GMC Motorhome Radial Tire Selection and Recall
History
by Chuck Botts (4-8-02)
Study Summary
GMC Standards and Notices
Recall Review
Approved Tires & Alternates Left
After the Recall
My Coach Handling Problems and Manufacturing/Industry
Standards
Load and Pressure at 75 mph rating
Year of Manufacture
When to Replace Tires
Tire Balancing of Center Located Wheels
Alcoa Wheels Tightening Torque
My conclusions
There has been so much confusion and differing opinions on the GMCnet about which tires to buy and what their pressures should be; I thought I would document my own investigation to establish a rationale for choosing my coach LT225/75R16 E tire pressures and 50,000 miles experience at those pressures. I also used this information to determine my next set of tires for my 1977, 23', lightweight coach.
More than half the original GMC coaches produced were delivered with D rated Bias tires, which were never recalled. Bias wheels should not have radial tires installed. In late '76 radial tires were phased in, of those, only specific serial number General radial tires were recalled.
For those who over load (exceed) the GMC original axial loading specifications of 2,100 lbs/front tire or 3,750 lbs/rear tire set, or those who do not properly weight their coaches, then E rated tires at 80 psi should be used. For the General Tire radial recall of specific serial number tire, Cinnabar Engineering went one step further and recommended all steel sidewalls in their recall recommendations. All steel E rated radial tires at 80 psi is a safe recommendation for radial wheels.
If, on the other hand, you do a proper weighing of each maximum tire load and determine the maximum axial loads then you might want to follow the current industry standards of tire selection and inflation pressures like I did. This may lead you to either D or E rated tires at varying inflation pressures, depending on your weighing results.
A’Weigh We Go, www.aweighwego.org , is a commercial RV safety organization that specializes in weighing RV’s at rallies and providing weight analysis and recommended inflation pressures reports. Tire load and inflation tables are also supplied without charge. Send a Self addressed, stamped business size envelope and include : tire manufacturer, tire size and load range and the rating data from the sidewall – both maximum load and inflation pressure for single and dual
Chuck Botts, San Diego, Ca. remodeled 23' 4/8/2002
During the middle of the 1976 model year, GMC changed from bias to General radial tires for their coaches. Bias ply nylon tires continued be an option for Transmodes.
To start with, I returned to the original GMC Owners Manuals to see what the specifications were on weight, tires, and pressures.
X-7512b Front GAWR = 4,200 lbs, rear GAWR = 7,500 lbs for a total of 11,700 lbs with everything including people but without fuel or coolant. (Strange standard, 50 gals of fuel would add 300 lbs or 12,000 lbs fully loaded. The 76 manual ends up with the same totals but states curb weight is without driver, passenger or cargo, but does include fuel & coolant)
Tire replacements were specified as 8.75 16.5 load range D, bias-ply polyester cord or bias-ply (Jumbo) steel belted.
Tire pressures were specified at 60 psi cold except when operating at continuous high speed, over 65 mph. Under those conditions the tire pressures should be increased to 70 psi. That standard was specified in all operating manuals until the radial tires were introduced in 1976.
The X-7725 (77 & 78) Section 10 Wheels and Tires specified 8.75-16.5 LT bias belted or 8.75R16.5 LT, both load range D. For 77 models, steel belted bias-ply (Jumbo) was standard with options for steel belted radial (Jet) tires. For 78, steel belted radials (Jet) were standard. Transmodes had an option of bias-ply Nylon tires for both years.
Tire pressures for bias belted tires remained 60 psi with radial tires inflated to 65 psi. Again, for continuous high speed operation over 65, cold pressures should be increased 10 psi above recommended inflation pressures. (Here, both tires would be above the current Tire and Rim Association recommended pressures of 65 psi for load range D tires at 75 mph. Tire technology advances or changes in the standards in the last 25 years?)
X-7780-A for my 1977 23' Transmode, "Front GAWR = 4,200 lbs, rear GAWR = 7,000 lbs for a total of 10,500 lbs with everything including people but without fuel or coolant." My fully loaded coach weights in at 4,160 lbs front and 6,180 lbs rear for a total of 10,340 lbs with both of us aboard.
"It is important that the tires on your vehicle be of
the proper size, and be properly inflated. It is important to
avoid over-inflation as well as under-inflation. See SERVICE AND
MAINTENANCE section for proper tire inflation pressures."
