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1985 GM M1008 - GI Gyp: Part 7

Posted in Project Vehicles on October 29, 2014
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Bumpstops are an often overlooked, yet important integral part of your 4x4’s suspension. Improperly located or a lack of bumpstops can result in poor performance off-road, torn-up tires, or broken and bent steering and drivetrain parts. Most lift kits come with new bumpstops or extensions, although many enthusiasts leave them off in an attempt to increase wheel travel. Unfortunately, this practice can do more harm than good in some cases. Over compressing coils and leaf springs can cause them to sag and lose lift height very quickly. Also, without properly placed bumpstops, it’s possible for the shocks to bottom out, damaging them or causing the mounts to tear off of the frame or axle.

Tiny factory rubber bumpstops were designed to work in a stock application. It’s easy to understand why they need to be upgraded once a lift kit, bigger tires and wheels, heavier axles, and thick, weighty body armor is added to your 4x4. With the tires fully stuffed into the trimmed fenders of our truck, the stock bumpstops were still about 4 inches away from making contact with the axle.

The biggest problem on our CUCV project was tire rub. We wanted to keep our truck low with as little lift as possible to clear our wide 39-inch tires. The result was quite a bit of tire rub at full suspension compression. There are lots of options when it comes to bumpstop selection. What you choose for your 4x4 will depend on the vehicle and how it’s being used. We compiled some of the more common bumpstops available to give you a better idea of what works where. We also went to work on our M1008 to find a way to keep our expensive tires from tearing themselves up in the fenders.

An unrestricted suspension can allow expensive tires to get caught up in the many sharp edges of an inner fenderwell. In some cases, it can be so bad that forward progress becomes impossible. It’s possible that you may be able to go in and trim more metal for clearance. We had cut as much as we were willing to trim, so bigger bumpstops were the most logical option.

If you have an RTI ramp available or even a short, twisty trail near your home, flex out the suspension to see where the tires hit. Use a tape measure to estimate where the bumpstops should be relocated to. Check all four wheelwells and look for components that might run into something. Inspect around the engine oil pan, transmission, axles, steering linkages, driveshafts, and so on to make sure there are no contact points. Vehicles that have undergone extreme suspension modifications or engine and axle swaps will need more careful attention.

Prior to hitting the trail, you can install a zip tie on each shock shaft to see how much of the shocks travel you are using. If the zip tie gets crushed by the shock, then you have a pretty good idea that your bumpstops need to be lowered.

Many off-road enthusiasts can’t resist the temptation of using expensive trick-looking race parts. Nitrogen-charged bumpstops such as these King 2.5-inch-diameter air bumps are really better suited on vehicles that see higher speeds or abusive air time off-road. They offer up to 4 inches of travel with internal valving to help slow heavy suspension compression. At full pressure, they will limit slow-speed articulation and wheel travel. Of course the nitrogen pressures can be adjusted to be less harsh for regular trail use, but ultimately nitrogen-charged bumpstops are overkill for most enthusiast 4x4s. Although, application-specific air bumps such as those from Light Racing can make an IFS 4x4 perform much better on- and off-road.

The Daystar Stinger is an adjustable cylindrical bumpstop. You can swap out different foam and urethane inserts to adjust the bumpstop stiffness depending on the weight of your 4x4 and how you use it. There are no seals to blow and there is no nitrogen or oil that can leak out. The compact footprint makes the Stinger a good choice for vehicles that don’t have a lot of undercarriage real estate. The Stinger inserts are good for about 5,000 cycles, so they are not suitable for race use.

The Daystar Stingers fit into typical 2-inch-diameter mounting cans that Daystar also offers. If you decide to upgrade to 2-inch air bumps down the road, they will fit right in your existing Daystar mounting can.

A simple threaded weld bung cap is also available for the Stinger (top right). This comes in handy when mounting the bumpstop inside a coil spring. You’ll also notice that the Daystar Stinger has adjustable travel. You can assemble it for 3 (left) to 4 inches (right) of compression.

For general on- and off-road use, you can’t beat the cost-effective simplicity of a large Daystar urethane bumpstop (PN KU09009BK). If you have the space to fit them and a solid flat mounting surface, the single mounting bolt makes installation a painless operation. Many applications can be fit without welding.

Originally, we planned on completely rebuilding the bumpstop mounts, but then realized we only needed to drop the new larger Daystar bumpstops 2 inches. We spaced the bumpstops with a short section of 2x2-inch, 0.120-wall square tube. For our truck and how we use it, this will work fine. For high-speed bashing and air time, you’ll want more reinforcement than this. It’s a good idea to build a mount that keys in with the bumpstop, as sometimes single-bolt bumpstops can rotate and come loose.

Here is an example of a very solid way to mount a large urethane bumpstop for abusive conditions. The well-reinforced mount was fabricated by T-Bilt with 3⁄16-inch, 2-inch-wide strap steel and gusseted in all the right places.

Out back, our bumpstops rest a long ways from the axle. Our tires don’t rub the fenders, and our shocks don’t bottom out at full stuff. The 1-ton leaf springs will probably never flex enough to cause spring sagging, so we simply left it as is.


Phoenix, AZ 85043
SPC Performance
King Off-Road Racing Shocks
Garden Grove, CA 92843
T-BILT Super Duty Fabrication

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