1987 V30 Dual rear wheel axles?

[Turbo Dog]

Vbb199 - I am currently planning on putting the second axel springs inside the frame and that's a great idea to just weld the spring mounts an inch higher than the stock ones. Thanks!

BTY - A short wheel base dually 4X4 never goes down a paved road in a straight line unless you are helping it. 6 mud tires probably don't help either.

 

[Vbb199]

You may consider riveting or bolting the hangers vs welding, just for the sake of anti-stress lol

 

[Turbo Dog]

Yes, I figured that since the factory rivets them to the frame that I will bolt them to the frame. The plan (so far) is to box the rear part of the frame where the second axel will go to beef it up and make a surface to mount the hangers.

 

[Vbb199]

Yes, I figured that since the factory rivets them to the frame that I will bolt them to the frame. The plan (so far) is to box the rear part of the frame where the second axel will go to beef it up and make a surface to mount the hangers.

Out of curiosity, what's under the hood? Being an 87 I would imagine either a 6.2 or a tbi 454

 

[Turbo Dog]

TBI stroker...Bored .040" over giving it 388 cid and making 420 ftbs at only 2000 rpm. Built for work, forged crank, 4 bolt main, 10.5:1 compression with hypereutectic pistons, custom Comp cam, full roller valve train, with stock heads, intake, and exhaust manifolds into a balanced dual exhaust.. Just as that was being built I saw a 454 for sale not to too far away...but the build had already started.

 

[Grit dog]

Vbb199 - I am currently planning on putting the second axel springs inside the frame and that's a great idea to just weld the spring mounts an inch higher than the stock ones. Thanks!

BTY - A short wheel base dually 4X4 never goes down a paved road in a straight line unless you are helping it. 6 mud tires probably don't help either.

Moving the springs inboard solves the length issue I suppose, with leaf springs.
I could see your idea working, conceptually, especially if you mounted the tag axle to not touch the road, when empty.
I “think”, trying to visualize this as a 10 wheeler squarebody just sounds cool, I would want less/softer leafs on the tag, and let the bags do more of the work.
Even better yet, while contrary to typical airbag installation, and the overall function they serve in a typical application, I would probably plumb them together to aid in articulation over uneven surfaces. Less ability to take the weight off of the wrong side of the drive axle in that scenario of uneven surface.

 

[Nonstop]

So just curious, rather than dealing with mounting springs and such, why not just mount a pair of trailing arms and air bags? That way you could also control the amount of down force exerted on the tag axle?

 

[Bextreme04]

That's exactly what I was going to suggest. If I was going down this path, I'd be looking at a a 3 or 4 link setup with air bags for the tag axle.

 

[Turbo4whl]

There is another issue to consider, that is steering ackerman. I work on medium duty trucks, many 10 wheelers.

The geometry of steering, when turning the inside front wheel makes a tighter turn radius than the outside wheel. Steering ackerman is the center line of the rear axle to each turning front wheel's radius of the turning circle.

Without changing the ackerman, when making a turn the rear axle/tires will need to slide to stay in line. This won't happen with the two rear axles and 8 tires loaded. Instead one or both front tires will skid to stay in line.

So to maintain the stock steering ackerman, you need to move the current axle forward and then add the back axle.

 

[Turbo Dog]

So you are saying that the distance between the front and rear axels will be too long for the vehicle to turn correctly? Is the second axel moving the pivot point of the rear axles behind the stock axel? That is interesting because I have only done alignment work on light duty trucks with single rear axels, and of course they were basically stock.

 

[Bextreme04]

So you are saying that the distance between the front and rear axels will be too short for the vehicle to turn correctly? That is interesting because I have only done alignment work on light duty trucks with single rear axels, and of course they were basically stock.

What he is saying is that the front steering geometry is set up for the rear axle centerline to be at a certain point. If you just add another rear axle onto the back, the new rear centerpoint shifts back to be in the middle of the two rear axles. This will make the turning circle bigger and the front steering will no longer be correct for the new turning circle radius. In order to maintain the same turning radius, you would need to put the center of the two axles at the same point as the center of your current axle, or you need to change the steering geometry of the front axle to match the new wheelbase(which seems much harder TBH)

 

[Turbo Dog]

If I do move the rear axel forward it negates what I am trying to do. I need more rear support.

Will this only be a problem on tight turns? Most on road turns are broad radius turns, then when I am positioning the truck to empty the tank I am on dirt which will be tighter turns on a more forgiving surface.

 

[Bextreme04]

If I do move the rear axel forward it negates what I am trying to do. I need more rear support.

Will this only be a problem on tight turns? Most on road turns are broad radius turns, then when I am positioning the truck to empty the tank I am on dirt which will be tighter turns on a more forgiving surface.

It won't negate what you are trying to do. You would still have a contact point moved much farther back on the frame, but your effective wheelbase for turning would be the same. Here's a page talking about it with pictures.
https://en.wikipedia.org/wiki/Ackermann_steering_geometry

 

[Grit dog]

So you are saying that the distance between the front and rear axels will be too long for the vehicle to turn correctly? Is the second axel moving the pivot point of the rear axles behind the stock axel? That is interesting because I have only done alignment work on light duty trucks with single rear axels, and of course they were basically stock.

I certainly don't understand the intricacies of Ackerman, but the principle applies here and may make your idea a non starter. I feel that it would not pose issue in long sweeping turns, but any tighter turn would be an issue.
Think about all the different types of tag axles and drop axles on large trucks. The common theme is either they are free to turn (steer) so that they trail well, and they all have the ability to be picked up, even under load. Tag axles that steer, like the back of many mixer trucks, make tight turns with the tag axle down, but you don't see any fixed tags that don't get picked up (or should, at least) when making tight turns.

 

[Turbo Dog]

I measured the approximate distance between the front wheel steering line and the stock rear axel, then added an estimated center created by mounting the second axel. My stock short wheel base truck has about a 133" wheel base that would get extended to about 153". What is the wheel base of a long wheel base truck?

This picture is used to show the different hypotenuses of the triangle created by the different wheel bases using a steering angle of 20 deg.. The short wheel base 141" radius creates a 885" circumference circle and the 161" radius creates a 1011" circumference, that is a 14% difference. Is this going to greatly change the front steering ability?

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