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Rear Axle "Torquing" Up and Down?


bmzero

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Ever since singling, when the truck is accelerating the rear end torques up and down when going through the gears. This is even more evident when hooked to the trailer. It's pretty unsettling when the rear abruptly drops when changing gears.

 

Those who are running a single in the rear, do you have a similar experience with yours?

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Zero,

 

How far does the axle travel in inches?

 

Have you placed a cam under the frame to observe that yoke angle movement?

 

What is your static airbag position?

 

Drive on.......(No hop......)

 

I have not put a camera on it, but I suspect it's moving quite a bit due to the feel. The next time I go out, I'll probably put a camera on it.

 

The ride height is set at 8.5" from the weld line on the housing, per Volvo sepc.

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I have the Pete air leaf they tend to jump around a lot with torque input especially when we're running empty without any plates/boxes on the deck. The new KYB shocks helped with it as did adding another 2-3K on the bed.

 

Have not been under a Volvo as new as yours, so have no clue what kinds of angles your links operate at. Maybe it's time you fire up the auto cad and design your own 5 link!

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So let's see...10k plus on front axle. Somewhere around 5 or 6k on the rear axle with no bed and singled mid with that long driveshaft. Suspended by air bags that really aren't doing much as far as weight handling goes. Lots of torque as your iShift does its job going through the gears. I'm not any kind of enginneer(I did stay at a Holiday Inn recently) so I am not qualified to even guess but I bet others can help you out. It will be really interesting reading. Would be an awesome round table discussion at Hutch.

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Ha ha. It's all I can do to not be doing that exact thing right now.

 

I want to understand the problem first, though.

 

Basically, I have the original rear setup under the truck now.

The pics I found on the net of what I assume is under your truck is a similar design to the Airleaf. Just by design it will raise the frame under torque and lower with negative torque. All you can do is damp the oscillation with good shocks. I assure you it will get better when you build your bed.

 

I'm thinking your air hitch is probably making the lifting more noticeable as it's isolating the 5er. If the 5er were rigid to the truck, it would likely stop the truck from lifting as much when the torque spikes.

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Seems normal to me. Every truck I've owned that's had an air ride suspension, does that. Current KW T600 does it, Pete 387 TC with tandems did it, and all three Sportchasis did it when you abruptly released the throttle. Seems like, the more weight over the rear axle, the more it does it. I see loaded semis do that all the time. I don't think it's an issue.

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Did you flip the torque bar over when you moved the axle forward?

 

I don't believe this suspension system has a torque bar, as I understand it. It does, however, have a panhard bar, and that was relocated.

 

In my opinion, adding weight to the bed only masks the issue. The more I study this, the more I realize the torque wrapping just a side affect of the suspension design Volvo uses. In the commercial truck world, especially with tandem axles and the limited packaging options, there aren't a lot of options. There are even less options capable of supporting a high weight capacity. So, that being said, I don't think this is necessary a "flaw" in the suspension design. I just think that we are using this system in a way that it was not designed, or at least I am.

 

I spent some time today crawling around the local Volvo dealer's lot and even drove a new I-Shift 780 for comparison. The new 780 suffers from the same issue as my truck, so I don't think it's a setup issue.

 

You'll have to excuse the dirty truck. I haven't had a chance to wash it from our trip to the beach last weekend.

 

Volvo design - basically, a two link plus panhard bar design:

97da26d3-675e-43f2-b766-8177d935b572-s.j
480501ea-5dd1-45ff-8449-594cf4f820bf-s.j
In this design, the only element protecting against the wrapping is the weight on the rear bags. That's why some say it's better with the bed on it, and why the commercial world doesn't care about this issue.
These were taken from a Mack, sitting right next to the Volvo trucks at the dealer. This system *should* all but eliminate torque elevating and in my opinion is a more "proper" suspension design for what I am using the Volvo. I was not able to drive the Mack due to work obligations, but I will when I go back to pick up my truck on Thursday.

 

 

A few things I like about the Mack system that I think would be beneficial in my use of the Volvo:

  1. No "helper" spring. I suspect that the Volvo system relies pretty heavily on the helper spring and hits it a lot sooner than it needs to (need to verify with camera), taking weight off of the bags. In a commercial use, this is probably good. However, in my use, I suspect it alters the ride quality more so than it should. I would like to see more travel out of the rear and have it "float" more.
  2. The upper triangle link (not a true four link), should eliminate the torque issues as well as axle "wrapping" as it's commonly called in the light truck world. Most light truck manufactures handle this with a torque arm.
  3. Adjusting pinion angle. In the Volvo setup, the only way I see to adjust the pinion angle is to change out the spring pads or raise/lower your bolt holes in your chassis. I don't know if changing out the spring pads is even an option. I don't see a need for this once the proper angle is determined, though, so this is probably a non-factor. The axle is designed to run/lubricate at a specific rotation, so I doubt we would benefit from changing that angle anyway, but just a thought.

Tony, it's inevitable, Solidworks to the rescue!

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Yup--all in the weight. I have a '97 WIA I use for hauling grain. 80K load, you don't notice that at all! Unloaded with a aluminum trailer, moves up and down real good! The first time I pulled onto the scale empty and got out of the truck to hear a loud hiss, I got worried! Just the ride height valve releasing air after the rear end went up 3" during the stop.

 

You gotta think about it right. Thats POWER man--thats how to tell its running right! Like a '68 OLDS with a Rocket 350...the left front fender is supposed to go up 6" when you step on the gas! :D

 

Seriously doubt it bothers your trailer, your hitch cushions any sharp jerks, so a smooth down/up isn't hurting anything.

