ADDING LENGTH TO A FRAME.
By Brad Sears
Question: I have a super c with the I-Beam front suspension on the Chevy Kodiak chassis. While I love the chassis, the body (Four Winds) has issues. Four Winds lengthened the frame by 30″, giving me a 24.5″ box behind the cab. This is on a 213.5″wheelbase.
While GM recommends against extending the frame where the cargo loading is less than 7% front axle/93% rear axle, Four Winds did it anyway, giving me a 0% front/100% rear loading ratio. In other words, all of my CCC loads the rear axle.
My question is are there any federal laws/guidelines against overloading the rear axle on non-commercial vehicles, and are there any laws that should have prohibited what Four Winds did on this coach?
Denny.
Answer: This one modification is one that I have never technically understood. Adding on to the back of the chassis is one thing and will not, other than adding to load to the back thus changing caster unless added springs are installed, but adding length to the frame between the wheels is something else.
The angle that is effected is what is called toe-out-on-turns. When a vehicle is driven around a corner both front wheels will travel a different distance around the same center point. This means that the inner wheel must turn sharper than the outer wheel to prevent scuffing. However they have to be coordinated with the center line of the rear axle. When the Ackerman steering was engineered it was found that the front steering arms could be angled inwards to accomplish the needed toe out on turns.
The steering arms are the arms that the tie rod ends are attached to and transfer steering input to the spindles. If the steering arms are angled in so that a line drawn through them intersects the cent of the rear axle, the steering angle will be correct. In adding to the frame length between the front and rear axle the lines drawn through the steering arms will now intersect with each other in front of the rear axle. This will make the front tires scuff on turns and could change the handling of the rig.
The loading the rear axle with all of the CCC instead of having some on the front axle will add more positive caster as described in last weeks blog. This could again change the way the rig handles and effect tire wear.
To answer the question about what laws may have been broken, the only answer is maybe. And again the laws my not apply because of the fact of two manufactures building one vehicle. I have asked the question of my contact at Chevrolet and he says that he will get back to me. I will publish his answer as soon as I get it.
But the up shot is that modifications to a frame without correcting any of the engineering is not a chassis that I would want to have.
Stay tuned because next week we will look at changing tire sizes on your rig.
Brad Sears
jimnlin
a question was asked about what fed laws are broken if rear axle capacties are exceded for non commercial vehicle. Our 49 CFR motor vehicle regs on RAWR/FAWR/GVWR/ tire caps are same for commercial (FMCSA) or non commercial (NHTSA/FMVSS/DOT). See definitions in 49 CFR part 571.3 for GAWR/GVWR in any of the administration booklets or website that gives the full up to date regulation (not just a shortened version). Also part 567.7 gives some more input on legalities on vehicle alteration. See FAQ on NHTSA homepage under laws/regulations and FAQs for some input. There are 24 questions/answers and a few may give insite on vehicle alterations. NHTSA does have a contact email number.
As a former commercial hauler one thing I found in looking at regs is contact the state the vehicle is registered in for their input on how they interpet and enforce a particular NHTSA reg. Most states will be simular in enforcement as fed monies can be witheld for non compliance of those 49 CFR motor vehicle rags.
JIM
Denny
Brad,
I just logged on and found your blog post… thanks for addressing my questions. Did you hear anything back from GM? I cannot get answers from either Four Winds, Thor, or GM on this matter.
To Fred, I had RVSEF weigh my coach a couple weeks ago… about 85% of max on the front, and 100% on the rear… actually, I was 300 lbs over max on the drivers side due to poor weight distribution by Four Winds. Drivers side is more than 1500 lbs heavier even when empty except for fresh water. So much so that the coach leans 4″ when it is empty.
It would be interesting to know the front/rear axle scale weight as it leaves GM as an incomplete vehicle.
Denny
Fred Golden
So what you are saying is you think that the manufacture only added weight to the rear axle when the motorhome box was added and the front axle weight stayed the same as it came out of the chassis plant. As the final manufacture, they are allowed to do that. Some manufactures even build RV’s with axles that are almost overloaded when they leave the factory.
As long as the front and rear axle weights are not exceeded, and both have a nearly equal percentage of their full capacity, then everything should be safe. But loading the rear axle to say 95%, while only loading the front axle with 60% of it’s weight rating might cause handling problems due to not enough weight on the steering axle. This can cause further problems if the owner wants to tow a trailer with a heavy hitch weight, and the ball is several feet behind the rear axle.
That is why taking your motorhome to a scale will inform you of what you can do to properly balance your RV. While some manufactures actually have 400 or more pounds difference between the two front tires, most do a much better job of balancing the RV.
Fred.
William A. Hart
I have a problem with a Four Winds, Dutchmen Dorado 26 foot that doesn’t track at interstate speeds. It is a Ford V-10, short wheel base with a queen sized rear slide-out. The rear overhang, like many Class C or B and a half approaches 10 feet. The rig is equipped with Mitchlin 16 inch wheels. Compounding the problem is the large drinking water tank at the extreme rear of the passinger side of the coach.
The alignment was checked by a truck shop in the first 500 miles. I have installed Safe Steer on the front axels and a hvy sway bar on the rear springs to stiffen the rear sway. With a small reduction of air pressure in the front tires (79 vs 80lbs) the rig will mostly track at 60 MPH. I still have to correct the steering more often than is disirable. At 65mph I have to pay attention to the lane useage. Passing 18 wheelers or cement trucks are only a small bother. I have have a variety of rigs (Class Cs and an A) for seven years. This “downsized” rig is most troublesome! Any ideas? Any other Dorado drivers with similar problems??
WH