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June 2012 Nuts & Bolts

Custom Truggy
Fred Williams
| Brand Manager, Petersen’s 4Wheel & Off Road
Posted June 1, 2012

Your Tech Questions Answered!

Brake Debate?
Q In some of the pictures of trucks, on the axle by the differentials is something that looks like a disc brake. What is that called, and what is the purpose of it?
Brian A.

A What you are looking at is a pinion brake found on many 2½-ton Rockwell axles. The Rockwell axles have very heavy drum brakes from the factory, and by removing them you can lose up to 150 pounds of weight. This also removes issues of mud contamination of the drums and shoes. Adding wheel-end disc brakes isn’t impossible; in fact, companies like Boyce Equipment (800.748.4269, offer both pinion and wheel-end disc brake conversions. The pinion brake kits are much cheaper at around $400 while the wheel end brakes are closer to $1,250.

If you run a pinion brake it works similar to a wheel-end brake except that the brakes are applied prior to the axle gear reduction. A pinion brake also works better with some sort of locking differential like a Detroit Locker or spool to disperse the brake to both wheels evenly. A rotor mounted at the pinion will also spin much faster than wheel-mounted brakes, which causes them to heat up more at speed. On the Rockwell the driveline spins 6.72 times for every one tire rotation. This means at 55 mph the rotor will be turning at nearly 370 mph! Driveline brakes are a great off-road–only option, but should not be used on the street.

Suspension Contention
Q As an off-roader and mountain biker with a full suspension mountain bike, I have always wondered what the significance of the uptravel/downtravel (or sag) is and how it impacts off-road performance. For instance, I see desert race trucks that look like they are set to ride around 80 percent into their travel. It seems like it would be harder to bottom out the suspension and would put unnecessary stress on components. If more uptravel was allowed to absorb the impact from landings, wouldn’t it work better? I realize this would raise the center of gravity and increase the possibility for a rollover, but it also makes sense that if the truck sits at 80 percent into its travel, when it lands a jump the first 80 percent of the travel won’t resist compression enough to slow down the landing, which will be left up to the last 20 percent. This is how it seems to me, and I would love for someone to clear this up for me and correct me if I’m wrong.
R. Kennedy

A There are a lot of different reasons for how the suspension of a 4x4 is set up. Most stock vehicles run the suspension halfway between full droop and full compression, so that half of the shock shaft is showing. Competitive rockcrawlers often run the suspension with mostly downtravel, so very little of the shock shaft is showing. This keeps their center of gravity low, and since they rarely go extremely fast but need the suspension to droop out into holes and over boulders, it keeps them stable. The go-fast desert type off-road racers often err on the side of 50 percent or more uptravel, so a larger amount of shock shaft will be showing.

This is contrary to what you have seen, so yes, you are wrong, but maybe the trucks you viewed were on display and not ready to race. In fact, the less wheel travel a desert racer has the more uptravel they will factor into the suspension. This is so that if they are bombing across the desert and hit a big ditch or washout, the suspension is able to soak it up quickly.

For a trail rig I would recommend a 50/50 split between up- and downtravel from ride height, but you could adjust this slightly depending on whether you like to run rocks (more downtravel, less shock shaft showing) or run fast (more uptravel, more shock shaft showing).

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