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FRAME MODS. Tony Foale - Nov. 1987. - 2002

High on the list of priorities for many specials builders is to improve the handling of their machine. Particularly since the advent of the UJM. there have been many bikes available with super fast engines housed in chassis packages that provided interesting (to say the least) handling characteristics. In several cases just travelling in a straight line at speed would be enough for dangerous wobbles and weaves to manifest themselves. But in others it would take the sight of a corner to excite them into a frenzy. Deficiencies in handling may take many other forms as well. e.g. Speed of response to the rider's input may be sluggish or even too rapid in some cases, the front end might be a bit too lively over certain types of bump, the steering might be too light or too heavy for a particular rider's taste, ----- the list is almost endless. The problem is, --- how to improve things. Much to the delight of some specialist manufacturers, one often trodden route to better handling is the simple substitution of some parts with those more suited to the intended use. The extreme example of this technique is the complete replacement of all or most chassis components with a chassis kit from one of the low production manufacturers, such as Harris, Spondon, Nico Baker, Me, et al. This is often (but not always) a successful route to take, but the main disadvantage is of course the high cost involved, which may easily exceed the initial cost of the original UJM. For most people, parts substitution is restricted to such items as tyres, suspension units, steering dampers, front forks, "braced" swing-arms, fork braces, etc. On some machines the selection of the right combination of these bits can totally transform it, while with others the results may be equally disappointing. There may be many reasons why the stock parts are inadequate, perhaps plain simple cost cutting by the original manufacturers, or it may be the inevitable compromises that have to be made to make the machine as universal as possible. For example, some bikes have soft damping to limit the forces trans- mitted back into the frame from bumps in the road. Some believe this leads to improved comfort. Personally I find the increased pitching and the bouncy ride causes more discomfort than the slightly harsher ride obtained with the correct damping. Don't get me wrong, I am not advocating hard suspension as such. This used to be the cure-all for the cafe-racers of the '50s. and '60s. Many a road test praised the handling benefits of fitting a hard pair of "matched" Girlings to the rear end, one cannot help wondering about the quality of the manufacturing processes that made matching necessary anyway. The reason that handling could often be improved in this way was the stiffening effect on the chassis as a whole, and the swing-arm in particular. These items were relatively flexible and the use of stiff springing helped tie the frame and swing-arm together, the matched units helped to eliminate differential swing-arm flex. It is a legacy of that period that many people today still advocate a hard ride, the real answer is to have a chassis structure that is stiff enough in it's own right and then fit suspension soft enough to soak up the bumps without bottoming out too often, with just sufficient damping to stop it behaving like a pogo-stick. Generally speaking most specials are less compromising in nature than the run of the mill production machine, This has handling benefits as it usually means that several components are surplus to requirements, so throw anything away that you don't need on your bike and add lightness. Centre-stands are heavy so toss it and use a workshop stand when necessary, steel fuel tanks don't help so get an alloy one made. Production bikes can be lightened considerably if you put your mind to it, and the results can be well worth while. The handling is improved on two fronts, viz;-- a lighter bike will respond more easily to the rider's bidding and the reduced mass will load the chassis components less and hence flex will be reduced. There are disadvantages of course, a light machine is more susceptible to disturbance from side winds, and the ratio of sprung to unsprung mass will usually be affected unfavourably, thus making it harder for the suspension to keep the wheels in contact with the road. So any weight saving programme will concentrate on reducing the unsprung mass as well, i.e. wheels, brakes, chain, sprocket etc,. ----------------- So, after throwing away all those unnecessary bits, it seems that the best place to start any handling improvements is with the frame and swing-arm to ensure that you have a rigid base from which to work. The main objective is to keep the wheels in line with one another, both torsionally and laterally. To do this the main frame loop must maintain the steering head and swing-arm pivot in their correct relationship to each another. Probably, most frames are still of the double loop type, pioneered on the 1950 Manx Norton, designed by an Irishman with the name of McCandless, this machine was renown for it's good handling, but we must remember that it had only 50 bhp. to con- tain, it weighed only 300 lbs. and the tyres of the day were only capable of feeding in much lower forces than those with which we are now accustomed. Despite it's past successes this type of frame is not very structurally efficient, that means for it's weight it does not provide much stiffness. Fortunately, it is often possible to improve on this quite substantially with the minimum addition of weight and with only a small amount of work.

