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Tires & Wheels/General Tire Info - Hummers:

By Jerry Jorgenson


Hummer tire information

I don't have a Hummer (yet, unfortunately), but I do have sixteen years of experience as a tire applications engineer (in a former life, I'm now a computer nerd), and I have had over 120,000 miles experience in a Land Rover (The old variety not the new wimpy kind. LOL), and it occurs to me that your site doesn't have a technical explanation of tire pressures. Here is a rather lengthy discussion covering all the points that I've seen talked about on any Hummer site that I've visited. Note that tire sizes, brands, etc. aren't listed here because there are already a zillion other sites where you can find that information.

How to determine the proper air pressure and other little known air pressure facts:

All tires are designed to be inflated to a certain pressure depending upon the load carried by the tire. You can find out what this is by using the 'Static loaded radius' (which I will call just loaded radius from now on). Here's how you do it.

Procedure:

  1. Find the loaded radius from your tire manufacturer's catalog.
  2. Perform the next steps in the morning after the vehicle has been sitting overnight.
  3. Park the Hummer on a flat level hard surface, fuel tank between full and 3/4 full.
  4. Load the Hummer like you would normally drive it (You can substitute the appropriate number of sacks of cat litter for yourself).
  5. Measure the distance between the center of the hub and the ground at every wheel position.
  6. Compare this distance with the loaded radius specs.
  7. Inflate or deflate until the loaded radius is equal to the specs. If there is a noticeable difference between left and right sides check your Hummer's loading and your measurements, as well as the levelness of the surface you are on. Then use the higher pressure. A difference between the front and rear axles is normal depending upon how the vehicle is loaded.
  8. Write the pressure down somewhere for future reference. An empty Hummer may well require less pressure in the rear than in the front, particularly if there is a winch mounted. Carrying three spare half-shafts and a complete toolkit in the back will go a long way to even out weight distribution.
  9. If you have several 'standard' configurations (e.g. Commuting empty, Off-road trip loaded with spare parts and camping gear, Trailer towing), repeat the above procedures.

Caveats:

There is some additional information that you should know about the above method.The loaded radius is for an ambient temperature of 65F. If you're environment is significantly lower or higher than this you need to consult a pressure temperature chart (Usually found in the Earthmover tire databooks) to get the adjustments (I can provide this information if there is any demand).

The tires should either be at new tread depth, or you must subtract the missing tread depth from the loaded radius (steel belted radial tires only, bias ply and fabric belted radials grow with use).

Tires normally increase in pressure during driving. Do not reduce pressure. If you have more than a 20% pressure increase there is a problem that needs to be checked into, or your initial pressure was set too low. A 10% pressure increase is normal.

In vehicles without CTIS, increase the pressure calculated by 2 or 3 psi to offset the fact that you don't check pressure every day. I haven't seen anywhere where there is a method to check the calibration of the CTIS, but certainly this should be done. Everyone should have a good dial pressure gauge (Not cheap but well worth the expense when tires are $500 each).

For off-road use on tracks and washboard surfaces you should reduce the pressure by 25% from the calculated amount and be sure that vehicle speed does not exceed 30 MPH.

For maximum flotation use you should reduce the pressure by 45% from the calculated amount and not exceed 10 MPH.

For emergency use you should reduce the pressure by 60% (Or more, depending upon the circumstances) from the calculated amount and not exceed 5 MPH. Increase pressure as soon as practicable after the obstacle has been cleared. U-boat commanders should note that higher pressure is likely to be of more help than lower pressure.

Because the tire pressures will be warm when you lower inflation for off-road use, be sure to add in the difference between the calculated pressure and the pressure just before adjustment. Example: Calculated pressure=30 psi. Pressure after driving to jump off point=33 psi. Calculated pressure less 30% adjustment (30-9)=21 psi. Add difference (33-30) 3 psi. Pressure to use=24 psi

When going home for the day, inflate to measured pressure. (In this example 33 psi.)

This method is for normal sane to semi-psycho driving. For totally psycho driving and racing you are on your own.

Using this method I was able to routinely get double the mileage that others were getting on the same tires.

Tire pressure vs. revolutions per mile vs. inflation.

With radial tires the revolutions per mile are dictated by the steel belt regardless of inflation. Therefore any difference between rpm at various levels of inflation is due to either surface deformation or tread flexing (most likely the latter). With bias ply tires the difference in rpm can be dramatic 30 or 40% is not uncommon.

Tire pressure vs. wear.

The best wear characteristics come from tires running at their loaded radius height. Deviation from this measurement tends to wear tires faster. As a general rule higher pressures increase the rate of wear less than lower pressures.

Too much pressure has the following results:

  • The tires are more susceptible to shocks to the tread area.
  • The ride is harder.
  • Tire squirm is reduced.
  • Fuel mileage increases.
  • Casing stress increases.
  • The tire runs cooler.
  • Cornering stiffness is increased.
  • The tires are less likely to unseat from the 15 degree bead seat.
  • Footprint area is reduced, which reduces traction and flotation on soft surfaces.
  • The tire's ability to envelope objects is reduced further reducing traction on uneven surfaces.
  • Tire bouncing on washboard surfaces is increased, reducing shock and suspension life as well as traction.

