5/2003

Steve Malysiak, Michelin America's Truck Tires Field Engineer, will present a seminar specifically aimed at the GMC Motorhome's wheel and tire safety. The information will cover:
1) the original 16.5" bias and radial tire combinations
2) the common replacement 16" Alcoa Classic wheels with LT225/75R16 tires
3) load range D and E and different tire constructions of tread and sidewall materials of fabric and/or steel.
4) how to get the most from your tires, what to inspect for, criteria to use to determine when tires should be replaced
5) information for you to use in selecting replacement tires.
6) driving safety tips including how to best correct for rapid pressure loss (blowout) and the resulting changes in steering forces
7) major changes and improvements in tire technology over the last 30 years as they pertain to the GMC Motorhome

Individual tire loads (weighing) will be conducted during rally registration on both Sunday, for those who come a day early, and on Monday the 28th. It is free and optional. It will be a drive through method so those with tows do not need to disconnect and may also weigh their tow. For those who want to be weighed, come in full travel weight for the best maximum weight measurements. All sites have full hook ups so full liquid tanks can be dumped after hook up.

A tire information sheet has been added to the newsletter. We are requesting that this sheet be filled out and mailed to the address shown above, by every GMC owner, not just those attending the rally. This will provide the largest database of questions for Steve to answer at the rally. After the rally, the results of the seminar, questions, and answers will be published in the GMCWS Newsletter. It will also be placed on the GMCWS Technical Information web page for future new GMC owners to access.

Keeping the tire information on one sheet of paper required that subjects be abbreviated. The abbreviation explanations are expanded here:
1) wheel placement is either on the Driver's side (D) or Passenger's side (P)
2) wheel placement along the coach with Front (F), Middle (M), and Rear (R).
As an example, Driver's side Front wheel becomes DF while the Passenger's side Middle wheel becomes PM.
We are requesting data for each tire position for this safety seminar. The spare was not included because it is usually a temporary wheel/tire combination.

Wheel: All GMC's were delivered with 16.5" steel wheels and 8.75 16.5 bias ply tire from 1971 through 1975. Starting in 1976, 8.75R 16.5 radial tires with radial steel wheels were a major option, but some vehicles were still delivered with the bias ply tire and wheel combination. Steel wheels for radial tires are all stamped "RADIAL" on the outer rim and were designed to resist the larger wheel loads created by the radial tires. Radial tires mounted on non-radial approved rims can cause wheel cracking resulting in slow or even rapid pressure loss. This is a major safety issue. We are requesting each owner to inspect and record each steel/aluminum wheel to assure that it matches the tires on the vehicle. The 16" Alcoa Classic is approved for all E load rated radials.

Tires: Brand name/style is molded in raised letters on the sidewall.
Size and Load range molded on the sidewall. For standard size tires, this will be either 8.75 16.5 D load range, 8.75R 16.5 D or E load range, or the LT225/75R16 D or E load range.

DOT #: The last group of digits molded on the sidewall is the tire manufacture date. There are changes in the meaning of the digits over the years. 1980 and 1990 used the last three digits and starting in 2000, four digit were used.

In the 1980's the first two digits determined the week of the year it was made and the last digit determined the year (198X). If it has 019, the tire was made in the first week of the year 1989 and the tire is more than 14 years old.

In the 1990's, the same code was used plus there was a small triangle to the right of the three digits. A tire with 019 <| was made in the first week in the year 1999. That tire is now more than 4 years old.

Starting in 2000, the first two digits remained the week of the year manufactured but the last two digits are the year. A tire with 0101 was manufactured in the first week of 2001. As of the 4/29/2003 rally, that tire would be 2 years and 17 weeks old.

F/S or S/S: Tire construction with the number of plies and material is molded on the sidewall for both tread and sidewall.
Tread plies: Some tires have a combination of tread plies such as X plies fabric (F) + X plies steel (S). This should be recorded as F+S/. Some will only list Steel and be recorded as S/. Slash (/) separates tread from sidewall.
Sidewall plies: X plies fabric (F) or X plies steel (S). This should be recorded as F or S.

A radial tire with steel tread and sidewalls would be recorded on the sheet as S/S while a radial with steel tread or combination of fabric and steel with fabric sidewalls would be recorded as F+S/F.