Back to Basics

Before getting into actual details, let's look at some basic structural principles, a knowledge of these will enable you to make a better job of modifying your own frame, regardless of type. The most important concept to grasp is the difference between the degree of flexing that occurs in a tube depending on whether we are trying to bend it or stretch it (called tension). A couple of examples will illustrate this. Consider two pieces of tubing with identical dimensions subject to identical forces, but one loaded in bending and the other in tension as in fig.1. You'll just have to take my word for the accuracy of the figures in the table, because the techniques of calculation, whilst not difficult, are outside the scope of this type of magazine.

There are several things that we can deduce from these figures, the most significant being the enormous difference in the degree of flexing and the stress levels between the two types of loading. For the larger of the two tubes considered, the deflection is over 1000 times as great for the bending case and the stress is 85 times as much, whereas for the smaller tube the deflection is nearly 5000 times as much with the stress up by a factor of almost 200. Also important to note is the influence of tube size. In the tension case if we reduce the diameter to half then the weight drops to a little under half and both the deflection and stress rise to a bit over double. But when those same tubes are subject to bending. the deflection of the smaller is over 9 times that of the larger and the stress is up by a factor of nearly 5. Another major influence on the structural properties of frame members is their length. The deflection of tension members is proportional to their length for a given loading, i.e. if we double the length we also double the flexing but the stress level will be unchanged. Whereas the situation for bending loads is much more critical, the deflection is proportional to the cube of the length, i.e. if we double the length then the deflection will rise to eight times and the stress level will be doubled, and for three times the length the flex will be twenty seven times more. From this we can conclude that where possible it is much more efficient if we can design our frames so that the material is subject to tension (or compression) loads as much as possible, whilst avoiding the bending situation. However where bending is unavoidable we should use the largest sections practicable, in the shortest lengths and keep the wall thickness down to avoid excess weight. This objective is best served by using a technique known as triangulation and fig.2. shows the principle.

A four or more sided structure can only resist the loading shown, by virtue of it's members bending stiffnesses. As we have seen this may allow considerable flexing depending on the size of the tubes. On the other hand the triangular arrangement is very stiff even with small tubing, in practice the major source of flex in this type of structure may well be due to local deformation in the wall of the tubing where joined to the others, this can be a quite significant problem and an area of potential failure when using large thin wall material. But with the practical sizes of tube used in triangulated frames, we can to all intents and purposes consider such a single triangle as totally rigid. It is not always possible to avoid the use of four sided structures but in many cases stiffness may be dramatically improved by the addition of simple bracing struts as in fig.3..

However, practical considerations may physically prevent this type of stiffening;--- engines, batteries and the like have a habit of getting in the way. Fortunately there are other approaches that we can use in some cases as illustrated in fig.4.. If any of these approaches are not possible all is not lost, we still have what is probably the most common method if least effective solution;--- the gusset, forget ladies underwear, I'm talking about local reinforcing at the junction of frame tubes. The way in which a gusset stiffens a structure is not always properly understood. Imagine a tube attached to another and subject to a bending load, as shown in fig.5..

If there were no gusset to support it, the tube would bend over it's entire length, whereas with a gusset, the bending is virtually restricted to the unsupported portion. As we have seen, the amount of flexure is proportional to the cube of the unsupported length, so reducing that by a half increases the stiffness by a factor of eight. Hence even small gussets can stiffen a multi-tubular frame considerably. However where a tube is stressed only in tension or compression a gusset will only have a minor effect. For that reason, they are seldom found on well-designed triangulated structures, except to provide mounting points. There is still one problem that may crop up with such well braced structures;--- rigid as they now are to forces acting within their own plane, unless the tubing is of large section the structure will have little resistance to torsional loads or twisting, to put it another way. This can be easily illustrated with a rectangular piece of cardboard, any attempt to flex the rectangle into a lozenge shape is strongly resisted, but just try and twist the board and you will experience little difficulty. There are two ways to improve this situation, one is the use of large diameter tubing (round tubing is best for this type of loading) which is torsionally rigid in it's own right, and the other is to triangulate the structure outside the flat plane of the four (or more) sides, a pyramid as shown in sketch 2. of fig.4. is an example of this. This type of pyramid can be a very effective method of stiffening, provided that the apex can be brought far enough out of the plane of the main tubes. Enough of all this theory, let's see how we can put it to practical use.

