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Wheel Cut Limits


GoldRush

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What is the limiting design feature of the various mfg's wheel cut?

 

Is the limit to prevent wheel/frame/tire contact?

 

Are the pitman arm lengths similar between mfg?

 

Is the ratio of pitman arm to steering knuckle similar between brands?

 

What allows the wheel cut of a Volvo be so much greater than a Pete or KW?

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Guest Lostinaz

Set back axle tractors all turn sharp.

It's not a Volvo thing .

The limiting factor on my KW is the tire will hit the drag link in a sharp right turn. It stops real close to tire / drag link collision.

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Wheelbase is Wheelbase. In order for one truck with the same wheelbase to turn inside of the other would require the steer wheels to wheel cut/turn more than the other. So are the Set Back axles themselves engineered differently or are they just the same standard axles set further rearward on the chassis?

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Guest Lostinaz

An interesting question...

I guess it comes down to how far the springs are set in on the axle, the tie rod arms, the pittman arm, and the location of the steering box, and drag link clearance.

This axle here advertises 55 degrees, but I think I have seen up to 58 degrees http://www.demanddetroit.com/pdf/Axles/0814-Front-Steer-Axles.pdf

 

At a certain point it will scrub the tandems too much and push the front wheels too much sideways.

 

My truck turns so sharp it always amazes me. It almost pivots on the rear axles.

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First time out with my IH MDT after towing with my Ford I cranked the wheel over to the lock to make a U-turn back to the road. With the Ford I'd likely had to make a couple cuts, with the IH I almost ran into the side of my fiver!

 

It is fun to go out to a gravel or dirt lock and make a few circles, aside from knowing how much room you'll need to turn, looking at the pattern the back axle leaves is interesting.

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I like when you have a short wheel base, a tight cut and sitting far out on the outside lke with the Volvo or a cabover. I dislike the classic long nose Peterbuilt's and Kenworth's for backing into a tight dock. While they may look pretty, It is hard to see (some guys open the door and lean way out). The big shiny exhaust stack is in the way also.

 

My fave so far is the Argosy.

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It used to be that SBA's were the ones that got you a puller gear (gear ahead of axle), long taperleaf springs, and the integrated knuckle that would make it ride better and turn sharper. But the SFA's have had that for over 20 years now so that difference has been gone for awhile. With the wide track axle nowadays the tie rod to axle beam and draglink to radiator crossmember are the limiting points. I always imagined the splayed rails of a Volvo was one of the keys to getting a wide cut as it puts the gear at an off angle to the axle beam, kicks the draglink inward, and gives you more space. When they took the 389 and the W9B to a 55 degree cut a few years ago putting a slight angle on the gear is one of the things they did to get it there.

 

The problem with all that is that you are messing with sacred geometries. When you build 1 million trucks over 90 years you pretty well figure out where the sweet spot is. That spot leaves you at 42 degrees but they drive like they are on rails. I ordered all my trucks with the fancy stuff when 50-55 degrees first came to KW and they are cool and all but they just don't drive the same and for whatever reason I find myself dry steering much more than I should as well. I dunno, I think if you've been driving a truck a good length of time then you know what it takes to get it done and all the new stuff just isn't worth it. But I guess it all kind of depends on what the truck does for a living

 

55 degrees is about the practical limit for steer tires so I don't think you'll see much more than that come out. I've done 62 in an IFS truck and it was pretty cool in a parking lot but you forget all about it when you are out on the road. Oh, and we can't forget about marketing numbers vs the real world as well..... If it can do 55 turning one way but only 47 the other, can you call it 55? And if it can only squeak out 55 on one side of one wheel at a 300" WB or above Ackerman setting, can you call the whole model 55 degrees? That tends to muddy the waters as well.

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