"Note: The cold inflation pressures for your tires are:
Bias-ply or Bias-belted. . . .60 psi
Radial Tires . . . . . . . . . . . .65 psi
For sustained driving over 65 mph, cold inflation pressures should
be increased 10 psi above the recommended cold inflation pressures."
Note that two different manuals for succeeding years specify different pressure increases for speeds over 65 mph, `76 specify adding 5 psi and `77, & `78 specify adding 10 psi.
"When replacing tires, you should use size 8.75-16.5LT or 8.75R-16.5LT, load range "D". Also, the construction type must be bias-ply, bias-ply steel belted, or steel belted radial."
GMC #77-1M-1 Nov 76, "The General Jumbo Steel Belted Radial Tire or its equivalent is currently the only tire approved by GMC for optimum ride and handling characteristics." The notice also contains the addition of 2" valve stem extensions with the detailed instructions "Inflate tire to 60 psi." I'll assume from this dealer notice that two-inch valve stem extensions where on production vehicles by November 1976.
This was 1976 tire technology. There was no difference in the manuals between the lighter loaded rear tires on the 23' from the 26' coaches. All tires on the coach were specified by GMC to be at the same pressures and increased for over 65 mph speeds, which is different from current tire technology of matching maximum axial loads to tire pressures at 75 mph rated speeds
I reviewed the General Tire February 18, 1981 Recall notice and GMC Motorhome service bulletins.
The original bias D load range tires on non-radial rims that were produced on GMC's between 1971 through 1976 were never recalled, nor were the transmode optional tires of bias-ply Nylon on all years production. The General Tire February 18, 1981 Recall notice specifically states, "The only suitable replacements on vehicles with non-radial rims is a bias tire. (Refer to Vehicles Owner's Manual or vehicle tire Placard for proper size, load, and operating pressure)"
More than half of the GMC Motorhomes were produced with bias ply tires with non-radial rims. Radial tires exert additional wheel loads which can split a non-radial rim, so those coaches should never have radial tires installed.
Reviewing the Recall notice, the difficulty to easily check tire pressure without removing the wheel covers may have led to coaches running with low tire pressures and increased the possibility of tire failures. The first item on the General Tire Recall Notice was a free set of valve extensions so the tire pressures could be easily checked and air added without removing the wheel covers.
The Recall Notice affected selected S/N of all General Tire's "8.75R16 Jumbo & Jet Steel Radial Tire" on the GMC Motorhome. The Recall also states, "In the immediate future, a consumer letter will be sent to the owners of those GMC Motor Homes affected by the recall". The affected coaches were not published to the public but the wording states that not all the coaches with those tires were recalled. This is very important; it was not a general recall for all those tires but linked to specific serial number tires.
The Recall recommendations were: "If a customer comes in with or without his recall letter, inspect all tires and rims. If the vehicle is equipped with General Radials, make sure he has radial approved rims. If only some of the tires are of the recall serial number, for optimum tire /vehicle performance and consistency (all tires of the same design, construction and load range) replace all tires regardless of serial number with the 8.75R16.5 Jet Steel Radial (LR-E) WSW, Product Code 0103284 only".
This wording confirms that the Recall was not a complete recall of all General radial tires, but a recall of only specific serial number tires. By that statement, General D load rated radial tires, outside the specific serial numbers, continued to be acceptable, and considered safe when inflated to the owner's manual values.
General Tire knew which serial numbers were suspected of being of questionable manufacture quality D load rated radial tires and which ones remained acceptable for use on the GMC Motorhome. The Recall elected to go to an all steel E rated replacements, but, again, only for those specific serial number recalled D rated radial tires.
Wes Caughlan, whose Cinnabar Engineering did the tire testing for General Tire, stated in his June 1999 Coach Talk, "Before General Motors would allow General Tire to recall the tires, we had to demonstrate the technical need and the replacement tires had to meet GM Tire Performance Criteria (TPC). Since handling and ride quality are two of the primary TPC considerations, we had to run a blind test on 65 versus 80 psi cold inflation pressure. Surprisingly there was no difference, and the recall went forward."
E rated tires have to be inflated to E rated pressures to achieve E rated load carrying capability. Since these tires have a maximum pressure of 80 psi printed on their sidewall, there would also be a good possibility that some customers would inflate their tires to that level. Part of the testing would have been to verify that the General E rated Jet Steel tires at 80 psi could still achieve the GMC Motorhome TPC.
