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#1234646 - Wed Sep 13 2017 08:27 PM Re: 1-ton front disc brake dumb idea? [Re: Jay Zed]  
thoroco  Offline
Shop Shark
Joined: Dec 2006
Posts: 670
Houston, Tx
On a 3600 you could have an offset of 7/16 on a 17" wheel or 3800 with the same offset both being SRW. But the 3800 DRW that had 18" had an offset of 4-9/16" with a DRW front hub. the axle length was the same , bearing position the same, The DRW just has the hub extended. If you look at the alignment section of the manual it shows the kingpin sitting at the inner edge of the wheel about 3" from the wheel centerline. You just can't have that much pin inclination and still drive it. Does it actually incline into the tread area , yes, That's the scrub area and it only changes that slightly. you can get a larger change for swapping to late model wheels.

Dave Chapman
The Hollister Road Company

#1234691 - Thu Sep 14 2017 12:53 AM Re: 1-ton front disc brake dumb idea? [Re: Jay Zed]  
Grigg  Offline
Joined: May 2005
Posts: 9,072
Lexington, VA
I'm looking at the alignment section in the 1949 100-450 GMC maintenance manual.
The picture with alignment chart is illustrative, not accurate.

Note that king pin inclination is 7-1/6* and camber is 1-1/2*
Added together the kingpin is at an 8.6667* angle with wheel centerline.
Figure a 32" tire, or 16" radius.
Tan 8.6667* X 16" = 2.438" meaning the kingpin centerline at the ground is that much further out than at center of wheel.

I measured best I could front hub and spindle on a 3/4 ton (single wheel) AD axle.
At center of wheel the wheel mounting surface (hub surface) is 3.25" outside of center of kingpin.
Subtract from that the nominal 7/16" inset of stock wheels (is this right?) gives 2-13/16" center of tire tread to kingpin center at center of wheel.

Difference then in the calculated offset due to camber and kingpin inclination and physical offset of hub from kingpin is about 3/8" between center of tire on ground and point it turns on the ground.
Any less offset in wheel or adding wheel spacers increase that offset, more scrub.

So the factory setup is pretty much ideal and good for ease of steering and better steering tire life.

Sorry if that's hard to follow in words. I can make up a drawing if it helps anyone, just ask. (And if I made a mistake it'll become apparent in a drawing)


1951 GMC 250 in the Project Journals
1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-

#1234707 - Thu Sep 14 2017 04:08 AM Re: 1-ton front disc brake dumb idea? [Re: Jay Zed]  
thoroco  Offline
Shop Shark
Joined: Dec 2006
Posts: 670
Houston, Tx
I think that s pretty much my point, The change minimal at best for an SRW kit and no change on a DRW kit. If that little of an offset and scrub difference mattered an entire industry would close down because we couldn't use aftermarket wheels and every time someone used a ford rim, which is a rim of choice on these, with a 5 or 6 inch back space vs the 4.5 original I think it would be very common knowledge that wheel bearing were failing and steering was greatly affected. But I do see that anywhere of hear about is much past theory. It would certainly show up here on stovebolt if it were happening.

Dave Chapman
The Hollister Road Company

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