Soft luggage racks

Soft Luggage versus Hard Luggage

The perennial debate, soft versus hard - luggage.  I must confess at the outset that I am of the soft luggage camp and so anything I may say about hard luggage will be one below statistics on the universal scale of lies (lies, damned lies and statistics).

Luggage for off-road riding

The requirements for off-road riding are far more severe than for road riding. Even small jumps greatly increase the forces experienced by the luggage. Combine that with the forces created by fore-aft rocking of the bike and the design load must be increased again. I use a design acceleration of 4g, if I have a static load of 20kg then the luggage and carrier must be capable of supporting 80kg without permanent deformation. Fatigue cracking can also be a problem. Fatigue cracking occurs when the stress is less than the yield point but is frequent and reversed.

Reasons for a luggage rack

If you ride a purpose-designed adventure bike then the rear sub-frame may be sturdy and have enough attachment points. Alternatively there may be commercially available racks.

If however you are using a true off-road bike then the sub-frame will be light (possibly aluminium) made to support a tail-light unit and not much else. Commercially available racks almost invariably attach to the sub-frame itself by the mudguard and exhaust mounts.  The racks themselves are limited in load capacity, are heavy and prone to failure (loss or breakage of the attachment bolts even when thread-locked). For almost all true off-road bikes a purpose-built luggage rack is required.

Attachment Point Criteria

Frame attachment

The first thing to resolve are the frame attachment points as they are the foundation on which the rack is built. The rack must be attached directly to the frame itself, no other point is as strong. It is preferable to use an existing mounting point to avoid welding on the motorcycle frame which may affect the heat-treatment or locally degrade the alloy from which the motorcycle frame is made. Existing mounting bolts should be at least M8.

Well-spaced attachment points

The attachment points should be as far apart as possible. The rack is subjected to weight forces and rotational forces. The further the attachments are apart then the more leverage they can exert in resisting those forces. The existing sub-frame attachments are a good place to start. The foot-peg mounting bolts work well or even the foot-peg pivot bolts.

Countersunk bolts

Countersunk mounting bolts are often used for attaching the rear sub-frame to the bike to get a low profile. It is important that the rack has a matching countersink on the inside where it mates to the existing sub-frame. This gives the strongest connection. Countersink bolts seem to come loose so thread-locking is essential.

Design Criteria

The design of the rack must be well thought out to provide strength, good support for the luggage and so that it is slim. Luggage on bikes is always intrusive and there is no point making a rack which stands out from the bike. This is one of the arguments against hard luggage, you cannot buy hard luggage which conforms to the shape of the bike.

Straight members

For maximum strength, the tubing members should be straight. This is especially true for members in compression. All compression members will collapse at some level of stress but bent members fail at a far lower stress.

Triangulation

The frame members should form triangles between them so that they are inherently rigid. Frame members should never be cantilevered.

Forces in all planes

Even though the primary force is gravity, the design must be able to resist forces in all planes, fore-aft, up-down and side to side. Neglecting any of these will cause the rack to fail or be forever wobbly.

No sharp edges

Every part of the rack should be smooth without sharp edges. After 500km of potholes, the chafing will destroy any sort of luggage, let alone your expensive dry bags. Luggage sitting directly on the bars of the rack is the best. Unfortunately having enough bars to support the load adds weight. Luggage resting against flat panels allows dust and grit to collect and generalized chafing can occur. This suggests that a corrugated surface would be the best compromise although difficult to achieve in practice.

Minimise Joints

Keep the number of joints to a minimum. If possible bend a single section to shape rather than fabricating it out of several sections welded together. The trickiest part of the whole job is jigging and welding so keep joins to a minimum. The added advantage is that a bent section is inherently smooth without catch points.