Tool little air pressure has the following results:

  • The tires are more susceptible to shocks to the sidewall area.
  • The ride is softer.
  • Tire squirm is increased.
  • Fuel mileage decreases (much more in bias ply than in radial tires).
  • Casing stress increases, but due to excessive heat rather than excessive tension.
  • The tire runs hotter.
  • Cornering stiffness is reduced.
  • The tires are more likely to unseat from the 15 degree bead seat.
  • Footprint area is increased, which increases traction and flotation on soft surfaces.
  • The tire's ability to envelope objects is increased further increasing traction on uneven surfaces.
  • Tire bouncing on washboard surfaces is reduced, increasing shock and suspension life as well as traction--Your back and passengers will thank you too!

The biggest enemy of tires is heat. Heat shortens the life of the tires by softening the compound making it easier to wear, and changing the compound chemistry and possibly delaminating the materials in the tire. Some freight trucks install tire drip bottles which drip water on the tires to keep them cool. The worst combination for heat in a tire's life--given proper inflation--is high ambient temperature combined with high tread depth. When possible install tires in late fall so that when summer comes around the tread depth will be lower. This can result in dramatic increases in tire life (depending upon the kind of tire). A general rule is that heavy off-road traction tires benefit more than passenger car ribbed tires from this procedure.

Traction in snow and ice.

To determine the correct pressures for traction in snow and ice is difficult because the mechanical properties of snow and ice change depending upon temperature. The colder the it is the stickier the ice and snow on the road surface are, higher pressure helps here. When temperatures are close to freezing 32F (or you make it that way by wheel spinning), there is a film of water over everything making it extremely slippery. only studs and chains help, when using chains higher pressure is better to avoid damaging the tires. Studs may benefit from lower pressures as more studs will be in contact with the ground. For most highway conditions (after the snow has been plowed) a 10% increase in pressure helps. If you're paving the way through deep snow, lower pressure helps. A road that was passible may not be passable in half an hour (or vice-versa).

Tire mounting.

The basic philosophy is if it's hard, you're doing it wrong.

  1. Equipment: Try to get some Michelin tire levers and slide hammers. They work much better than the ones that the tire stores usually have.
  2. Cleanliness: Clean the rim surface removing any foreign objects such as rubber. A power wire brush works well for this. Paint exposed steel surfaces to prevent rust. You really don't want a wheel to fail with pressure in it.
  3. Lubrication: Don't use the liquid kind because it's for car tires--it dries out too fast to be of any use on Hummer tires. Don't use products that contain petroleum--any petrochemical based product will decompose the tires. The two products that I have found to be good are Murphy's Compound and Michelin Tiger Grease.
  4. Where to lubricate: Lubricate the entire rim base--that's right all of it, the exterior and interior bead area of the tire.
  5. Mounting: Follow the instructions that come with the levers, and in the Hummer manual. I know, this is a cop out, but I don't have good Hummer specific pictures.
  6. Seating: The easiest way to insure proper seating is to inflate the tire to the pressure on the sidewall, deflate the tire to zero (do not break the bead--air seal--doing this), then re-inflate the tire to the pressure on the sidewall and back down to the operating pressure.
  7. Guide rib: After the tire is mounted, spin the tire and make sure that the guide rib--a small ring just above the bead area--is equal distant from the edge of the rim flange. If it's not, re-lube and try again making sure to do the seating step above. If there is still a problem you have a bad tire (this assumes a new tire) and the dealer should replace it. My experience has been that many dealers do not mount tires properly.
  8. Torque wrench: Be sure to use a torque wrench to tighten the wheel nuts properly. Use a criss cross pattern to tighten the bolts and use two steps of torquing. After a couple of hundred miles re-torque the wheels. Use of an air gun to torque the wheels is a common source of vibration.

Tire rotation.

Rotate tires if they are showing uneven wear. If this wear is due to alignment or suspension part breakage fix these too. Otherwise if one axle is wearing more than the other rotate when there is about 1/3 of the original tread depth difference between the tires. An early first rotation (1500/in summer 3000/in winter miles) will do more to keep tire wear even than all the rest of the rotations put together. There are actually studies that have been done proving this. The reason that this is so is: tires are most likely to start odd wear patterns when they are new and the tread is most wiggly. An early rotation breaks up any incipient wear patterns reducing the chances of them developing. There is no preference between X rotation and back-to-front rotation. Purchasing two spare tires and wheels--instead of one--makes rotation easier (Well as easy as a 150+ pound tire rotation can be). It also means that you won't have a new--but possibly sun damaged--spare when the other four wear out.

Tire balancing.

There are two kinds of tire balance: static and dynamic. If you divide the tire in half looking from the side of your Hummer (through the axle), and both halves weigh the same the tire is said to be in static balance. If you divide the tire into quarters looking from the front of your Hummer (through the axle and through the centre of the tread) and all four quarters have equal weight then the tires are said to be in dynamic balance.