The General Recall also states, "Although the replacement 8.75R16 (load range E) tire inflated to 80 psi provides additional reserve load carrying capacity, the Gross Vehicle Weight (fully loaded) SHOULD NEVER EXCEED the Gross Vehicle Weight Rating or the Maximum Front and Rear Gross Axle Weight Rating specified. As stated in your owner's manual, the only way actual weights can be accurately determined is by weighing the vehicle."
The E rated tires at 80 psi carrying capacity exceeds both the GMC coach Gross Vehicle weight ratings (E rated tires are capable of 16,080 lbs versus the GMC 26' 11,700 lbs rating or 12,000 lbs with full gas tanks. The capacity on those tires is 156% of my 23' 10,340 lb fully loaded coach.
Approved Tires & Alternates Left After the Recall
From the data, there are five alternate wheel & tire combinations
left after the recall:
1) 8.75 16.5 load range D Nylon bias-ply, option on all Transmodes
2) 8.75 16.5 load range D, Polyester bias-ply (Standard thru mid
76)
3) 8.75 16.5 load range D, bias-ply (Jumbo) steel belted (non
recalled serial numbers)
4) 8.75 16.5 load range D, steel belted radial (Jet) (non recalled
serial numbers)
5) 8.75 16.5 load range E, all steel radial (Recall replacements)
Cinnabar Engineering currently supports the GMC Motorhome under license from GMC and recommends the E rated all steel radial inflated to 80 psi for all GMC Motorhomes with radial wheels. It is easy to overload the 26' coach's in excess of the GMC ratings of 11,700 lbs or exceed a tire's D load rating. In addition, many owners do not properly weigh their full coaches to determine the proper inflation pressure. Hence, the safest recommendation.
My Coach Handling Problems and Manufacturing/Industry Standards
Like many GMC owners, I upgraded my coach wheels to the Alcoa 16" Classic wheels with LT225_75R16 tires concurrent with the remodeling. I followed the Cinnabar Engineering recommendations to buy all steel E rated tires and initially operated them at the maximum 80 psi pressure. I purchased Goodyear rather than Michelin tires because of availability.
The remodeled coach's initial handling was very unsatisfactory, darting in truck ruts, uneven tracking due to patches on uneven roads and easy rear wheel lock up, particularly in the wet. Adjustments and replacements of steering mechanism parts, four-bag suspension, and multiple alignments helped but did not solve the problems. Discussing my handling problems with a Goodyear Truck Tire Distributor, he logged on to the Goodyear Tire web page and printed out the recommended load versus pressure chart for my tires.
In agreement with his suggestion, I filled all my liquid tanks. Since we were traveling, the coach was then loaded to its maximum. We took it to a truck weighing station and weighed the coach at each of the four load points. With these loads, we lowered the tire pressures to match Goodyear Tire's printed recommendations for the maximum weight on that axle plus 5 psi as recommended.
My front tire loads measured 2,100 lbs on the driver's side and 2,060 lbs on the passenger side. My rear tires were more uneven because the majority of the heavier equipment is on the driver's side. The rear tire loads were 3,271 lbs on the driver's side and 2,908 lbs on the passenger side. Dividing the shared load on the rear tires resulted in rear tire loads of 1,636 lbs for the driver's side and 1,454 lbs for the passenger side. Using the industry standards, all the tires on a given axle should be inflated to match the maximum loaded tire on that axle.
Using the charts and interpolating between the data points:
Maximum front tire load of 2,100 lbs results in a minimum inflation pressure of 57 psi + 5 psi or 62 psi for both front tires.
Maximum rear tire load of 1,636 lbs results in a minimum inflation pressure of 40 psi + 5 psi or 45 psi for all four rear tires.
What a change in character! Later, I found these values were identical to Michelin and the Tire and Rim Association standard values. Most of the objectionable handling problems were greatly reduced. The following is documentation of those standards.
Load and Pressure at 75 mph rating
Michelin's Recreational Vehicle Tire Guide book, Goodyear web page and the Tire and Rim Association Rating agree on tire pressure versus load on these LT225_75R16 tires.
http://www.goodyear.ca/tires/tirecatalog/G159LTLT225_75R16___EData.html
For 75 mph rating, Load and Pressure for correct load/cold air
pressure.