Gusseting

All joints are high stress areas.  All joints in the primary frame must be gusseted just like a bicycle frame. The gussets act to increase the joint contact area and to stiffen the roots where the bending forces are greatest

Materials

High strength to weight

There is a general love affair with aluminium and its alloys in motorcycle construction. This is true when the part can be aged and heat-treated. It is generally not true for home construction. Aluminium for extrusions has been selected primarily for its extrudability (ductility and dimensional repeatability).  The trend in motorcycles is for specialist cast or forged components without too much welding (which locally interferes with the metal composition). Aluminium and its alloys have low strength but acceptable strength-to-weight. Aluminium’s lower strength means that whatever you make, it will be physically larger than the equivalent steel item.

A material with high strength-to-weight is Chromium/Molybdenum steel (CrMo steel). The most commonly available, 4130, is an alloy that is about twice as strong as “mild” steel. It is a favourite with race car and bicycle frame builders.

Workability

Whatever you use, it must be workable without too many high-tech tools. If you have a TIG welder and can afford the gas hire, then aluminium may be the way to go. Aluminium can be cut with a tungsten-carbide tipped blade on a table saw or skilsaw. If you must use a hacksaw, it is tedious work, the sections being quite thick and aluminium has a tendency to clog. Abrasive blades do not work with aluminium.

CrMo steel has thinner sections for the same weight and cuts cleanly with hacksaw or abrasive blades (I use the 1mm thick “Zip” disks on an angle grinder. My preference is ½” tubular 4130 CrMo steel for strength-to-weight, working, welding and electroplating

Connection method

The success of your rack will depend on how well the connections between each member are made. The most efficient method is welding or brazing. Bolting is time-consuming, adds weight and produces a joint which has strength in only one or two planes.

Welding or brazing requires that the members are in close contact so that no filling is necessary. If rectangular sections are used then only straight cuts are required as long as the rack is made of planes at right angles. Unfortunately motorcycles are seldom like that. Rectangular sections also limit the plane of bends so that any out-of-plane bends must be cut and welded. For easiest creation of complex shapes, I have found that round section tube which can be bent in any plane is best. Where tubes meet, a round file of the same diameter as the steel section produces a perfect joint. A new file works quickly with 4130 CrMo steel. Use the file to achieve a perfect fit. After fitting, mark the position with a scriber so that when you take the rack off the bike for welding, less jigging is required. A well-fitted joint often only requires a wide-mouth Vice Grip to hold it for welding.

Brazing or welding? Welding with CrMo steel is generally done with a TIG welder using the correct filler rod. Joints must be prepared by undercutting the tube ends so that the filler rod can completely fill the joint. Some build-up is possible to add joint strength. While it is possible to use a MIG welder, the high deposition rates mean that you will end up with large unsightly welds. With MIG welding you only get one chance and a good finish is very difficult to achieve on the small tubes.

Brazing on the other hand can be done with oxy/acetylene welding equipment using high silver brazing rods and the appropriate flux. Other than good joint contact, no special preparation is required. The joints are very strong, especially when gusseted. The process is also reversible, if you make a mistake, you can reheat the joint and pull it apart without damage to the base metals. The only problem with brazing is that the flux leaves a hard glassy deposit which must be removed in a caustic bath (24 hours at least).

Manufacture

Zip disks

Zip disks are God’s gift to constructors. They are abrasive disks that are only 1mm thick. They cut quickly and cleanly leaving a good finish. They are especially useful for making all the odd-shaped gussets. Normally a quick swipe with a file to remove any burrs is all that is required after cutting.

Jigging

All the welding must be done with the rack off the bike. Some means must be provided for securely holding the components in position while they are welded or brazed. The stronger and more accurate the jig, the less rework will be required. A strong jig will ensure that the parts hold position as they cool.

After each weld, the rack should be refitted to the bike to check that it hasn’t moved. Put some alignment marks on the bike and the rack to ease checking. Centre-line marks at the rear end are invaluable. It only takes a millimetre of error at the attachment points to make 5 - 10mm of error at the rear end. The countersunk attachment points also assist in repeatable  fitting.

Infill Panels

Infill panels are required in some places especially around the exhaust so that no part can ever come in contact with the exhaust. I have heard of luggage catching fire or the contents turning into a large sticky mass by exposure to heat. In my case, the loss of a single rivet caused heat to be directed towards my bag, scorching the bag and melting the insulation inside. My tent was the next in line. With this in mind, continuous infill panels are preferred rather than wire mesh or perforated metal.

