P Chassis for Motorhomes General Information for Alignment http://users.sisna.com/cebula/P-Chassis-AlignmentProcess.pdf Version 1.
Introduction The intent of this document is to provide both reference information and guidance to get the Chevrolet P Chassis to perform as it was designed to perform and maybe even make a few improvements. I've tried to highlight these in bold. A majority of the information is taken from the 1995 and 1997 “Chevrolet Motor Home Chassis Service Guide” for the P Chassis.
adjustment/maintenance information from a Forward Control Chassis manual for your Class A motorhome. Some things are common, some things are not. The motorhome chassis is basically a light-duty truck model and there are modern improvements that can be made. These will be noted in the text in bold. It is unfortunate that even GM's documentation varies in what it names these chassis. The “P30” term is simply too generic to help when specifics are needed.
Table of Contents The Process.................................................................................................................. 1 Worn Parts................................................................................................................... 1 Air Bags....................................................................................................................... 6 Weight......................................................................................................
The Process The first step to maintaining proper vehicle handling is through the regular inspection and replacement of suspension bushings, ball joints, tie rod ends and just about everything else that makes up the front and rear end suspensions of the motorhome. The second step is to perform a suspension alignment. Sounds straightforward enough, but the P Chassis has been given a reputation for not handling very well, even after an alignment has been done.
axle housing and suspension parts too. Proper lube of ball joints means lifting the motorhome by a frame member so the suspension hangs free and then lube the zerks. Don't forget to wipe the zerks clean before you lube. Otherwise, you will be pushing gritty dirt into the joint. If the motorhome is lifted for lubrication by driving up on wheel ramps, the ball joints are under enough tension to inhibit full flow around the entire ball socket. When lifted, it is a good time to check ball joint wear.
lower control arm bushings get pretty bad after 10-20 years. These are cheap to buy and labor is the real cost. If you can replace them yourself, do it. Also, using a spray-on rubber lube on suspension points on a regular basis is cheap insurance – once you know they are in good shape. An improvement you can make is to use the newer poly bushings. The steering linkage is located forward of the front cross member. The P Chassis linkage is illustrated below.
storage checklist to help prevent this situation. Check the damper attachments to make sure they are tight. Rubber bushings should be replaced if they are worn, cracked or crushed. Check for leaks or noisy operation(it's a shock absorber). In 1991 the GM part number of the damper changed to 22011982. The steering is a recirculating ball type and needs basic checks for fluid level and condition (ever think of changing that fluid?), drive belt tension, loose mountings, and loose pump pulley.
frame mounts. Make sure those mounts are tight and all the bushings are in good shape. An improvement to this stabilizer bar is to replace it with a much stronger version such as the IPD 1 5/8” version and use polyurethane bushings. The 1 5/8” bars available are 10 to 12 times stronger than the stock 1 1/4” sway bar. The front wheel bearings require lube every 12,000 miles with high-temperature grease – really.
Doesn't sound like something you can easily determine? Take it to a mechanic you trust. Air Bags Air bags are an interesting item to folks with this chassis. Leaks can be located by removing the bag from the vehicle, inflating it and submerging it – just like looking for a tire leak. Actually, if it doesn't hold pressure, you have a leak. The logical repair is a replacement. If you replace, replace bags as a pair.
matched to the actual axle weight of your coach and you can eliminate the air bags when done properly. One improvement you might want to consider for the rear is installation of a rear track bar. This will help with that “tail-wagging-the-dog” feeling you might be experiencing, this is a good modification if you tow something heavy. Weight Wait a second. Before you start throwing new springs, bags, etc. at the motorhome, have you checked its weight? You're looking for the GAWR.
the suggested improvement parts. Almost. How about those tires? Tires No brand or type tire recommendation is presented here. However, the first rule is that you do not mix different types of tires on the vehicle such as radial, bias and bias-belted tires except in emergencies. Best option is that all tires on an axle are the same brand, type of tread and same age. A 'highway' (also known as 'steering') version for the front and a 'traction' version for the back is OK.
