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Old 02-02-2020, 05:03 PM   #1
maxitoman007
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Default Why Do 7/9 Rear Subframes Twist?

I recently twisted my subframe and snapped the upper torque rod in my Volvo 740 turbo. I just can't seem to understand the mechanical reason it happened. I don't understand what force is actually twisting the subframe. Could somebody help me visualize this? Its been bothering me.

How does a setup like this:


Cause something like this?
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Old 02-02-2020, 05:19 PM   #2
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That subframe contains the torque reaction of the rear end. When you are "banging gears" in a manual car, when your tires have grip and all that torque reacts with that tire applying the torque to the road, the subframe & those cast "dog bone" links must handle the torque (think equal and opposite, for the torque you apply to those tires).

So for every full torque application in lower gears (lots of multiplication from the trans & final gearing) one of those dogbones is in tension, the other compression. What you are showing is a "buckling failure" of that dogbone due to the big torque reaction it must transfer to the chassis.

Have a look at BNE's website and you will see how substantial those 7/9 subframe torque roads that he builds are. Not an aluminum beam..... they have higher buckling strength that what Volvo designed.

Yoshifab has a steel plate reinforcement kit that you can "box" that subframe with. I did mine, even tho my 9 series is an auto..... I can't apply the same torque as a manual with those steep 1st gearing sets.

Your car a manual or auto?
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Old 02-02-2020, 06:57 PM   #3
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*Weak* design that isolates some NVH from the cabin.
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Old 02-02-2020, 07:11 PM   #4
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It twists because the only thing fighting twisting forces like from large side loads are the lower trailing arms. The subframe does resists some side forces but it's central location is more like a pivot point than something to resist side loads.

With your lifted car. Not only have you increased by a large magnitude the back and forth torque reaction. You have also greatly increased the side twisting forces. An example of these large forces are one tire on a much higher location in the dirt than the tire on the other side. As mentioned it is a very good design for noise, vibration, and harshness. But not as good a design to resist twisting. It's actually pretty good at the torque reaction.

Also like I suggested before. Having a much lower/higher numerical rear dif gearing will reduce the forces on back and forth torque reaction.

PS the panhard bar is supposed to help side loading and keeping the axle centrally located during suspension travel. But you need a longer bar for large travel and to have that travel start with a centrally located axle.

Last edited by dl242gt; 02-02-2020 at 07:13 PM.. Reason: add PS
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Old 02-02-2020, 09:35 PM   #5
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Quote:
Originally Posted by DET17 View Post
That subframe contains the torque reaction of the rear end. When you are "banging gears" in a manual car, when your tires have grip and all that torque reacts with that tire applying the torque to the road, the subframe & those cast "dog bone" links must handle the torque (think equal and opposite, for the torque you apply to those tires).
I initially didnt understand what you were saying, however after looking at this diagram:



And imagining the propeller as the wheel and the plane body as the axle, it makes sense now the compression/tension you explained.

The part I'm still having issues understanding is why the "ears" of the subframe often twist out of shape. almost like wringing out a towel.

Last edited by maxitoman007; 02-03-2020 at 03:46 AM..
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Old 02-03-2020, 12:34 AM   #6
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That torque link is designed for maximum luxury NVH specs for hauling grandma to church.

It was not designed for dumping high loads into it or altered ride heights, and especially a manual trans or de-accumulatorized automatic.

The short links have a lot of leverage against the trellis and only have a short range of travel before things get wierd.

I'd try to build a torque arm that is close to the same length as the rear driveshaft, however it will transfer more road noise to the body.
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Old 02-03-2020, 02:38 AM   #7
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Quote:
Originally Posted by maxitoman007 View Post
I just can't seem to understand the mechanical reason it happened. I don't understand what force is actually twisting the subframe. Could somebody help me visualize this? Its been bothering me.

How does a setup like this:


Cause something like this?
Here is a possibility:
See how the panhard rod is connected to the body left side, and the axle right side? So you car is already lifted, pulling the axle over to the left, even while at rest. The more the suspension goes into rebound, the further the axle gets pulled over to the left. Add aggressive cornering forces coupled with bumps/ruts/oversize tires/braking/acceleration/body lean and something gave out.

Notice how that dogbone is bent over to the left also, where it snapped? Axle was forced way over to the left when it encountered violent forces far in excess of what it was designed for.

Since you are getting serious in your game, you have found some deficiencies with the rear suspension. You are receiving some good suggestions, several solutions you can implement to improve the system. Until that uncovers the next deficiency waiting somewhere else. Motorsports philosophy.
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Old 02-03-2020, 03:53 AM   #8
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Quote:
Originally Posted by Dirty Rick View Post
The short links have a lot of leverage against the trellis and only have a short range of travel before things get wierd.
Not sure I understand what torque links you’re talking about. Like the torque rods you mean? Also not sure what you mean by the trellis. I’m sure your explanation makes sense, I just need to understand the terms being used.
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Old 02-03-2020, 04:33 AM   #9
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standard shockup stroke?
seem way too long til torque can bent the rod.
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Old 02-03-2020, 11:03 PM   #10
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Quote:
Originally Posted by zhor View Post
standard shockup stroke?
seem way too long til torque can bent the rod.
not sure I understand what you're saying...
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Old 02-03-2020, 11:08 PM   #11
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The important thing is to box them. They are known to bend and we have a known fix.
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Old 02-03-2020, 11:34 PM   #12
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Time to read this thread. There is also another one with a lifted wagon but I can't find it yet. https://forums.turbobricks.com/showthread.php?t=352784
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Old 02-04-2020, 02:08 PM   #13
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The important thing is to box them. They are known to bend and we have a known fix.
That's the plan.
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Old 02-04-2020, 05:38 PM   #14
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The important thing is to box them. They are known to bend and we have a known fix.
While boxing them helps, the real real fix is to also solidly attach them to the body of the car. What will start happening when the car gets abused is that the subframe will start to pull out of the rubber bushings it is mounted in and then is has less support and gets twisted up.
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Old 02-04-2020, 06:00 PM   #15
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Yeah the fingers part is a weak point where the subframe goes into that dogbone part that is bolted to the rear of the car and the rear of the subframe slides into those rubber bushings. Linuxman eliminated those rubber bushings. Maybe he can post pics of what he did. He posted pics on facebook. Not sure if that helps here, maybe they are here in his build thread. Here is my subframe thread.