Press: List your current tire pressures (psi).

Truck ruts sensitivity: This is a subjective value with 10 meaning highly sensitive and 0 meaning not bothered by truck ruts. People who are dissatisfied with their coach's handling often bring their suspension wear and alignment up to snuff first. It truck rut sensitivity is still high, some owners modify the suspension. Tire selection and pressures also seem to impact sensitivity. The idea is to have each owner rate their coach's truck rut sensitivity and list what suspension modifications have been made. This data will be used to relate tire selection, pressures, and suspension changes to truck rut sensitivity values. Collecting data from as many owners as possible will make the truck rut sensitivity correlation more accurate.

Suspension modification: List any known changes from the original design.

Questions for the Michelin Tire Engineer will be collated, grouped, and answered at the seminar. Here is your chance to ask the tire expert any question that applies to the GMC Motorhome.

Steve Malysiak, Michelin America's Truck Tire Field Engineer, presented the one-hour seminar and answered all 37 questions submitted on the Tire Information sheets. Every attendee received a new version of Michelin's "RECREATIONAL VEHICLE TIRE GUIDE, HOW TO GET THE MOST FROM YOUR TIRES". To get your own free copy call 1-800-677-3322, Option #2.

Steve covered all sizes that owners filled in on their tire information sheets and explained tire size, aspect ratio, and DOT Nomenclature.

He covered why tire pressures should match the tire size maximum load on that axle to the pressure in the book's load chart. The book shows how to determine each tire's load. All tire manufacturers must meet or exceed the same Tire & Rim Association standards for a given size and load rating, regardless of construction materials.

Overpressure causes different tire failures than underpressure. Overpressure reduces tire patch size, traction, and tread life. A chart was presented showing how much overpressure reduces tread life. Example, a tire load/pressure that should be at 65 psi but run at 80 psi will have 75% of its designed tread life while one that should be at 60 psi and run at 80 psi will have 70% of its designed tread life due to the smaller footprint.

All steel construction is more robust for commercial applications, including multiple recapping. The downside is a LT225/75R16 E weighs 10 pounds more, cost $30 more, has stiffer sidewall and harsher ride. If a steel tire is 20% below its recommended pressure, it should be remove from vehicle & taken to dealer to check for ruptured cables. Do not re-inflate on vehicle. Fabric sidewalls tires of the same load rating have the same capacity as steel sidewall.

Tire pressures should be checked when the tire is cold and driven less than one mile. Steve used his master gauge to check some attendees gauges after the seminar and found many were not accurate. He recommended using an accurate gauge and verifying against a master gauge. He also recommended the use of metal valve caps on stems to seal and protect the valve.

Replace tires when the wear strip shows or when sidewall cracks are 2/32". Have dealer check older tires when cracks are more than 1/32". Steve recommended replacing the driving tire at 2/32" tread depth for greater traction. Tires age differently in different environments and conditions. As the tire rolls and flexes, the waxes and emollients migrate to the surface, replenishing protection. There is no set age to change tires, but he recommended closer inspections after six years.

Tire manufacturing date is on one side of the tire sidewall and are the last numbers in the DOT molded in information. See the book for details. Not all manufacture's complied to this recommended standard on all tires before January 1, 2002 (0102)

Extend tire life by washing before long term storage, increase storage pressure to maximum on the sidewalls, place barriers between storage floor/ground surface and cover tires to protect from UV damage. Michelin does not recommend any tire protection dressings because they may contain petroleum products, alcohols or silicones that can accelerate the aging process over a long period of time. Tires in use last longer than those in storage.

Michelin has no restriction as to the method of rotation but recommends including the spare and changing direction of rotation to achieve the maximum life of a set of tires. If wear is even, there is no need to rotate tires. Spares should be covered to protect from direct sunlight.

Tire repairs from a puncture should be made from the inside with a patch and plug/insert and the casing inspected. Never repair a nail hole "on the wheel". An LT tire with Perforations larger than ¼" diameter should be scrapped.

A slow leak with Aluminum wheels can be caused by oxidation between the "O" ring, valve stem, and the wheel. Bubble check the valve stem where it comes out of the wheel.

