P Chassis for MotorHomes General Information for Alignment DRAFT http://users.sisna.com/cebula/P-Chassis-AlignmentProcess.pdf 1 Version 0.
Introduction The intent of this document is to provide both reference information and guidance for getting the Chevrolet P Chassis to perform as it was designed to perform. A majority of the information is taken from the 1995 and 1997 “Chevrolet Motor Home Chassis Service Guide” for the P Chassis. However, the information has been presented in what is felt to be a logical sequence to accomplish that most elusive item for this chassis – good handling. Age is the common enemy of the chassis.
The Motorhome Chassis also has different frame rails. Be careful to NOT use adjustment/maintenance information from a Forward Control Chassis manual for your Class A motorhome. Some things are common, some things are not. 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. If the 5th thru 7th positions in your VIN number are “P37”, then you have the motorhome version of the chassis.
Table of Contents Introduction.......................................................................................................................2 The Process........................................................................................................................5 Worn Parts.........................................................................................................................5 Air Bags.......................................................................................
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.
hoses or the bottom of the radiator. You can drill right through, just like you can drill through your own hand if you point it incorrectly. Don't forget to clean the rear axle housing 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.
Look at all the rubber washers and bushings for cracks, bulges and wear. The upper and lower control arm bushings get pretty bad after 10-20 years. These are cheap to buy and labor is the real price. 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. The steering linkage is located forward of the front cross member. The P Chassis linkage is illustrated below.
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. If the power steering pump needs to be replaced, be aware that the motorhome version is different from the standard Commercial P-Series version. It is a stronger pump with higher pressure ratings. The wrong one will definitely affect steering effort.
The front wheel bearings require lube every 12,000 miles with high-temperature grease – really. Check the bearings by raising the motorhome and supporting it at the front lower control arm on each side. Now you can spin the wheel and check for noise or roughness. To check for tight or loose bearings, grip the tire at the top and bottom and move the wheel assembly in and out on the spindle. If hub assembly movement is less than .001 inch (too tight) or more than .005 inch (too loose), adjustment is needed.
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. There are basic guidelines for inflation depending on which version of the P Chassis you have.
'ride height' check we will do in a bit and the charts in Appendix A and B will help with this determination. 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. At a minimum, you should be able to find the label that gives the rating for your motorhome for the front and the rear axle. Front 4,300 lbs. and rear 7,500 lbs. is a common combination for a short chassis motorhome with a 137 inch wheelbase.
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. Good option is that all tires on an axle are the same brand, type of tread and same age. A 'highway' version for the front and a 'traction' version for the back is OK. Current recommendation is to replace any tire that is 5 to 6 years old.
Looks like 50 PSI would be a good choice for the radial tire in the chart below from Michelin. Bumping by 5 – 10 PSI will get you to 60 PSI for the same reasons noted above. Note that this chart says “per position”. That means the left rear, right rear, left front or right front. That also means that the capacity listed for the rear duals is for a PAIR of tires. These Radial tires are maximum rated for 3,042 lbs. each – when inflated to 80 PSI.
8-bolt wheels (9/16 bolts) get 140 ft. lbs. 5-bolt front wheels and 10-bolt rear wheels get 180 ft. lbs. We're still not quite ready for that alignment. There are two more items to check. Ride height and frame angle. Both affect the caster that will be set during the alignment. Without these numbers, you CANNOT come up with a valid CASTER setting. Ride Height Ride height is checked at four points. Tolerance for manufacture of the springs is ± ½ inch.
One last item to check before attempting to measure the ride height. You are looking for a bent rear axle housing. Indications are found by noticing the inner dual tires wear more than the outer ones. Also look for grease lube leaks at the bottom of the axle housing at the differential. A split gasket near the bottom area almost always indicates an overload, or flex and housing distortion, which destroys the gasket between the carrier and the housing.
