The Stovebolt Site Forums The Parking Lot Big Bolts Motorhome IFS for 2 ton truck

I'm in the planning stages of my '48 Chevy big bolt build and have a motor, trans, and rear ready for the stock frame. The motor is 12V Cummins backed by an Allison 4 speed auto and the rear our of an International 4700 LPX with disc brakes. I've seen the P-30 front end swap that Grigg wrote up,(which is very good) but also saw on marketplace an IFS setup with disc brakes from a class A motorhome. Has anyone adapted IFS from a larger truck/shuttle bus, etc to their truck? Another idea was a tubular front axle from a late model F450/550 which I would be comfortable with cutting and sleeving to shorten.
Thanks
-Bill

If you can find the GVWR tag on the motorhome, it should have GAWR for the front and rear axles, that would give you the weight carrying capacity of the front suspension.

Be very careful when adapting a different front end to a frame it wasn't engineered to fit. There's a built-in angle called "Ackerman" (AKA toe-out in turns) that must be considered, and it varies with wheelbase. To over-simplify things a little, as the front wheels turn away from the straight ahead position, both front wheels and the rear axle must follow an imaginary radius several feet long that lets them pivot smoothly without fighting each other. That is determined by the length of the steering arms at the front spindles, and their distance away from the pivot point of the spindles. Changing wheelbase has a pretty radical effect on Ackerman, so a rig that steers well in a straight line can have very unpredictable handling in a sharp turn at highway speed if the wheelbase is shortened (or lengthened) in comparison to the original application of the front suspension.

As a general rule of thumb, a line drawn from the spindle pivot points to the center of the rear end with the steering pointed straight ahead must also intersect with the pivot points where the tie rods attach to the steering arms. Do your homework on steering geometry before you encounter some nasty surprises in a tight turn.
Jerry

So on a vehicle with kingpins, would that mean the line goes from the center of the rear axle, through the centerline of the kingpins, then through the centerline of the tie rod end bolts? Or for rear steer, the centerline of the tie rod bolt, then the kingpin? Just making sure I'm understanding this correctly, thank you :-)

This page illustrates what Jerry is talking about (toe out on turns).

If you were to take the rear axle and move it rearward, the angles of the front wheels would have to change in order for the center point to intersect at the same point. That would require changing the angles of the steering arms.

Yes, that's exactly right. use a plumb bob and some masking tape on a flat, level floor to check those intersection points. On the race cars with altered wheelbase, we would bend the steering arms (cold) to make the Ackerman angles correct.
Jerry

I'm not planning on changing the wheelbase or trackwidth of the truck. I was intrigued by this ad and wondering if it would bolt on as-is or if it could be narrowed by cutting and welding the cross member and shortening the drag link. GM workhorse IFS from Class A motorhome

Side benefit is disc brakes and modern(available) off the shelf parts....

If you're swapping in a different front axle setup, unless the donor truck and the recipient have the same wheelbase, you'd probably need to adjust the steering arm angles to make it corner without scrubbing the tires. That would apply to changing the trackwidth of the front axle assembly as well.

@Hotrod Lincoln thank you for the clarification.

Amazingly if you go though the parts books for say a 77 Chevy C 10 pickup there is only one spindle for short bed, long bed and crew cab. Check it out. Ackerman is nowhere as critical as some thing if you are not building a race car.

Not so amazing, as it's the steering arm that controls the Ackerman geometry, not the spindles. Here's a snip from the 1929-1957 parts book for the steering arms. You can see that there are different ones for the different size vehicles.