https://forums.turbobricks.com/showt...ghlight=sbabbs
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Old 02-04-2020, 06:14 PM   #16
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Like another user has pointed out, track rod (aka panhard bar) is displacing your axle by L*(cos(theta1)-cos(theta2)), where L is the track rod length, theta1 is the stock angle the track rod makes with the horizontal plane (should be close to 0), and theta2 is the new angle your track rod makes after the vehicle is lifted. Now you have static stress in the links because of the horizontal displacement. Alone, it's probably not enough to bend your torque rods, but it has now reduced the overall F.O.S. of the link.

(Notice how the track rod pulls the axle towards the driver's side of the vehicle when you lift the suspension height. Keep that in mind for later)

Combine this now with the forces brought onto the torque rods by axle articulation (youtube videos of solid axle articulation if you're unfamiliar with the term/concept), and you'll have a situation where the rear (AKA side connected to the axle) of the lower torque rod is pushed severely towards the driver's side of the vehicle whenever the driver's rear wheel is lifted and the passenger rear wheel is lowered (clockwise axle articulation).

The amount that the rear end of the lower torque it is displaced is equal to [track rod induced displacement] + [articulation induced displacement]
...Or:
[L*(cos(theta1)-cos(theta2))]+[d1*(1-cos(phi))+d2(sin(phi))]

where phi is the angle the axle makes with the parallel, and d1 and d2 are geometric locators for the lower torque rod.

hopefully this diagram helps visualize how the torque rod gets displaced and can eventually fail due to rubber bushings not letting it articulate enough




based on the photos of the failure, I'll bet you were turning left and maybe hit a bump with your driver's side rear wheel.

I'd also bet that you could get away with not reinforcing the subframe if you converted your torque rod bushings to heims and optimized your track rod length.
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Old 02-04-2020, 08:36 PM   #17
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Yeah that's one idea another is the reason the sub frame breaks is the rear end twists and it breaks. Nobody really knows what breaks them. It twists turns the pinion angle down or up and that makes the subframe give up the ghost. Maybe put a camera under your rear and go do some burnouts with your manual trans stock rear subframe turbo 740 and post the video of you breaking one and then I'll believe your theory.

Last time my buddie hit a curb going sideways it broke his axle end where the wheel bolted onto it... 1978 gold on gold 10th anniversary trans am. Or it breaks the wheel or the studs..

Easy to bet on other peoples cars instead of betting on your own...

Neat pic of your 240.. Break something on that, anything and post back what you broke..

Me, my bodies broke from first time I ever drove to high school hit black ice and slide into a bus sideways. Lost my b-balls shot. I coulda been the next Ainge.. Had to play pac-10 footballs instead.. Now I just hit the dirt on my dirtbike, with all the gear of course. I mean who plays ball with out the outfits?


If you look at all the busted sub frame pics from the last 20 years they mostly bend and break at that finger spot...

Sure I had 240's for years and years way back in the 90's, know why? Cause I couldn't afford any 740 or 940 turbo.. But then I was given one. What a difference. The auto doc where i worked kept telling me you should be racing these volvo's instead of those 351c fords..

Course this guy wanna jack it up and wonder why it breaks, most of us lower it and wonder..

Cheapest easyist is just weld it up and then try and break it. Best to get IPD adjustable panhard also. YEah and those kerplunky rods or full kerplunky.

Last edited by sbabbs; 02-05-2020 at 04:12 AM..
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Old 02-04-2020, 10:20 PM   #18
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On my list of parts coming in the mail are the Kaplhenke adjustable Torque rods and Panhard bar. Also going to get a buddy to brace up my new (junkyard) subframe and hopefully we should be good to go.

On a side note, a CD009 is also getting swapped in.
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Old 02-05-2020, 01:59 AM   #19
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Quote:
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Easy to bet on other peoples cars instead of betting on your own...
Very true

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Neat pic of your 240.. Break something on that, anything and post back what you broke..
I'd be real surprised if I do. The 4-link design with heims/johnnys should keep me out of trouble for anything that I'll be doing... but then again I'm not going fast




My doors were opening and closing all weird when I took that picture due to torsional strain on the chassis
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Old 02-05-2020, 04:16 AM   #20
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YEah fun stuff. I remember taking my 240 wagon up to high lakes to fish on crazy roads to get there. Amazing what stock 240 will do. Even crazy was my buddie in his diesel 240 made it. Back in the 90's of course..
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Old 02-06-2020, 07:03 PM   #21
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So is the damage usually associated with stick shift models?

Or will a AW71 slush box B230FT with 20psi boost bend the sub-frame? (Lowered but not slammed 760) #nodrifting

I'm replacing bushings, so now is the time to weld it up before it gets bent. If it'll likely bend it.

I felt the rear end squirm last time i drove it, jacked it up today and both the torq link rear bushings are showing signs of age and the top one is split.
Everything else looks good, already has an adjustable pan hard with rod ends.

Last edited by Dirty Rick; 02-06-2020 at 07:26 PM..
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Old 02-06-2020, 07:38 PM   #22
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I would reinforce it and I think you can get poly bushings for the fingers. That should set it up for more better fun.
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