Dear Mr. Lao:
I was referred by a friend to your column here on TopGear.com.ph. With your expertise in motor vehicles, I am sure that your knowledge is not confined to just light vehicles, such as cars and SUVs, and that it also extends to medium or heavy vehicles.
I want to ask about the type of under-chassis suspension of certain 30-seater passenger buses. I would like to know about the difference between a laminated and a parabolic leaf-spring suspension, specifically regarding their respective performance and advantages. Which is the better choice?
I would appreciate your valuable technical advice on this matter.
Thank you and regards.
William A. Enriquez
Hi, William. When people think of leaf springs, the image that comes to mind would be of the more common laminated leaf spring. This design has been around almost forever, and we not only see it on the older cars from the early 1900s, but also on horse-drawn carriages.
It gets its load-bearing capacity from each of the leaves--or sections of spring steel--that comprise the entire assembly. It is relatively simple to manufacture and can support heavy loads. It also has the added advantage of making the entire rear-suspension design a bit simpler compared to using coil springs.
Parabolic leaf springs are similar to laminated leaf springs in how they are installed on a vehicle. The primary difference is that a parabolic leaf spring, by virtue of its basic design and construction, has the advantage of giving better ride characteristics compared to a laminated leaf spring. It supposedly approaches the comfort level of coil springs, while retaining a simpler suspension design over a coil-spring suspension. The whole assembly will also be likely lighter than an equivalent laminated spring. However, also because of the way they're constructed, parabolic leaf springs are likely to be more expensive than laminated leaf springs. The former is used on certain types of buses wherein passenger comfort is a priority, unlike heavy load-bearing trucks wherein heavier cargo capacity is needed.
I'm thinking that perhaps you have a bus that needs spring replacement, or you may be building a bus from the ground up. If you're replacing springs, then go with the same type that originally came with the vehicle if the current ride comfort and handling have no issues. That would be the best bet.
If you're building a bus from the ground up with a money-no-object budget, or if you have a specific type of build in mind, then you'll need to work with a number of spring suppliers and engineers as to what type of spring and load rating best suit the total vehicle weight, when fully laden or at its normal operating weight. I'd probably go with a parabolic spring if possible. Take note that you will need to match them with the appropriate dampers (shocks) as well.
Hope this helps you.
Do car problems keep you awake at night? Send questions to firstname.lastname@example.org.