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