If your vibration starts small and increases linearly with speed then you have a static balance problem. An example is if the tire was mounted with water in it and the water froze--Don't laugh, I've seen it happen.

If your vibration comes and goes--e.g. at 30 mph and then at 60 mph--you have a dynamic balance problem.

Properly used by competent tire technicians either kind can do a good job.

Off-the-Hummer balancers: These balancers are typically easier for the tire shop personnel to use because they don't have to know anything except how to press the buttons. Things to watch out for with OffTH balancers:

  • Some wheels are designed so that the centre bore--that's the hole in the middle--is the center of rotation, others are designed so that the center of the bolt circle is the centre of rotation. This means that you have to use either the cone attachment or the finger attachment depending on which it is (Sorry, I know of no list that says which wheels are which kind).
  • Some OffTH balancers don't properly balance tires larger than X diameter properly.
  • This kind of balancer can be out of calibration.

On-the-Hummer balancers: These balancers balance the wheel tire and axle assembly together, theoretically resulting in a better balance job, but because the Hummer's brake rotors are inboard there is likely less advantage to this than on a normal car. They require the technician using them to know more than just Here kid, push these two buttons and align this pointer to the wheel. The things to watch out for are:

  • Don't use the Hummer engine to spin the wheels; make sure everything is in neutral.
  • Be careful spinning the balancer engine against the tire, the small balancer wheel can melt the tire's tread.
  • If you remove the tire--later in service--be sure to mark the bolt holes and wheel so that you can replace them the same way.

Common points for both on and off types:

  • If the weights go half way around the wheel you may as well take them off as they aren't doing you any good.
  • If it seems like the technician is having problems, inspect the guide rib--on both sides--to make sure it is concentric. If it's not--and most tire manufacturers specify a 2mm tolerance between measurements around the rim flange--have the tire reseated as described above.
  • Truing the tires is not recommended as truing is done at the factory (at least for quality tires), and the problem is most likely seating--it could also be a warped wheel.
  • The wheels can also be warped. This seems to be more common with alloy wheels than with steel wheels. Use a dial gauge to measure concentricity at both bead seats--Sorry you will have to dismount the tire for this one. It is also possible that the bead seats are concentric but that the right and left sides don't rotate around the same center. There isn't a really easy way to test for this.
  • The automatic balancing fluids, powders, and devices you see advertised at best do no harm. At worst they damage the interior of the tire, degrade the tubeless liner, oxidize the wheel, retain heat making the tire run hotter, make the tire hard/impossible to repair and inspect for damage. They may even effect the CTIS. I don't say that there aren't some that are the greatest thing since sliced bread, but I don't recommend them because if you need them there is probably some other problem that should be fixed first.

Dynamometer testing.

Many places use dynamometer testing to fine tune the engine and other drive train components, however dynos can destroy your tires in minutes--faster than the T-Rex in Jurassic Park. It depends on the size of the dynamometer roller, the speed and the time spent. The symptom of dynamometer damage is hollow tread blocks--it looks like a rubber worm went inside the tread and ate a big hole (1/2 inch) in it. You can't see this damage from the outside until the rubber wears away and you have cavities or the tops of the tread look like flaps. In the worst cases three minutes is enough to destroy your $500+ tires. The best way to avoid damage is to get a set of slave tires and wheels and mount them. On one of the web sites I saw some military tires and wheels for $50, these are the kind to use. Perhaps if you belong to a club you can pitch in and get a set to pass around. Bias ply tires stand up better than radial tires. Full inflation pressure--the pressure that is marked on the sidewall of the tire--is required.

Electrical problems.

Depending on where you take your Hummer, it is possible to run over some high voltage power lines. What to do: Don't leave your Hummer, back off until you are sure you are not going to be fried. Inspect your tires. You can tell electrical damage because the tires look like they have been shot with a machine BB gun, usually in just one or two areas. There are a number of holes the size of a BB in the tread and possibly by the bead area. If you look closely you may also see little 'lightning bolts' in the surface of the rubber.

Fuel problems.

Dumping fuel or oil on the tires is about the nastiest thing you can do to them. Keep your shop/garage floor clean. Wash any spills off the tire with soap and water. Use a grease absorbent material on the floor.

Plugs and nails.

Plugging a hole in a tire is an emergency measure to get you buy until you can have the tire repaired properly. The problem with plug repairs is that you don't remove the tire from the wheel to inspect it properly. It is entirely possible for the penetrating object to have scratched the inside of the tire making the tire unsafe. This condition can't be detected without disassembling the tire and inspecting the sidewall. Plugs are fine to get you going, just don't rely on them for a long term fix. Self-sealants inside the tire have the same problem, and are worse because you don't even know that anything has happened--of course no one would put them in their Hummer since they would increase the effort of tire disassembly and increase the weight as well.

Congratulations!!! You made it to the end--finally.

Jerry (jerry@penreal.ca)


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