I'm not familiar with the normal E36 rear end. Can you guys do something like this:



And....curious if you think what we've done would be enough to avoid a similar fate?

Epic fix btw!!!

Dave

Dave, no there is no good way to do a similar fix on the E36. The whole arm is cast iron and welding would be problematical plus there isn't anything else near that shock mount to weld to. That's why the E30 rear suspension is actually better suited to rallying.

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john vanlandingham

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Doivi Clarkinen

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john vanlandingham

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Tom B
proof that the Hintz Brothers are awesome.

And that cast iron in single shear isn't.

Yup, this is the number one part of the car that has given me the most concern and it finally did (perhaps predictably) fail. Derik Nelson is already in the process of making custom chrome moly tubular trailing arms with double shear shock mounts. The stock cast iron arms are heavy as shit anyway. In the meantime we'll go to the separate shock/spring in the stock location setup which should put less stress on the lower mount.

All the stress comes from the damper.
Spring only transmit the force it sees, the damper charges it---and the only change any damper makes is to slow things down,
and that is where the stress comes from..

So if the spring is here or there , a coil spring a torsion rod or an old wet noodle, it is the degree---how much it slows things down that stresses mounts.

And usually its compression damping, specifically high shaft speed compression damping that if wrong, is hardest on mounts.

Yes, I knew this would come up and the shock is putting the greatest loads into the mount but not having the mount holding the entire car up (well, ok, just that corner) should certainly make it last longer. For example, if the shock mount did break off again having the spring in the stock location would prevent the whole suspension from collapsing to the ground. You would be bouncing around back there but you wouldn't have to stop and find a piece of wood to prop the suspension up.

However, it's wrong to completely dismiss that with a coilover the lower shock mount is also supporting the weight of that corner of the car.

I can't believe that BMW would put that in single shear. Dumbfounded.

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krisdahl
I can't believe that BMW would put that in single shear. Dumbfounded.

You better believe it. It's an old Germanski fad... and their faithful imitators the Japonaise did the same on many cars, at least they hhung big herking 20mm pins (which meant they needed bogger herking bottom eyes on the bottom of the shock.
Old 2002--I think
cf Eclipse/Gaylant
plenty-o-Nissans

Ford Focus hangs their shock in single shear with a miserable little M10 I think, I think..

Works fine for grocery getters...like the ultimate driving machine..

That looks like a serious problem

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.