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SUSPENSION AND BRAKES

PROGRESSIVE RATE BUMP STOPS

Supple suspension gives great wheel travel and a comfortable ride, but load and road conditions can sometimes cause ‘bottoming’. There’s a procedure to control this undesirable situation.

Bottoming occurs when the effect of a sharp bump or load mass causes the springs to flex to the point where the solid rubber bump stops engage with the axle or suspension components.

Rubber bump stops are fitted by vehicle manufacturers to prevent too much spring travel that could break a spring, or cause metal to metal contact between chassis and suspension components.

Bump stops are the last line of defence and, although protective of spring breakage or metal to metal contact, can be quite harsh and noisy in action.

There are after-market progressive bump stops that take up earlier in suspension travel and progressively resist too much suspension compression.

Hollow rubber springs are often fitted as standard to the front ends of leaf-sprung European trucks.

However, before venturing down that path, it’s important to ensure that your vehicle’s springing and damping are correctly set up.

 

Correct springs and dampers first

Step one is to ensure that your spring rates are correct for the load you’re carrying. These days a 4WD maker’s springs are usually correctly rated for load, but if you have a full-time load on board you might be better off with re-rated springs that resist the ‘sagging’ that often occurs with standard springs. A 50mm lift is possible at the same time.

Standard dampers (shock absorbers) vary greatly. The standard items fitted to the Ford Ranger Raptor (Fox racing shocks), the Mercedes-Benz X-Class and, surprisingly, the LDV T60 Trailrider (Chinese-made Sachs) are all excellent out of the box, but all other ute shocks need upgrading.

Wagon dampers are generally fine at light loads, but we’d upgrade all of them for loaded bush work.

Many owners fit air springs to their rear axles and we’ve done that with our LandCruiser 75 Series, but if the bags need high pressure to maintain ride height the rear axle springs need beefing up (provided the maker’s rear axle maximum weight isn’t being exceeded). A need for more than 30psi in air springs probably indicates under-springing.

Air springs are also effective as progressive bump stops, because, as the suspension deflects, the air bags resist compression in a rising rate.

 

The need for progressive bump stops

After making sure your springs and dampers are optimised for your load, it’s time to check whether progressive bump stops are required.

Apart from the aforementioned air spring situation, most 4WD owners don’t need progressive bump stops. However, competition vehicles are invariably fitted with them, because they allow long wheel travel and smooth bump response at high speeds, while protecting expensive racing shocks and springs from excessive compression.

The springs can be softer and longer-travel than would otherwise be possible. However, longer-travel coils can suffer from ‘coil-binding’, where the coils compress into a solid ‘tube’, so progressive bump stops are used to prevent that happening.

Some competition vehicles have progressive bump stops integrated into their dampers or between the damper body and the upper mount. That works fine if the damper mountings are purpose designed to handle the compression loads: most standard shock absorber mounts aren’t strong enough for that.

Many competition vehicles have robust shock absorbers with rose jointed, metal to metal, spherical bearing mounts.

Some non-competitive 4WD owners fit ‘comfort’ suspensions, with softer spring rates than load-carrying versions. These springs can flex excessively on bumpy terrain, risking suspension damage, so progressive bump stops may be in order.

It’s unlikely that standard or after-market conventional leaf springs will flex enough to need progressive bump stops, but spaced-leaf taper leaves or parabolic springs flex much more and can certainly benefit from progressive bump stops.

We’ve fitted parabolic leaf springs to the front end of our trusty old 75 Series LandCruiser and the superior flexibility of these springs improved ride measurably, but also resulted in more spring travel.

Sharp bumps, such as parking lot speed bumps, sent the front end to the bump stops, so we looked for more progressive bump stops.

 

Types of progressive bump stops

Standard bump stops are hard rubber and while progressive, start with a high compression rate and then move to solid state.

More progressive bump stops are formed from rubber or urethane and usually have some voids in their design that deform as the bump stop is compressed.

These stops engage with the compressing suspension components much earlier in suspension travel than do the standard bump stops. Some of them that have extremely progressive action are actually in light contact with the suspension before it compresses at all.

The downside of rubber and urethane progressive bump stops is that they absorb energy when they compress, but feed that energy back into the suspension as it extends, after the bump action.

This means that the dampers must control this extension and that normally means much better quality shock absorbers than standard items, with improved rebound damping. We have Bilstein monotubes on our 75.

However, too much rebound damping can result in a suspension that doesn’t extend back to normal ride height after a bump and if that bump is followed by closely successive ones, the suspension eventually compresses to the point where a massive, uncontrolled rebound occurs.

That’s why many of-road racing people use gas-pressurised hydraulic bump stops.

Often called ‘air bumps’ or ‘hydraulic bump stops’ these designs are similar to a gas-pressurised shock absorber, but have shorter travel. They act progressively, with rising resistance to suspension compression, but do not feed this compression energy back into the suspension as it extends after the bump.

 

Progressive bump stops on test

We purchased pair of hollow rubber spring bump stops from Timbren and fitted them as replacements for our front bump stops. We’ve had experience with this type of hollow rubber spring on European trucks fitted with long, parabolic front leaves and with our previous torsion-bar suspended HiLux, so we knew the principle worked.

The Timbren units are extremely progressive and can be compressed a few millimetres just with hand pressure. That deformation requires more and more pressure before the ‘waist’ in the middle of the bump stop actually disappears.

As fitted to our 75 Series they have around 5mm clearance from the axle wear pad with the vehicle loaded.

It’s early days for the test, but they’ve already passed out parking lot speed bump test: giving progressive, non-jolting response to a set of bumps that caused the front end to bottom out before.

We’re heading off on a long desert jaunt in July 2019, so we’ll update this story in August with our bush travel findings.


 


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