13 July 2023
The chassis and underbody of Discovery 3/4 and early Range Rover Sport have been corrosion-free for years, but their time is up. Ed Evans explains how to extend their life
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Time: 5 hours
Difficulty: 3 out of 5 stars
Models: Discovery 3 and 4, Range Rover Sport 1.
Tools needed: Scrapers, wire brushes (hand and rotary), soft brushes, cloths.
• Neutrarust 661 rust convertor.
• Dinitrol Converust RC900 rust converter and primer.
• Dinitrol underbody wax.
• Dinitrol cavity wax.
• Use barrier cream or gloves to protect hands, as preferred.
• This work needs to be done with the vehicle raised on a suitable vehicle lift. It is not practical to do this work with the vehicle supported on stands, nor is it considered safe practice.
• Wear eye protection when clearing rust and dirt debris, whether by poking, brushing, grinding or wire brushing.
• Wear thick gloves to protect against abrasion, and when using power tools.
• Wear a particle filter to avoid inhaling dust.
• Painting and other surface treatments should only be carried out in a well ventilated area wearing hand and eye protection and with strict adherence to the manufacturers’ safety recommendations.
Early examples of Discovery 3 and sister ship Range Rover Sport can now be up to 17-years old and so, in the traditional manner of the old beam axle models, chassis corrosion has become a significant problem. And unlike the days when a chassis was made mainly of rectangular hollow sections and flat steel that was easily repaired or patched by anyone reasonably competent with an angle grinder and a welding machine, modern materials, complex shapes and lack of accessibility make a rusted chassis on D3/4 and RRS 1 a different proposition.
When all is lost, a new chassis is available. But whereas the replacement of a classic Defender chassis costs around £2500 and a few days’ work to fit, a modern Land Rover chassis will set you back around £6000 plus a massive amount of time to strip the vehicle and rebuild it with all the additional parts that will inevitably prove necessary along the way – frankly, it’s not financially worthwhile.
So the best approach is preservation and, if repair is needed, the services of a good professional specialist should be used. And I mean good. There’s a lot of weight and power in these vehicles, and that means plenty of load is applied to the chassis in various directions – the wrong sort of flex can adversely affect handling, braking and stability, even more so if carrying or towing heavy loads. And there’s the accident scenario to consider: these machines are designed to fold up in the best way possible to absorb impact force and to direct it safely away from occupants both in the Land Rover and whichever other vehicle is unfortunate enough to contact it. So it’s not reasonable to mess with the structure by welding in a haphazard way. Additionally, the complex shapes of this chassis, especially at the rear where the axle shafts pass through circular portals in the main longitudinal chassis sections, together with the close proximity of the body structure, restrict working access to many areas and would demand accuracy in forming replacement sections.
Integrated body frame
These trucks use a monocoque safety cell body but, that alone is not intended to deal with the forces inflicted during driving and collisions. The separate chassis, or T5 integral body frame to use its correct title, provides much of the rigidity and collision crumple zone behaviour.
Around the time of the Discovery 3’s introduction there was much shouting about its hydroformed chassis – a first for Land Rover. In reality, only four of the 140 component parts of the chassis are made by the hydroforming process, though these include the main longitudinal sections running along each side for almost the length of the vehicle. Unlike earlier rectangular chassis sections, these structures have three-dimensional curvature. They are formed from seam-welded tubing in which internal hydraulic pressure forces the material outward against a die to produce the complex and seamless (apart from the original tube weld) structure which is optimally shaped to support the body, suspension and powertrain, and also to provide impact crumple behaviour in conjunction with the body structure. The individual parts of the chassis are joined to form the complete assembly by laser welding, an accurate and fast process, as is hydroforming, giving production benefits and also weight saving which translates to fuel and/or emissions savings. The finished chassis received sophisticated corrosion protection internally and externally but, as we’re now finding out, even the best protection has a finite life.
The chassis type is similar for Range Rover Sport 1, and Discovery 3 and 4, the Range Rover version having a shorter wheelbase. But remember we’re not just talking about the chassis on these ‘integrated body-frame construction’ vehicles – the underbody is also likely to be corroded and in need of repair to preserve the full structural properties.
Given the impracticalities of safe repair, or the expense of replacing the chassis, the best plan is to do whatever is possible to prevent corrosion, and to arrest the corrosion that will have already started. In this feature we’ll take a look around the vehicle undersides to see what can go wrong, and how to deal with that or, better still, prevent the corrosion in the first place.
The difficulty is access, both for dealing with existing corrosion and for applying protection, due the many restricted and remote spaces underneath. It’s essential to do this work on a vehicle lift which is certified for the load and in good order, or over a properly ventilated pit. Attempting this work with a vehicle on the floor or supported on stands is impractical and potentially dangerous, bearing in mind the vehicle weight and air suspension behaviour.
Removing surface corrosion by hand using wire brushes and scrapers is time consuming and hard work, but you do become intimately familiar with the condition of the underside, quickly learning the areas that need most attention. Individual vehicles seem to corrode in different places, so it is useful for you or your garage to get to know your vehicle underneath. The easier, and arguably more effective, alternative is to have the underside media blasted, but this involves much masking off parts such as fuel lines and electrical harnesses, and a lot of muck and mud will still need to be manually dug and poked out of crevices beforehand. After making any repairs, a rust convertor should be applied to neutralise existing rust and form a protective layer, followed by comprehensively spraying with a penetrating wax preservative which, ideally, should be topped up on a regular basis. There are few places where fluid can be injected inside the sections, though if internal surfaces have formed layered rust flakes that treatment may not reach the remaining metal surface. We do what we can with what we’ve got.