7.50-16 Bias Ply 30 PSI C,D, E Lbs per 1620 tireSingle Lbs per 1430 tireDuals 35 C,D, E 40 C,D, E 45 C,D, E 50 D,E 55 D,E 60 D,E 65 E 70 E 75 E 1770 1930 2060 2190 2310 2440 2560 2670 2780 1565 1690 1815 1930 2040 2140 2245 2345 2440 Looks like 50 PSI would be a good choice for both the front and rear tires in the chart above. The placard for this particular motorhome said 60 PSI.
duals, are properly spaced from each other. If the duals touch sidewalls, they won't last and are a hazard. (If you do decide on bigger or wider tires, work with a pro. Substituting LT235/85R16 tires in place of 7.50-16 tires is probably not going to work in the duals position without changing rims too – they would be too close together.) By the way, a bent rim can cause problems, but should be caught when you have the tires balanced.
height. Another option is to take the measurements, mark the position of each corner tire and turn the motorhome around 180 degrees, park in the same marks facing the opposite direction and check the four points again. An average of each reading should bring you close to actual. Of course, you need to start this with an almost-level location. This won't work very well if you do it on the side of a hill. REAR: The reason the rear ride height is important to us is because it can affect the handling.
is full to the appropriate level and changed on a regular basis. All lubricants lose effectiveness over time. Every four oil changes ('normal' or 'dusty' cycle) will do the job. The differential can run a temperature approximately 100 degrees above the ambient temperature. Air passing over the housing is the only thing that carries away this heat. Make sure the housing is not severely covered with dirt or dried mud which will insulate and help retain and increase the heat.
measurement equal. (This supposes that the weight is not in excess of the axle rating and you have moved your movable stuff around to help balance the side-to-side weight.) Spacer blocks are fabricated in local machine shops. NOTE: The addition of a spacer block ( 2 ½” wide, 6” long, thickness as needed) can actually improve overall ride quality while the addition of a leaf tends to reduce the ride quality of the vehicle. A new motorhome manufactured with an extra spring on one side is a bad design.
to some extent. Once complete with the work in the rear, re-check the front. If you 'twist' the rear, the front will be affected. FRONT: Coil springs sometimes break near the end and wedge in place without obvious indication of the failure. Look hard. If you think this might have happened, the only way to prove it is to remove the spring. In the front, the measurement is checked on each side between the lower control arm rubber bumper bracket and cross member flange.
down to the “MOTOR HOME” row to get the degree setting. This is the initial point for the CASTER setting. This chart is from the 1995 “Chevrolet Motor Home Chassis Service Guide” for the P Chassis but applies to all manufactured. Note the various vehicles in the first column that use the P Chassis. Also note that there is a separate line for the MOTOR HOME version. It is different from the “P-20, 30” line.
(a) A down-in-rear frame angle must be subtracted from a positive caster specification. (b)An up-in-rear frame angle must be added to a positive caster specification. (This is the most common situation.) (c) A down-in-rear frame angle must be added to a negative caster specification. (d) An up-in-rear frame angle must be subtracted from a negative caster specification. (You can make up the measuring tool by picking up a cheap plastic protractor, file a small notch at the 0 center on the flat side.
MODEL CAMBER TOE-IN (IN.) G10, 20 .5° 3/16” G30 .2° 3/16” .25° ± .25° 0° to .06° CLASS A MOTOR HOME (32 – 52) Toe-in was reduced from 5/16 inch in 1985 as part of a GM trend reducing toe-in. If equipped with radial tires, some tire manufacturers would suggest toe-in specifications of 1/32 inch to 1/8 inch. Basically, you want the tires parallel, if you can't get them exactly on zero, make sure it is toward that .06 degrees of toe-in and NOT toe-out.
CONGRATULATIONS! Your Class A motorhome should no longer be yanking on your shoulder sockets and you can enjoy the next trip a little better. You also should now have a better feel for why some maintenance needs to be done, the airbags should be consistently inflated to the same pressures and the tire pressures maintained properly. They all affect handling and safety. The Appendices following this are for further information. A and B help in determining if your springs are up to snuff.
Appendix A – Front Load Height Curve Known weights compared to actual dimensions can determine if the spring is performing according to its rating. Actual measurements will be ±½-inch on the chart and normally considered within the spring makers production capability.
Appendix B – Rear Load Height Curve Main Body Page 20 of 38 Total Pages
Appendix C – VIN 1979 - 1980 C P Y 3 7 9 3 1 0 0 0 0 1 1. Selling Division 2. Chassis Type 3. Engine Description 4. Series 5. Body Type 6. Model Year 7. Assembly Plant 8. Vehicle Sequence Number 1. Selling Division C - Chevrolet T - GMC Truck 1973-75 Models Q - 250 Six T - 292 Six X - 307 V8 V - 350-2 V8 Y - 350-4 V8 M - 400-4 V8 Z - 454-4 V8* L - 454-4 V8** 2.
