Bryan H

Jeff: I did some work in Phoenix, AZ in 2004 and visited the Rush Peterbilt dealership there. I was told they were spec'ing single 11-5/8" frames for Super 18s then. Recently, I noticed a couple Super 18s on Rush's web site showing the frame spec as a single 13-3/8" frame. This makes me wonder if they took the "low tare weight" thing a little too far with the single 11-5/8" frame spec. I've researched the Maxle booster axle and Strong Arm booster axle quite a bit. They're both really interesting pieces of engineering and I would like to model each of them in the future. The two are quite different from each other. However, there was a pretty nasty patent-infringement lawsuit that went on for years between the developer of the Strong Arm and the developer of the Maxle. Bryan

Tim: I've seen variable section frames before but never an "inverted fishbelly" where the frame rises aft of the cab. Is this something you saw on one of your visits to the Peterbilt factory? Bryan

My compliments on your fine work. Did you create the frame rails by soldering flanges to the web or did you start with commercially available brass channel? Also, what tools do you use to fashion the brass pieces with "free-form" shapes, such as that mid-ship driveshaft bearing? Thanks, Bryan

Hi: I've actually studied this topic quite a bit, so I thought I'd chime in here. In the US, any group of axles spaced 96-inches or less is considered a tandem, so if your drive axles are spaced 54" apart, you'd want to put the pusher axle at least 42" ahead of the forward drive axle. I see a lot at 43" giving an extra inch to be sure the spacing is at least 96". Here in Minnesota, many pusher axles on tri-axle trucks are placed 55" or 56" ahead of the forward drive axle due to a state statute that says any axle in a tridem spaced 9-feet or less may carry no more than 15,000 pounds. Making the tridem measure 9'-1" ensures the two drive axles can still carry a total of 34,000 pounds. Quad-axle, quint-axle, and superdumps (sadly, we have only a few of those here) generally have the pusher axles spaced close together to make use of limited frame space. Any group of axles spaced 40-inches or less is considered a single axle, so pushers on multiple lift-axle equipped trucks generally have the first pusher at least 43 inches from the first drive axle (see first paragraph) and successive pushers spaced 40.5", or 41" apart. This is, however, not universal. I've seen quad-axle dumps with their pushers spaced further apart to ensure the steer axle isn't overloaded. If you wanted to build a super dump, you'd probably want to shove the pusher axle as far forward as possible as the booster axle really loads up a steer axle. However, if you wanted to build a boost-a-load mixer, the pusher axle could remain close to the drive axles as the mixer drum locates the load in a consistent position whereas on a dump it's more variable. Heavy-haul tractors typically seem to have the pusher axle spaced similar to the that between drive axles, or about 54" ahead of the forward drive axle. This may be more than you were looking for, but I find the whole weight-distribution exercise quite interesting. It's part of the reason I left a job of 17 years to become a full-time mechanical engineering student. Bryan

Very nice Chris! Quite impressive. I would be interested to hear about your milling machine. Is it a manual machine or CNC? If CNC, what CAM and controller software do you use? Bryan