Why do big rigs have front wheel rims that stick out while other rims stick in?

[DaveC426913]

Why do rigs have their front wheel rims sticking out while other rims stick in?
http://www.discountramps.com/truck-wheel-risers-1.jpg

My first theory was that it allowed them to use a common axle length for all axles on a rig but subsequent observation reveals that that's not the truth. Front wheels and rear wheel are flush with the sides of the vehicle, meaning that the rear axles must be shorter (see diagram 1).


My second theory has to do with making room for the steering/pivoting bearing (which is unique to the front wheels). Having the rim stick way out allows the mechanism, and most importantly the actual pivot point, to be vertically over the contact point on the road (see diagram 2).

Am I close?

 

[lisab]

Is this something you've been pondering while you were in the Caribbean?

 

[Danger]

I've always just assumed that it's because both rear axles are dualies. The inner wheels and outer ones have their (flanges?) face-to-face for a solid connection.

 

[DaveC426913]

I've always just assumed that it's because both rear axles are dualies. The inner wheels and outer ones have their (flanges?) face-to-face for a solid connection.

That was one of my other theories. But look how far the front rim sticks out - several inches beyond the tire. If you placed another tire face-to-face with this one, there would be a gap of about six inches between the two tires, not the 1-2 inches I see with the rear tires.

 

[Office_Shredder]

Are you sure they use the same rim for the front and rear tires?

 

[Danger]

That was one of my other theories. But look how far the front rim sticks out - several inches beyond the tire. If you placed another tire face-to-face with this one, there would be a gap of about six inches between the two tires, not the 1-2 inches I see with the rear tires.

You've got me there. While the ones in the photo are clearly intended to look good, I doubt that the shape is purely cosmetic. Maybe it does have something to do with the braking system or steering geometry after all.
Maybe you should post a link to this thread in the Mechanical Engineering section. It might have a better chance of an expert responding.

 

[Andre]

Unfortunately I have no idea about the technical terms, but from what I hear and see, is that the secret is in the number of wheels. Frontwheels are single, but most of the time the wheels on the back are doubled up and are been connected both with flush sides of the rim to each other, which makes the flush side of the outside wheels going to the inside.

 

[Astronuc]

Why do rigs have their front wheel rims sticking out while other rims stick in?

My second theory has to do with making room for the steering/pivoting bearing (which is unique to the front wheels). Having the rim stick way out allows the mechanism, and most importantly the actual pivot point, to be vertically over the contact point on the road (see diagram 2).

Correct. The driving/tractive effort wheels do not steer, while the steering wheels rotates about the pivot, and it has to clear the tie rod and suspension.

See the illustrations here
http://en.wikipedia.org/wiki/Steering
http://en.wikipedia.org/wiki/Ackermann_steering_geometry
http://en.wikipedia.org/wiki/Kingpin_(mechanics )

 

[turbo]

I assume it's because the convex front rims allow the use of longer axles for the sake of front-end stability, especially when turning. The rear end of the tractor is heavily loaded, but the front is not, so for best traction during steering, a wide stance (like Larry Craig!) is preferable.

 

[DaveC426913]

Unfortunately I have no idea about the technical terms, but from what I hear and see, is that the secret is in the number of wheels. Frontwheels are single, but most of the time the wheels on the back are doubled up and are been connected both with flush sides of the rim to each other, which makes the flush side of the outside wheels going to the inside.

That's the obivous answer at first blush, yes. But it doesn't add up. The rims on the front wheels stick out much farther than they should if your theory were true.

 

[DaveC426913]

I assume it's because the convex front rims allow the use of longer axles for the sake of front-end stability, especially when turning. The rear end of the tractor is heavily loaded, but the front is not, so for best traction during steering, a wide stance (like Larry Craig!) is preferable.

Would longer axles make any difference if the wheelbase is no wider? See diagram 1 in my OP. Rear wheels have same base as front.

 

[DaveC426913]

Is this something you've been pondering while you were in the Caribbean?

These are things I ponder during idle brain cycles, such as on the daily drive to work.

 

[Andre]

That's the obivous answer at first blush, yes. But it doesn't add up. The rims on the front wheels stick out much farther than they should if your theory were true.

Are you sure? Have you tested that? We did. The rims of the truck of my brother in law are identical, front and back, it's just turned the other way around.

 

[pantaz]

Well, http://www.alcoa.com/alcoawheels/north_america/en/brochures/docs/Alcoa_LvL_One.pdf" for their latest offering says one wheel fits all positions...

... This exclusive two-side finish is the brightest finish available without buying Alcoa’s
traditional mirror polish. Mount the LvL ONE™ wheel on any axle of the truck or trailer and reduce the need to order single side inner diameter and outer diameter wheels.

 

[hypatia]

Other then stability,my answer is, hehe...that it just looks cooler.

 

[Ivan Seeking]

I've always just assumed that it's because both rear axles are dualies. The inner wheels and outer ones have their (flanges?) face-to-face for a solid connection.

How else could you mount a pair of dualies while minimizing the total width and mounting space? Also, this allows one to use the same rim for the inner and outer wheel.

It is done the same way for smaller trucks, but not so for truck running singles.

 

[DaveC426913]

How else could you mount a pair of dualies while minimizing the total width and mounting space? Also, this allows one to use the same rim for the inner and outer wheel.

I'm still sayin' no. The front rims often stick out at least 4 inches beyond the tire. If they were using the same rims on the rear tires, there should be an 8 inch gap between the rear pair of tires.

[PLAIN]http://www.firenews.org/ri/PascoagRIM4.jpg

 

[SirAskalot]

Im no trucker but as far as i can see the extra width of the front rims are due to spacers and hub caps. Remove them and they will propably form a neat dually.

Extra clearence between the sidewalls is probably a good thing when the load increses and the sidewalls bulge. Though 8" migth overkill.

 

[Ivan Seeking]

I'm still sayin' no.

Yes.

Note that the inner rear wheels have to slide over the drum or they would stick out beyond the side of the vehicle. You also want to minimize the mechanical advantage between the drum and the outer edges of the tires.

Note that the torque acting on the studs due to the inner wheel cancels the torque due to the outer wheel. This way the bearings see a [mostly] vertical load, with no side loading.

 

[Averagesupernova]

That's the obivous answer at first blush, yes. But it doesn't add up. The rims on the front wheels stick out much farther than they should if your theory were true.

The inner wheel and the outer wheel are not necessarily the same on the dually. I just measured and the distance between the sidewalls is about 3 inches on the rear duals. The reason they are not the same is simply because of looks. Why stick a shiny aluminum rim in behind where no one will see it? I measured the distance between the edge of the rim where the bead of the tire seats and the face of the rim where the other dual would set against and it is only 1.5 inches. This measurement was on the front and although it was too difficult to do on the rear I would estimate it is the same based on the cockeyed way I had to hold the tape measure. It probably has nothing to do with brakes since this setup was used long before semi tractors were required to have brakes on the front. It would have more to do with putting the steering pivot as close to directly above where the tire hits the road. This does have an advantage during braking too. If the steering pivot were more towards the center of the truck the wheels would want to pull back during braking and put unecessary stress on steering components.

 

[Ivan Seeking]

There are limitations for the front, but the key question is why different rims are used on the rear when dualies are used. Again, without dualies, they generally use the same rims on the rear as the front wheels. The dual wheel configuration is independent of steering concerns.

 

[Averagesupernova]

Only the inner rim on the back is different and I've already stated why.