Cab Design Plays Role In Truck Selection

Construction applications place unique loads and stresses on truck cabs. To withstand this environment, the cabs must be based on a robust design. But weight and ease of repair are also key issues ' reduced weight equates to increased payload, and trucks used in this environment are subject to crash damage.

Material Choices

Two of the most common materials used in vocational cab construction are steel and aluminum. 'For the most part, steel seems to be the preferred cab for the construction industry,' says Bill Sixsmith, director of marketing, severe vehicle service center, International Truck and Engine Corp.

But the proper choice for your application really depends on which trade-offs better suit your needs. 'Aluminum has two primary attributes ' it is weight efficient and can be more corrosion resistant,' says Jim Crowcroft, manager of product marketing, Sterling Truck Corp. and Western Star Trucks. 'Steel has the attributes of being a little more robust and a little stronger, and it is also lower cost.'

Sterling and Western Star cab platforms are both based on steel. 'They are all unit body welded steel construction in both cases. We don't use any rivets or any kind of secondary fasteners,' says Crowcroft. 'But we do offer aluminum components where it makes sense to save weight for specific applications.'

The trade-offs of using steel are addressed by using the newer high-strength steels to minimize weight, and double-sided galvanized and specialty-treated steels to minimize corrosion. 'You can prevent corrosion down to the point where it is almost nonexistent,' says Crowcroft.

International brand trucks also use steel cabs. 'If you need the weight savings and you want to go with aluminum, that's fine. But if you want longevity, then steel is the direction that you want to go,' advises Sixsmith. 'We feel that steel is ultimately the way that everybody is going to go.' This is due to the cost of tooling for aluminum.

Sixsmith also encourages customers to look at the construction techniques used to manufacture the cab. 'We feel that the lap joint seam with bonding holds up best long term as opposed to spot welds that can give,' he states.

To help resist corrosion, International does not use any exterior hinges on the doors. 'We try to maintain the integrity of the cab by not having any exterior holes drilled into it on the roof or on the side to keep moisture and sound out,' says Sixsmith.

Mack Trucks Inc. has chosen to manufacture cabs out of galvanized steel. 'When we introduced the Granite, we made steps forward in cab integrity to the point where we now have the strongest cab that we have ever produced,' says Steve Ginter, marketing manager for vocational trucks. 'Every day, all of the critical welds on the cab are tested with a pry test. Periodically, we tear down the complete cab to verify the integrity of every weld.' He adds that it was a conscious decision to use galvanized steel as opposed to aluminum or composite materials to maximize cab integrity.

The weight issue has been addressed by design. 'We are able to enjoy some weight savings with our galvanized steel cab [over previous designs] by varying the grades and strength of the steel that is welded at particular points. We also build the components as part of the structure rather than adding braces,' says Ginter. In addition to being galvanized, the steel is also E-coated to protect against corrosion.

Peterbilt chose a different approach. It has taken load measurements on various components across many vocations. 'We use that data in terms of optimizing the structures in the cab,' says Craig Brewster, chief engineer.

'Our material of choice is aluminum,' Brewster continues. It is light weight and provides adequate strength. 'But there are elements in the structure where we do use steel. It is in the areas where there are high cycling stresses, in which aluminum's fatigue properties are not as good.'

There are also some companies using composites on the cabs. Sterling Trucks uses a sheet-molded compound fiberglass material for the doors and hood. 'That has proven to be the most cost-effective and the most durable material to use for non-metal components, like the hood,' says Crowcroft. Sterling Trucks Inc. is one of the few companies that uses a composite door. 'It is lighter, stronger and absolutely will not corrode.'

Attachment Points

Cabs are constructed to be very rigid, which offers many benefits. 'It helps reduce noise and vibration, which can result in driver fatigue,' says Ginter.

The way the cab is attached to the frame has a definite impact on both durability and operator comfort. 'By its nature, a cab is a very stiff and rigid component that is mounted in the front and in the back,' says Brewster. 'The frame rails, on the other hand, are very dynamic components.' They must be able to flex when traversing uneven terrain. Due to these differences, the interface on how the cab ties to the frame is very important, particularly in vocational trucks where there is a lot of frame movement.

As the frame moves, you don't want loads to get transferred into the stiff cab structure and create high stresses that can fail the material. 'The killer of the cab is fine cracks,' says Sixsmith. 'It starts with fine cracks that get enhanced more and more. Sooner or later you have a cab that is completely losing its integrity and it usually has to be replaced.' This can be avoided through the proper attachment system designed to isolate the vibration.

The importance of this interface means most truck manufacturers have proprietary designs. Consider the elastomeric materials used in Peterbilt's cab mounts. 'On one hand, you want a lot of isolation, so elastomeric materials that will allow for that motion are preferred.

But too much motion is not good either,' says Brewster. 'The cab will have a lot of motion and be uncomfortable. So the real trick is to control it in a way that dampens out the loads coming into the cab, but not to the extent that the cab doesn't feel stable.'

Air suspension is a popular alternative for mounting the cab to the chassis. International's Sixsmith claims air-suspension is the best alternative.

'Air suspension is a really good way to go because you have a lot of options in terms of controlling the abuses that are fed from the chassis back up into the cab,' agrees Sterling's Crowcroft. 'The only downside to that is the air suspension system is typically more expensive than rubber block suspensions of the past.'

Sterling has addressed this issue by designing a simplified system with a lower cost. 'We pretty much make it standard on three-quarters of the applications,' says Crowcroft. 'We really see air suspension as the best way to go for long-term durability of the product.'

Mack uses widely-spaced air bags to support the cab. 'The air bags are outside the frame rail width. We also have canted shocks ' shocks mounted on an angle ' to provide stability,' says Ginter. 'An air-suspended cab, coupled with a quality air-ride seat, improves driver comfort. The payback is in driver retention.'

Easy To Repair

Crash damage is an unfortunate reality for vocational trucks. 'It is likely that it is going to be damaged at some point in its life by being run into, getting something dropped on it or the driver running into something with it,' says Brewster. 'A cab structure should be put together in a way that makes it easy to repair when damage occurs. A good design is simple. If it looks simple, then it is likely easily repaired. As you look at different products, you can see those that are complex in design.'

One example of an easy-to-repair design is Mack's two-piece windshield. 'Our vocational customers appreciate the efficiency of this design,' says Ginter. 'If this windshield gets damaged, you only have to replace half of it.

'And our windshield is not an integral structural cab element,' Ginter adds. 'It is locked in with a rubber gasket. So if the windshield is lost in a mishap, the structural integrity of the cab is preserved.'

Ginter also points out that the galvanized steel cab construction makes repair easier. 'All professional body shops know how to repair galvanized steel,' he states.

User Friendly

After you examine cab construction, there is the issue of cab functionality. This is even more subjective. There are issues of ingress and egress, comfort of the seat, the ergonomic layout of the wrap-around dash and visibility.

These are all addressed in unique ways by each of the manufacturers, whether it be as subtle as a drip edge on the roof, factory sun visors, tinted windows to aid the HVAC system, corner windows, easy to blow out interior designs or high-quality air-suspended seats. Prior to selecting your next truck, take time to carefully compare the functionality of the various offerings. The photos presented here only scratch the surface on some of the options available.

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