Here is a case study right out of the Chevy P Chassis manual: “GM CASE STUDY: A motor home was loaded to a maximum GVW, both front and rear. Sufficient air was applied to a typical after-market leveling device to establish a 'dead-level' frame. In this case study, wheel travel was limited to ¾ inch before the after-market device “went solid metal-to-metal” between the rear axle and the frame. This severe limitation on wheel travel promoted a “crash-through situation” on even the slightest bump.
hang loose. Be careful to check while doing this so you don't accidentally rip up that brake hose in case it is too short. Modifying the rear side-to-side measurements will affect the front side-to-side measurements 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. http://users.sisna.com/cebula/P-Chassis-AlignmentProcess.pdf 17 Version 0.
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. This measurement must be perpendicular to the cross member flange. It is from “iron to iron”. The rubber bumper is ignored. Look at the illustration below.
INCHES 1 1/2 1 3/4 2 G-30 2 3/4° 2 1/2° P-20, 30 2 1/4 2 1/2 2 1/4° 2° 1 1/2° 3° 2 1/2° 2 1/4° 5 1/2° 2 3/4 3 3 1/4 3 1/2 3 3/4 4 4 1/4 4 1/2 4 3/4 1° 3/4° 1/2° 1/4° 0° -1/4° 2° 1 3/4° 1 1/2° 1 1/4° 1° 1/2° 1/2° 1/4° 0° 5 1/4° 5° 4 3/4° 4 1/2° 4° 3 3/4° 3 1/2° 3 1/4° 3° 5 CLASS A MOTOR 3° HOME (32 – 52) But wait! This may not be the final version of the degree setting you will use for the CASTER.
(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. Then capture a knotted string in that notch with a weight at the end of the string hanging down to indicate the degree of angle - not rocket science. Of course, if your eyesight is as bad as mine, get a big protractor.) Also, note that early versions of the P Chassis Service Manual had the words 'added' and 'subtracted' in a & b above reversed and c & d above reversed.
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 and NOT toe-out.
minus .25 degrees and TOE-IN at 0 degrees or slightly plus up to .06 degrees. After the wheel alignment is completed take the vehicle for a test drive. Note any wandering, drifting or pulling that would indicate that the alignment is still out-of-spec. CONGRATULATIONS! Your Class A motorhome should no longer be yanking on your shoulder sockets and you can enjoy the next trip a little better.
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. http://users.sisna.com/cebula/P-Chassis-AlignmentProcess.pdf 23 Version 0.
Appendix B – Rear Load Height Curve http://users.sisna.com/cebula/P-Chassis-AlignmentProcess.pdf 24 Version 0.
Appendix C – VIN 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 http://users.sisna.com/cebula/P-Chassis-AlignmentProcess.pdf 26 Version 0.
Appendix E – Tire Inflation (as of 1995) http://users.sisna.com/cebula/P-Chassis-AlignmentProcess.pdf 27 Version 0.
http://users.sisna.com/cebula/P-Chassis-AlignmentProcess.pdf 28 Version 0.
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 12,000 12,500 13,500 14,000 14,500 15,000 16,000 19,000 REAR AXLE RATIOS ENGINES 5.7L (350) 11,500 3.08 3.23 3.42 3.73 4.10 3.73 4.10 4.56 4.56 V8 GAS 6.2L V8 3.42 5.13* DIESEL 7.
How do I determine my rear axle ratio? The actual ratio should be stamped into the axle housing. It will usually be found on the forward portion of the axle tube on the right side. Probably pretty gummed up and difficult to read too. The stamping on the housing is read as follows: First 3 digits indicate the rear axle ratio.
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 41⁄2K, 4.50E, 5K, 6K, 6L 5.50F (SDC), 6K, 6L, 61⁄2L, 7L LT225/75 16 6J, 61⁄2J, 6K, 61⁄2K, 61⁄2L, 7J, 7K, 7L LT245/75 16 61⁄2J, 61⁄2K, 61⁄2L, 7J, 7K, 7L LT265/75 16 7J, 7K, 7L, 8J, 8L LT285/75 16 8J, 8L, 8LB, 8KB LT215/85 16 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)
