Tire Pressure

[truk4u]

Tuga,

Generally, the 40 tag tire will come off the ground, but the 45 does not.

[Jeff Bayley]

I guess the definitive way to tell is maneuvering and testing in a parking lot as suggested prior. I'm not confirmed for the rally and we don't have much time left to decide but I wonder if enough people are curious to do testing of this there. Curious to know what others experience has been on wear of tag vs. drive tires.

[JIM KELLER]

I raise my tag on any 90 degree or more turn and tight parking maneuvers. Just stand next to someones tag axel when they are doing tight turns. Watch as the rubber is ripped from the tire and left on the ground. Observe the tire being pulled sideways from the rim. Think of the abuse the sidewall belts of the tire sustain not to mention the additional bearing load placed on the axel bearings. I know for a fact that when a car is involved in a side accident and it is pushed across the road sideways tire damage occurs. Insurance companies try not to pay for the tires but when the vehicle wiggles going down the road the problem is obvious. I suspect the ride of a Bus is compromised if tire belts are pulled and stretched sideways.

[Woody]

(this from Jon's post)
But lifting the tag when cornering is really not required because over the life of the tires, if they are inflated properly tag axle tires are not being scuffed sideways any more than the steer tires are during turns. I don't know this for sure, but I would suspect steer tires on average have a shorter lifespan than tag axle. When we turn the front wheels to alter the path of the bus those steer tires are resisting the forward motion of the bus and are being scrubbed sideways as the bus turns.

Just curious - is tire rotation ever considered?

[Kenneth Brewer]

(this from Jon's post)
But lifting the tag when cornering is really not required because over the life of the tires, if they are inflated properly tag axle tires are not being scuffed sideways any more than the steer tires are during turns. I don't know this for sure, but I would suspect steer tires on average have a shorter lifespan than tag axle. When we turn the front wheels to alter the path of the bus those steer tires are resisting the forward motion of the bus and are being scrubbed sideways as the bus turns.

Just curious - is tire rotation ever considered?

The steer tires are rotating along the direction of the turn and they define the turning circle and have the easier time of it insofar as scrub, the drive axle wheels are the privoting point that describes the other point on the chord of the described turning circle. The tag axle wheels are outside the turning circle and are have a (sideways) component more at an angle to the tangent and 'dragged' (or forced to distort) more sideways than the wheels on the steer and drive axles. This is why they are 'scrubbed' significantly more. They are at the opposite end from the fulcrumm (the drive axle). It is sufficient enough to cause widening of the turning circle and the reason for 'lifting' or removing weight from the tag. It is also the reason for steerable tags that tighten the turn circle by removing this geometric issue, as on the recent Newells, and at various times M.A.N., Daimler-Benzes, Setras, Tatras, and others on some of their commercial bus offerings over the years. Interesting to some may be the reasoning for, and physical constraints against, that invalidated Ackerman steering (where the steer wheels were articulated to describle the two different turning radii, but failed to take into account the greater slip angle(s) of weighted outside tire (and is related to tuning based on specific tire characteristics). Other more distantly related topics we could discuss informally at POG9 might include the geometry behind what is/was known as zero (and plus or minus) point steering, the geometry behind unequal length wishbone suspension systems, camber, caster, toe-in etc.. Or, I can keep my big mouth shut (probably best).

[Kenneth Brewer]

Fulcrum, fulcrum, dammit.

[Jon Wehrenberg]

I did rotate a set of tires every 30,000 and at 130,000 with 1/4" of tread remaining replaced them. At the time I was putting 20,000 to 30,000 miles per year on the bus. Because our wheels are polished on the inside for drives and outside for steer and tag it meant either breaking down the tire and wheel (which is how I did it) or polishing the dirty side of the wheel which was a lot more work.

Today I wouldn't think about rotating because I think I will end up with tires aged out instead of worn out. Rotating a tire on 45 foot coaches would require breaking them from the wheels because the inner wheel on the drive axle are steel, unlike the inner wheel being aluminum on the older 40 foot coaches with 12.00 tires.

As to tag tire scuffing, without a doubt they do scuff, but I think everyone is trying to over compensate. Yes there is considerable sideways force on a tag axle tire when it is not lifted, but the tires are built for that. If they are not every 10 wheeler truck or every tandem axle trailer on the highway today would be tearing up tires. Yes they are duals, but they carry loads per tire similar to our tag axle tire loads. I am sure we have all seen a truck in a truck stop parking lot make an extreme turn where the trailer wheels just sit in one spot while the tractor pulls the front of the trailer around in a circle.

As to our steer tires not scuffing as much, I agree totally with what Ken said, however he paints a picture with unrealistic conditions. As we travel down the interstate we drive on crowned roads and often have strong crosswinds. To keep the bus from tracking to the low point or to counteract the crosswinds we turn the steering wheel which keeps the steer tires in a small but ever present side skid. The same side skid occurs when making a turn. The tires are always pointed in the general direction of the turn, but are always pointed in such a way to indicate a smaller diameter turn than the actual path. That side slippage in the steer tires is to compensate for the resistance of the bus to a turn due to the drag of the tag, and because the drive direction is always straight ahead.

[JIM KELLER]

Jon, Do you think anyone in the " Prouds " would ever exhibit this much passion about Tires ?

Ken, I would attend one of your seminars.

[Kenneth Brewer]

Jon is correct when he points out that I did not, in my post, discuss crown, which does indeed influence tire wear, and (straight) tracking, and was not attempting to. And, further, I believe Jon will agree that side to side load imbalance, which he has mentioned on more than one occasion, is also a factor that can arise to criticality if not adequately addressed. I don't attempt to discuss crosswinds; another matter. But, in fact, there is compensation related to wheel alignment/adjustments on all heavy vehicles to help alleviate the tendency to pull to the right (of course, the opposite condition occurs in countries with left side driving) for general/standard crown(ed road) conditions, particularly as it can be factor in accidents where inattention leads to sideswipes and wandering off road. Some may be familiar with the compensation device, whose name escapes me at the moment, that is/was somewhat often seen where the adjustment was made to track straight by means a knob accessible to the driver. This can be a big deal. My Bluebird, that I loved dearly, pulled enough that it tended to wear the power steering pump and related equipment, and often could be heard to whine for extended periods, especially on 2 lane country roads.

[LA-HODAG]

My office overlooks the Staples Center and Nokia Theater and better yet, the lot where the entertainer buses park for concerts. I don't spend all day staring out the window, but when I do look I have noticed that the pros almost always lift their tags when maneuvering in that lot, even if they have plenty of room to make a wide turn.