1981 – 1983 1 G B J P 3 7 W ? C 3 3 0 4 6 1 1 Country 1 = U.S.
1984 - 1995 1 G B J P 3 7 N 8 M 3 3 0 4 6 1 1 Nation of Origin 1 = U.S.
Appendix D – An Alignment Primer Main Body Page 24 of 38 Total Pages
Appendix E – Tire Inflation (as of 1995) Main Body Page 25 of 38 Total Pages
Main Body Page 26 of 38 Total Pages
Appendix F – Towing The GCWR (Gross Combination Weight Rating) includes weight of both the motorhome and anything being towed. Only the V8 and diesel models are shown here. The combination of engine and rear axle ratios are the determining factor for the GCWR on the P Chassis. GCWR 10,000 10,500 11,000 ENGINES 11,500 12,000 12,500 13,500 14,000 14,500 15,000 16,000 19,000 4.10 4.56 4.88* REAR AXLE RATIOS 5.7L (350) V8 GAS 3.08 6.2L V8 DIESEL 3.42 3.23 3.42 3.73 4.10 3.73 4.10 4.
Next 3 digits indicates the day built - expressed in Julian Date form Last digit indicates the shift 1=1st shift, 2=2nd shift. (Don't have any idea what the purpose of this is.) Another way is to try and find the RPO codes sticker somewhere in the coach, your Chevy dealer can interpret it for you. The RPO info I have is: RPO 066 is 4.10 RPO 005 is 4.56 Don't have info for the 4.88 or any others. Those are the easy ways.
Appendix G – Chart for properly matching tires to rims/wheels. Information obtained from the 1994 Tire and Rim Association Yearbook. TIRE SIZE (1) APPROVED RIM CONTOURS (2) LIGHT TRUCKS 6.50 16LT 7.50 16LT LT225/75 16 41⁄2K, 4.50E, 5K, 6K, 6L 5.50F (SDC), 6K, 6L, 61⁄2L, 7L 6J, 61⁄2J, 6K, 61⁄2K, 61⁄2L, 7J, 7K, 7L LT245/75 16 LT265/75 16 LT285/75 16 LT215/85 16 61⁄2J, 61⁄2K, 61⁄2L, 7J, 7K, 7L 7J, 7K, 7L, 8J, 8L 8J, 8L, 8LB, 8KB 51⁄2J, 51⁄2K, 5.
Weights Worksheet From Plate: GAWR FRONT Actuals: Left Front Right Front Total ____________ ____________ ____________ ____________ GAWR REAR Left Rear Right Rear Total ____________ ____________ ____________ ____________ Tag Left Rear Tag Right Rear Total TOTAL GAWR ____________ ____________ ____________ ____________ ____________Total Actual Weight Brakes Capability Max Weight (digit 4 of VIN) ____________ VIN # _______________________________________ GCWR Capability (Appendix F) _
Understanding motorhome weights You've probably heard it said that a chain is only as strong as its weakest link. This theory applies to motorhome chassis as well. The chassis manufacturer establishes weight ratings and limitations based on the major components of the system — the engine, transmission, brakes, axles, tires and frame. These components are designed to accommodate a particular weight, and if they are overloaded, their life expectancy diminishes rapidly.
tongue weight of the towed vehicle that can be carried by this motorhome. (NCC can be determined by subtracting the UVW from the GVWR.) It's important to look at the definition of each term, particularly the UVW and NCC. Note that UVW is defined as leaving the factory with full fuel, oil and coolants. The NCC represents how much "stuff," including fresh water, the motorhome can carry.
When purchasing a motorhome, you may not find a place to weigh your motorhome wheel by wheel, as weighing experts recommend. However, you can weigh each axle. Then, you can evaluate the placement of storage areas, liquid tanks, major appliances, slideouts, generator, LP-gas tank, etc. to see whether the distribution of these heavy components could cause problems. If the capacity is not reasonably distributed, you may have difficulty loading the coach within its limits.
then calculate the axle loads and the actual gross vehicle weight. Add the wheel loads for each axle and compare the total to the GAWR for that axle. If the total is greater than the GAWR, then you exceed this rating. If the total is at or near the GAWR, a tire overload is probably. Now, add all of the wheel loads together and compare that total to the GVWR for your coach. If the total exceeds the GVWR, then you exceed that rating.