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Yup--all in the weight. I have a '97 WIA I use for hauling grain. 80K load, you don't notice that at all! Unloaded with a aluminum trailer, moves up and down real good! The first time I pulled onto the scale empty and got out of the truck to hear a loud hiss, I got worried! Just the ride height valve releasing air after the rear end went up 3" during the stop.

 

You gotta think about it right. Thats POWER man--thats how to tell its running right! Like a '68 OLDS with a Rocket 350...the left front fender is supposed to go up 6" when you step on the gas! :D

 

Seriously doubt it bothers your trailer, your hitch cushions any sharp jerks, so a smooth down/up isn't hurting anything.

 

It isn't just the up and down that bothers me. Granted, I don't like that at all, but torque wrapping applies extra pressure to the axle that bypasses the proper shock dampening process. It seems to me that there is a lot of ride quality left on the table this way.

 

As I'm designing the bed for my truck, I'm constantly battling adding weight on purpose, and keeping it light as to not take up any cargo capacity. It's commonly suggested here to add weight to the bed to improve the ride quality. To me, that's backwards of everything I know about suspension design. I do agree that with this suspension system that adding weight will improve the perceived ride quality. However, the REASON that improves ride quality is bad in my opinion, but I also acknowledge that these trucks need weight on the rear for weight balance and braking reasons.

 

On other threads, we discuss how having a single rear axle reduces our weight capacity. It seems like a never ending balance between adding weight to improve ride quality and not adding weight to improve carrying capacity.

 

I'm going to take a different approach with this truck. I'm not going to add anymore weight than necessary and I'm going to modify the suspension to work properly for my use.

 

We'll see how that goes.

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Yep, the Volvo is basically the same as the Airleaf.....Other than the simple design, the only other advantage I can see is anti-roll from the binding of the springs. The Mack system would be superior off road by far.

 

I was lucky that Specialty Alignment Products makes axle wedges to roll the pinion angle. They were only available in 2° though....had to stack a pair per side.

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Torque rise / fall is most noticeable at light rear weight conditions but often it can be mitigated with softer shifts and smooth power ramp ups.

 

The old Freightshaker remains tandem but ride quality is mostly a factor of balance more than actual weight in our case because we utilize two different bed / box configurations.

 

Our flatbed / coffin sleeper results in a total rig length of 27 ft

 

Our Dolly-Box is a 20 ft cargo box that increases out truck out to 34 ft

 

With the 20 ft cargo box our end of box is 11 ft behind the rear axle centerline and 400 lbs is removed from our front axle weight.

 

Both configurations have the same total weight but the weight shifts farther back with the cargo box and everyone is amazed how much better the truck rides with the cargo box installed. The Mass Distribution of the cargo box is such that the rear tandems suspension has a even load to work with so it functions much better.

 

A Air-hitch sitting on top of a lightly loaded truck air suspension system would likely drive a suspension engineer to drink more than just prune juice.

 

Mass distribution matters..........

 

Drive on...........(Mass is..........where you find it)

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That's the rub about lightweight highway trailing arms. The truck must also be an 1850 with a 12 or 13spd (ie: deep reduction) trans? That's when things get bad as a single. Usually you couldn't order such a thing and about the best thing you can do is add a Barksdale valve (you already have one) and some high rebound shocks. Hendrickson used to call that an Edge kit. The Neway AD123 was about the only suspension you could order in the 'big truck' single scenario. What's shown in the pics is a Neway ADZ, which is their newer version of the AD but still has their same underslung/barpin principles. You are right, it is all in the geometry and it is a problem waiting to happen as soon as you leave the pavement. All that bound up energy is held in by traction and is either gonna get you stuck or it is gonna come out as a broken axle shaft, driveline, or chip a pinion. Be careful when reapplying torque until it gets itself settled between gears. Most everything over 1000-1200 ft lbs in a single goes underslung nowadays.

 

I think I've posted it before but you can do a Watt's link on your truck side to match. :) We never planned on that truck carrying anything when we did it though...

 

https://www.flickr.com/photos/9105879@N05/2346969833/in/photostream/lightbox/

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Scrap has pretty much nailed it.

When I first started driving I drove a two axle international with an aluminum frame hauling aluminum bottom dumps. The truck had 500 hp with a 13 speed. Empty the truck and two trailers weight was 26,000 pounds.

Loaded the truck would torque the front end out twisting the frame almost lifting the front tire off the ground. But empty the rear would top out the air bags on every shift. And when you dumped the trailers in a windrow you also had to dump the air bags or the truck bags would top out and get you stuck. I had to learn how to progressive shift. And really be respectful and careful with the throttle.

 

Fast forward I now have a Volvo with the (I shift)

Once again I'm learning to use a very light foot going through the gears until past 6th gear.

Or I have to manually shift it.

The truck is tandem but wants to start many times in 2nd gear and then skip to 5th gear.

The time it takes for the truck to pause before shifting to 5 th gear. When the power rolls back on it really wants to jump the truck up in the air. I haul house paint for a living and paint will surge and is very easy to dump your load if you're not careful.

 

Bottom line you got a lot of torque my friend :) build the bed as light as you can.

As hard as it is for Racer. back off the throttle until you get through your first six gears or either manually shift it through the first couple gears or use the P mode when you upgrade your shifter.

 

torque is an awesome thing but you most respect it or plan on buying a lot of drivelines.

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