Load Placement

Even with the best rack, poor placement or attachment can lead to failure. Here are a few rules

Luggage type (discrete bags vs Giant Loop)

I have used 3 separate bags in the past and they have been OK. The advantage is that you can compartmentalize, cooking gear in one, tent in another and clothes/sleeping bag in the third. This means that you only need to remove one bag if you stay in a motel. The downside is that you need to attach 3 bags. The side bags tend to droop and the top bag slides around.
Recently I made a copy of the Giant Loop brand “Coyote” bag. It is a large horseshoe shape that naturally sits on the bike. Attachment is quick using 6 clips. Everything fits in the one bag and it is waterproof and rugged (I still keep my sleeping bag and clothes in a dry-bag inside). Over 5500km around the South Island, it never showed any sign of falling off or damage. It was used for some proper trail-riding and didn’t move around.

It is large enough for continuous camping in summer (tent, sleeping bag, stool, spare clothes, cooking and washing gear, rain-gear and 3 days food). A larger bag, of the same design, would be required  to carry extra warm gear for colder climates.

Clear of the seat

If you do any trail-riding, then the luggage must be clear of the seat are so that you can move back and forth as required. It is too easy to become trapped in one place by your luggage and all those good riding techniques go out the window.

Low

Keep the weight low. All the heavy stuff should go in the side bags (or the lower ends of the giant loop) and they should be mounted as low as possible. I only keep my sleeping bag and clothes on top. 

Items in bottom of the lower bags are first to sustain damage if you drop the bike (or it falls over). If possible have soft or deformable items in the bottom so that they are not damaged and do not cause damage to the soft luggage. I have found that cooking billies do not survive well here.

Near the centre of turning/gravity

The centre of turning for a bike is somewhere near the footpegs and about halfway up to the seat. The further any item is from this point, the greater the forces it will be exposed to as the bike rocks. If the load is as far back as the tail-light, then you will have problems keeping it attached. Obviously there is a compromise between seat clearance and distance from the turning point.

Weight distribution

When riding a dirt bike off-road, it is important to maintain weight distribution both around the bike and between the bike and rider. Control will be compromised if the bike is far heavier than the rider. Hence the BMW GS1200 effect as demonstrated by Ewan McGregor & Charlie Boormann in “The Long Way Round”. The weight of the rider (and his apparel) move with the rider and so can be used to  control the bike, weight forward to steer, weight back for traction, side-to-side for balance while standing.

A normal off-road rider:bike weight ratio is around 80kg:120kg. If you add a lot of weight to the bike (extra fuel and luggage) then to maintain the same rider:bike weight ratio the rider should share some of the extra weight. There is of course a limit to what the rider can carry but an extra 5 – 7 kg in a well-designed backpack is manageable long-term without excessive fatigue or back problems. I carry my tool-belt on the front and a backpack containing hydration pack, essential documents (in a dry bag), locator beacon and sight-seeing stuff. The stuff all fits in a 20 litre backpack and this is the bag that goes with me everywhere.

Load Attachment

Wide straps

25mm wide straps are the ideal size. There are several grades of strap, the best being the tubular webbing used by climbers. Do not use the thin hard webbing, its hard edges chafe on bags while it is easily abraded by the rack itself. The strap should feel soft to the touch and be at least 2mm thick.

I use “Fastex” buckles, the plastic ones that clip together. They are adjustable but are easy to unclip. The whole bag can be unclipped in about 15 seconds and be taken into a motel room. Reattachment takes under 1 minute. The whole thing is accessible in-situ by unclipping one or more buckles.

Do not use cord unless you must. Its small-radius round section will eventually wear away most materials.

No movement

When you have attached your gear, there should be no movement. Movement leads to chafing. Also if your gear can move, then the bags will be subjected to shock loads as the gear slides and then is brought to an abrupt halt.
The extreme of that was when I had a single item in a large bag. After a jump, the item ripped right through the bag!
Always jiggle the load as you cinch up the straps to remove that last bit of slack.