If a blow out happens, step on the accelerator and steer straight ahead. Do not brake. With control maintained, slow down and pull over.

Truck Rut Sensitivity Summary (0-10, with 10= high, 75 owners reporting)
Click HERE for a chart and HERE for the raw data in MS Excel format.
Click HERE for a chart showing suspension modification sorted by rut # and HERE for the raw data in MS Excel format.

There was a major difference in opinions of Truck Rut Sensitivity between a stock suspension with F/S (ragwalls) and S/S (all steel) tires. 2/3 of those with F/S tires had values of #4 or less. 3/4 of those with S/S tires had values of #5 or more, with none below #2. Tire pressure or tire size did not seem to have any impact. 92% were E load rated tires. 57% were S/S tires. Clearly, fabric sidewall produced greatly reduced truck rut sensitivity.

F/S tires (D & E, 16.5 & 16) with stock suspensions show the largest number of owners reporting truck rut sensitivity at #2 with nothing beyond #6. Only one owner with a stock suspension reported #0 truck rut sensitivity. His coach had the original combination of radial steel wheels with 8.75R16 D "radial construction" Cooper tires.

The S/S tires with stock suspension showed the largest grouping at #8 with some as high at #10. Suspension modifications dropped that grouping to #7 with nothing beyond #8. Changing from S/S to F/S tires indicated greater improvements than suspension modifications.

All others reporting 0 or 1 sensitivity had multiple modifications, with 2/3 having either 4 bag or TRU-TRACK type rear suspension modifications. One had both and another had a 4 bag in combination with a front wheel spacer.

11 of those reporting #2 sensitivity had Fabric sidewall radial tires with stock suspensions Two more had changed to a single heavier stabilizer bar. Only 3 reported #2 sensitivity with all steel (S/S) radials, 2 with a stock suspension and 1 with multiple suspension modifications.

There is no single magic bullet of suspension modification. Many with multiple modifications had lower values. Others with those same modifications had much higher values. All of these coaches are 25 years or older so worn parts enter into the values. The value itself is the owner's opinion and what one owner would see as a 4 another may see as a 6. Modifications were more successful in further reducing truck rut sensitivity with the F/S tires than the S/S tires.

I did not attempt to establish a relationship of the Truck Rut Sensitivity to the imbalance found during coach tire load weighing. There were to many imbalanced variables to reach any conclusions in that sample size.

Weigh-in Summary
Click HERE for a chart and HERE for the raw data in MS Excel format.

47 of the 123 coaches at the rally had their coaches weighed. The Michelin scales were available 2 days before the rally, but 1/3 of the attendees were already parked. The owner of every weighed coach was given a card with each tire load recorded. The scales had manufacture's calibration seals that were within their recalibration due date.

The main purpose of the weigh-in was to enable the owners to match axle pressures to maximum axle loads, maximizing tire life and safety. A second purpose was to find large side-to-side imbalances and misadjusted front torsion bars.

The results show some coaches were imbalanced and overweight.

34% front and 17% of the rears had side-to-side imbalanced loads of 200 pounds or more. The maximum front imbalance load was 550 lbs. This pointed out the need to weigh each GMC tire individually and not just front and rear totals as done by most drive on commercial scales.

58% of the coach's had a frame twisting front imbalance with a rear imbalance on the opposite side.

The maximum front tire load was 2,480 pounds along with an imbalance of 480 pounds. 6% of the coaches with more than 250 lbs imbalance exceeded the original D tire load rating. None of the rear tire loads exceeded D tire load rating. 92% of those 75 reporting on the tire information sheet had E rated tires.

57% of the front tire total loads exceeded the GMC specification of 4,200 pounds. 76 % of the total rear tire loads exceeded the GMC specification of 7,500 pounds.

It was suggested that those with high front tire loads or imbalanced loads look into shifting weighty items, adjusting the front torsion bars, or both.

Please click HERE to view the complete presentation as shown at the rally. And here are the documents suitable for printing. TireSafetySeminar.pdf and TireSafetySummary.pdf

HERE is some other good reading concerning tires and GMC Motorhomes

Michelin RV Tire Guide
Michelin Load and Inflation Charts for RV Usage

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