Because of the multiple curvatures and graded thicknesses, and the dependency of the vehicle’s collision and handling behaviour, any welded repairs need to be made by a competent welder with an understanding of the structure. The old methods of simply welding a good thick patch on are no longer appropriate.
Access and view points
It’s the rear half of the chassis and underbody that gives most concern, though the side sills also need keeping an eye on, which we’ll see later. Chassis corrosion is most critical, on the inside faces of the main longitudinal sections, just forward of where the rear axle’s shafts pass through the circular portals. The rear crossmember can also suffer, and especially the underbody aft of the rear diff, above the spare wheel. Fortunately, with the vehicle on a lift, there’s good access to the inner faces of those main longitudinal sections, and by removing the spare wheel the rear chassis, crossmember and rear body underside are accessible.
To see what’s going on underneath, we’ll take a look at a Discovery 3 on which the body has been lifted off the chassis which, incidentally, is the best way to deal with the situation if you have the space, time and facilities. It took a us a day to remove this body, though some specialists have it down to three hours. This Discovery was just nine-years old when we lifted the body off; think what it could be like after 18 years.
Rear is worst: A glance at our D3 chassis with body removed shows the main area of rusting is at the back, from that crossmember just ahead of the rear wheels.
Entrapment: This close-up of the fuel tank in picture one shows mud compacted between the tank and the chassis side rail, which will rapidly corrode. Mud needs clearing out.
Deep rust: This rear suspension turret remains solid, and those upper spring/damper nuts are easily cut off. But the complexity and tight packaging harbours dirt and corrosion.
Available new: The chassis rear crossmember is available and can be replaced after removing the rear bumper. There’s access to apply protection when spare wheel’s removed.
Difficult reach: The rear of the underbody, extending over the rear diff and axles, is difficult to reach. A good dosing of wax protection using a spray will help preserve the area.
Looking good: Side sills on Discovery 3 always look good because they’re covered with plastic. When inspecting or treating the metalwork, they need popping off for access.
Trouble below: With the plastic trims removed, there’s a different picture of a side sill. These are structural and their strength needs to be maintained – repair sections are available.
Internal access: The body is mounted directly to the hydroformed chassis side rails. This one is rusty, but solid. Holes in the rail provide access for internal fluid injection.
Surface troubles: The lower parts of the structural underbody attract surface rusting which needs to be halted. A proper repair
here would need the body to be removed.
Repair quality is vital
Critical: The main chassis rails can corrode badly on the inner faces immediately ahead of the rear axle portals, as seen here, and also on the underside face.
Still strong? This one has been repaired by welding, and sealer applied. Note the suspension wishbone is attached either side of this area – there’s a lot of stress here.
Reaching and treating the rusted surfaces
Festering: Removal of the spare wheel exposes early underbody and chassis corrosion. This car remains solid, but it’s time to clean back, stop the rust and apply preservative.
Winch trouble: This 15-year-old vehicle needed a new spare wheel winch due to seizure (common, so check), an opportunity to catch that rust on the underbody and chassis.
Accessible: The inner face and internal parts of the rear crossmember can also be reached from the spare wheel housing. Outer surface is accessible after removing the rear bumper.
Paint issue: Paint on this underbody has lifted and cracked, harbouring moisture beneath, which accelerates the corrosion. The cylinder is the air suspension air intake silencer.
Poke it out: By poking a tool into the manufactured holes, loose rust scale can be pulled out to give the injected wax a better chance of contacting the inner surface.
Scrape and chip: Initially, chip away at flaked rust to reach a steel (if still rusted) surface, and remove remaining debris with a scraper. This may expose perforations to be repaired.
Labour saving: Using different sized and shaped rotary wire brushes with a compact drill will make quick work of accessible surfaces after scraping and chipping.
Convert the rust: Quality rust converter applied over surface rust will neutralise existing rust and convert it into a protective layer. An aerosol achieves easy, effective penetration.
Tight spots: Fitting a tube to the aerosol gets the convertor fluid into crevices, remote places, and through production holes to the inside of structures and fittings.
Protection: When dry, a coat of wax treatment seals the job. This underbody wax dries fairly hard to resist abrasion and stone chips. Cavity wax stays flexible inside sections.
Internal, external: I prefer Neutrarust for converting the surface rust, and then Dinitrol cavity wax inside the sections, and hard-drying underbody wax on outer surfaces.
Kit to suit: There’s a choice of treatments, but stick with established brands. Suppliers offer acessories like this spray trigger and nozzle tube to reach inside cavities.
All over: The nozzle tube can be used to reach on top of the longitudinal chassis rails, outer side near the sill, and sills themselves to give comprehensive protection.
Crossmember: Normally, everything ahead of this transmission crossmember will be okay; everything behind needs attention and is the area to concentrate on.
Rear end: With spare wheel removed it’s possible to apply a thorough rust neutralising and protection treatment on underbody and rear chassis after cleaning.
Regular checks: With the spare wheel back in position there’s little to see, but great peace of mind. It’s worth inspecting every couple of years and topping up where needed.
Overview: A sills, B above axle and spare wheel, C rail near fuel tank, D main rails forward of axle portals, E suspension turrets, F spare wheel frame, G rear crossmember, H chassis rails inboard, crossmembers ahead and behind axle.
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