The Real World

From the forgoing we can see that from the structural view point the fully triangulated frame such as that on the Krauser BMW is potentially the best, but a cost conscious volume manufacturer runs a mile from the concept because of the labour intensive nature of the construction, after all there are a lot more joints to weld up. Easier from the production angle is the spine frame in all it's variations. this uses the stiffness that can be obtained in both bending and torsion from the use of one main large section member, and this concept has found volume application on some moped type machines in the form of welded up pressing. The Yamaha FS1E. is a good example of this technique. However, apart from small volume chassis makers, such as Egli and Moko in Switzerland and yours truly in the U.K. there have not been many attempts at using this type for larger machines. The Norton Commando is a notable exception, but that was spoilt in two ways, one;-- the backbone could have been larger, and two;-- the potential stiffness from this frame was thrown away by the method used for rubber mounting the engine AND swing-arm. Probably the main reason for its lack of use in production, is that on a large bike the space needed for the engine, airbox, battery, etc., is not compatible with the concept in most cases. The flavour of the month frame, at the moment, is the twin boom type (Delta box and the like), this can be considered almost to be a split back-bone which gives more room for the engine. Like the back-bone it can be quite rigid if done properly and I see it as a significant improvement in mass produced frames. I only hope it survives the ever present demands of fashion, which often dictates change for the sake of it. If you have any of the above types of frame it is unlikely that you will need to think in terms of modification to improve it's stiffness. As they are more common, it is likely that you have some form of multi-tubular frame consisting of a number of medium sized bent tubes, descended from the previously mentioned Manx Norton. These types of frames are traditional, reasonably cheap to make, and structurally inefficient, i.e. they are heavy for the stiffness that they provide, which is often minimal. Looked at from any angle they consist basically of an assembledge of a number of four-sided structures. So if we can use some of the triangulating techniques discussed earlier we have the opportunity of stiffening the frame considerably. Probably the best way to understand this is to follow an actual example. Earlier this year I was asked to stiffen two frames for the current model 750. Kawasaki, despite favourable comments on the handling of this machine in many road tests, deficiencies were soon brought to light when subject to the much higher rigours of the race track (just ask TM.). These frames were to be raced in a class where the rules forbade any mods. which entailed removing bits but it was permitted to add. As weight was obviously a priority, brute force was not the way to go, any added material had to earn its keep. I find it very useful to measure the frame stiffness, as this way you can keep a check on the effectiveness of your work. This is not as difficult as it sounds, because you are only after a comparative figure and great accuracy is not necessary for our purposes. I have a heavy piece of tubing which is machined to be a good fit in the swing arm mounting of the frame, this piece of tubing is then fixed to a rigid piece of machinery in the workshop, although any solid object will do, such as a wall. For convenience of loading this mounting tube is located vertically, so that when the frame is mounted on it the frame lies horizontally. The frame can be loaded in torsion and lateral bending by applying a force to the end of another piece of tubing through the head stock, this should be a good fit in the head- stock and if about three or four feet long it will be possible to significantly flex the frame with moderate hand pressure on the end of this tube. Frightening isn't it. Whenever I have done this in front of an audience, there is disbelief and amazement at the degree of deflection that can so easily be produced. If a constant load is applied through a spring balance always in the same place along the tube then we can compare the frame stiffness during the course of modification. Perhaps a more valuable consequence of this controlled loading is that we can actually see and measure the pattern of deformation within the frame. This makes it very easy to assess where its most important to put bracing tubes and where it would be largely ineffective. The photographs show the finished modifications that I came up with for the Kawa. 750., and incorporated are examples of most of the techniques mentioned above.