Chafing

Chafing can occur between the straps and the rack as well as between the strap and the luggage. All the straps should pass around tubular members so that the straps have an adequate bending radius. Small bending radii weaken the straps.

Chafing can also happen internally. I have had my Primus gnaw right through an aluminium billy over 500km of pot-holes. The primus now travels in a “Sea to Summit” neoprene bag.

The same applies to things like tent pegs and poles. The pegs should be embedded longitudinally in the folded tent, not in close proximity to the rack. Poles sets should have an end cover.

I line the bags that carry heavy stuff where they contact the rack with closed-cell foam (the blue stuff that cheap camping mats are made of). It’s amazing how misshapen it becomes from pressure points which would otherwise chafe.

Keep gear to a minimum

Gear naturally expands to fill the available space so that you always leave fully loaded. And then you need to buy food and carry water. What often happens is that you have a nice set of luggage with a whole lot of small odds and ends tied all over it. While this works for road-riding, off-road it is a disaster.  I have found that even 4 wide zip ties cannot hold a 300g fire extinguisher on a bike. The only things I have ever lost were external bags that could move slightly on the bike.

The rule is; take as little as possible. If it doesn’t fit inside the luggage, then either don’t take it or take out something else so it will fit. It is amazing how much you don’t need, forget the huge toilet bag, bath towel etc. If you want to shave, use shaving oil, 15ml will last weeks. And buy the food before you start so that it is factored into the initial packing.

Even camping gear can be reduced. Use a multi-fuel primus with a really small fuel container, just big enough to cook one meal. It can be refilled from the bike tank so why have more?  

Version 1. This is a high stress design using the lower sub-frame mount and the foot-peg pivot bolt (the upper sub-frame mounts were inaccessible). It takes some lateral support from the rear sub-frame and exhaust mounts. Left bag contains cooking gear and many spares for Russia, right bag contains tent and fold-up stool.
Version 1 left side view. Note the triangulation between the two bars from the mounting points and the gusseting of joints. The whole thing is over-kill but lasted 20,000km across Russia and Mongolia carrying 20kg.
Version 2. Much-simplified rack before electroplating. The main members follow the normal sub-frame. There is no contact with the bike other than at the 4 sub-frame mounts. It meets all the design rules above except that the compression members are not straight. This was overcome by the one-piece fibreglass infill contributing to the compressive strength. Air filter access is unimpeded.
Version 2. Almost complete rack and luggage. The rack has fibreglass shell separated from the plastics by 10mm. There is still an exhaust guard to fit by the lower strap. Fibreglass infill completely protects luggage from heat.
Version 2. Left-hand side view. Note that the luggage is asymmetrical because there is no kick start on the left. Also allows for the long (500mm) tent poles. All straps pass through the fibreglass and around the rack tubes for good bending radius and strength of attachment. Fibreglass was laid up over the rack in-situ and so is a perfect fit while still being removable. Attachment of the fibreglass is with 4 bolts through holes in the gussets. Side-stand folds in behind the rack.
Version 2. Rear view. Note the 10mm clearance between fibreglass infill and the plastics. 25mm wide straps with Fastex buckles. Luggage looks big but does not impede riding position. Light sleeping bag across the top to keep the weight low.

Completed Example

The Cr-Mo steel rack frame weighed in at 2kg with another 1kg for the one-piece fiberglass shell and straps. It is supported entirely off the motorcycle frame not from the mudguard or exhaust mounts.

The bag itself weighs in at 1.5kg because it is made of a heavy grade tear-resistant PVC with a substantial full length zip closure. No damage to the bag was experienced in many low-speed get offs.

This design proved itself over a 5500km ride around the South Island including the Dusty Butt 1000 in 2012. The only fatigue failures were the number plate and me.

A slightly larger bag would be required for colder climates or foreign travel where spares are not easily available.

Article Added By

IanMeyle
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