Michelin has some booklets specifically for RV owners
Michelin
RVTIREGUIDE1.pdf
Michelin
RVTIREGUIDE2.pdf
Michelin's book recommends measuring the load on each tire
individually for a fully loaded RV and then, "For control
of your RV, it is critical that the tire pressures be the same
across an axle".
The cold inflation tire pressures versus the loads for single
LT225/75R16 stated in the Tire and Rim Association Ratings applies
to all manufacturers.
35psi/1500 lbs
40psi/1650 lbs
45psi/1790 lbs
50psi/1940 lbs
55psi/2060 lbs
60psi/2190 lbs
65psi/2335 lbs Load limit of D rated tires
70psi/2440 lbs
75psi/2560 lbs
80psi/2680 lbs Load limit of E rated tires
Michelin's Recreational Vehicle Tire Guide also recommend increasing the tire pressure by 5 psi "to accommodate temporary shifting of load from side to side which is common in RV's." Goodyear recommends adding 5 psi to decrease tire wear and improve gas mileage. All the current pressure/load ratings are at 75 mph with no added pressure for speeds above 65 mph.
The Guide also states, "Over inflation will reduce the tire's footprint or contact patch with the road, thus reducing the traction, breaking capacity, and handling of the vehicle. A tire that is over inflated for the load that it is carrying will also contribute to a harsh ride, uneven tire wear, and will be more susceptible to impact damage."
Year of manufacture is currently the last three numbers of the DOT number system. The first two numbers are the week of the year and the last number is the year. That will be changed in the future to a four number system with the first two numbers being the week of the year and the last two being the year
Per Michelins RV book, "Replace tires when the wear strip shows or when sidewall cracks are 2/32 deep. If the cracks are less than 1/32" deep, the tire is O.K. to run. Between 1/32" and 2/32" the tire is suspect and should be examined by a dealer.
In cooler, clean air locations, the expected tire life will be longer than in high temperature, high ozone areas".
Tire Balancing of Center Located Wheels
The balancing must have a 4 or 8 lug adaptor that bolts the wheel to the machine instead of just clamping it. The common cone shaped insert that clamps the wheel to the machine will never properly center in the wheel.
Discount Tire uses a Coast IBS-2000 with an 8-lug adaptor.
Alcoa Wheels Tightening Torque
Torque nuts to 150 ft-lb. Re-torque after first 500 miles.
Alcoa Aluminum weight is 20 lbs versus the original steel wheels 39 lbs.
Operating all steel E rated radial tires at lower than maximum D pressure results in heavier than necessary tires with some added suspension loads and sidewall puncture protection due to stiffer all steel sidewalls.
The current industry published standard methods of selecting tires and tire pressures indicate tire loads that reflect D rated tires can still be used on a properly loaded GMC coach at variable pressures depending on the actual axle loads.
The results of lowering my tire pressures to match their measured load on our light 23' decreased the coach's sensitivity to truck ruts, cross winds and greatly improved the ride. The lower pressure also increased the tire footprint; greatly increasing traction and eliminating the easy lock up of the rear tires in the rain. I now use the recommended pressures of 57 psi + 5 psi on the front (62 psi) and 40 psi on rear + 5 psi = 45 psi. These pressures give me a safe margin over the manufacturers recommended minimum pressures, including the SmarTire warning buzzer at 5 psi loss.
Over the last five years of operating at lower tire pressures, the tires on the front, which see the majority of the individual tire load, were replaced at 50,000 miles before reaching the 2/32" tread depth in anticipation to another 10,000 trip and the rears look like another 20,000 miles will be easy.
Because of all the confusion in tire pressures and proclaimed unsafe high temperature at the lower pressures that I was using, I purchased SmarTire to monitor both individual tire pressures and temperatures. Operating at the lowered pressures, the SmarTire has never measured over 145F even at 75 mph in 117F desert heat driving on blacktop. The SmarTire temperature warning signal is set at 180F. The dire GMCnet warning about low pressure generating high tire temperature proved to be nothing but a false opinion.
Based on my 50,000 miles experience and this analysis, my next set of tires will be LT225/75R16 D rated polyester sidewall steel radials inflated to my current pressures (62 psi front, 45 psi rear). Other GMC owners have had great success going to steel radials with polyester sidewalls to further reduce truck rut sensitivity and directional control problems.