In front of the engine a large open area has been heavily triangulated, the actual layout of the addition tubes being dictated by the need to avoid the exhaust system and parts of the crankcase, there was not the space to use the pyramid method and all the bracing lay in a flat plane. This mod. was particularly effective, the torsional stiffness of the bare frame being more than doubled. The swing-arm pivot area is subject to high loads trying to deflect it in a fore and aft direction, these forces arise from the pull of the chain and any lateral loading on the rear wheel. This tends to try and twist the frame sections immediately above and below the swing-arm pivot, the section above is the most affected, simply because it is longer. To limit this flexing a pyramid was added. This is not always possible due to the location of large airboxes and/or battery. The large four sided area under the engine could be seen to be lozenging when loaded but the engine itself prevented any cross bracing and so the two rear corners where gusseted as much as room allowed. The similar area above the engine was approximating to a long thin triangle anyway, and so needed no similar treatment. So far we have considered only the bare frame, the engine unit is quite rigid and when bolted in place has great potential as an aid to stiffening the whole structure. Unfortunately, with modern bikes there are two factors that greatly reduce this effect. Firstly, the trend to rubber mounting, whilst good in other aspects, does nothing to help the handling. Secondly, to reduce production costs, mounting holes in crankcases are cast in rather than being drilled, this results in tapered holes that are a lose fit on the mounting bolts. These bolts are usually 10mm. in diameter, but the smallest part of the hole will be nearer 11 or 12mm.. Considerable stiffening can occur if attention is paid to these two areas. Rubber bushes can be replaced with specially machined aluminium ones, and the mounting holes can be carefully reamed out to either 7/16" (close to 11mm.) or 12mm. (this job is best done with the motor in situ, to ensure correct alignment) The engine bolts can then be replaced with ones of the correct diameter; I usually use stainless steel for these. Both of these methods were applied to the Kawa, and in addition two detachable tubes were added between the outside head mounting studs and the front frame downtubes; this largely braced the sides of the frame. So how effective were these mods? Well, less than 10% was added to the bare frame weight, which went up from 28lb. up to 30.5lb. I cannot give you accurate figures for the stiffness increase because it became so stiff that most of the flexing was then taking place in my supporting jig, but it was very hard to detect significant movement between any sections of the frame. If I was pushed to guess then I think that there was an improvement of between about seven to ten times. Just a word about the size of tube needed for this bracing work. In the figures for the relative flexing of a tube in bending or tension/compression, we saw that the stress levels and deformation were minimal in the tension situation, and so quite small section tubes can be very effective. It is not necessary to use tube sizes similar to those already in use on the frame. Unless the bracing tube is long and subject to compression loads which may cause buckling then 1/2" diameter with 16 or 18 gauge wall thickness should be more than adequate. It's all very well stiffening up the main frame loop in this way, but quite often it is the swing arm that is the major source of torsional movement. Unfortunately, I was not given the opportunity to either measure or modify the arm from the Kawa, so I don't know how the frame mods related to the whole assembly. Probably the most effective way to stiffen this item is with bracing similar to that used years ago on the Vincent and more recently the "cantilever" Yamahas. In reality these are just versions of the pyramid that have been compromised by the need to avoid tubes going through the wheel, etc.

There are a number of so-called "braced" swing arms on the market, but sad to say, many of them are of no more than cosmetic benefit. A lot of the flexibility in a swing arm is due to twist of the pivot tube, and hence those arms that only feature bits of added tube along the sides are not going to help much. The final link in the chain that is responsible for holding the wheels in line is of course the front forks. Regular readers will know my views on these abominations, but if you must use them, they can be improved. If money is no object, go out and buy top quality replacement units with large stanchion diameters and a large wheel spindle. If you are stuck with your originals then fit a brace above the wheel, but get a good quality one or don't bother. Like alternative swing-arms there are many ineffective ones on the market, make sure that it is rigid and equally important it must be accurately made or it may distort the fork alignment and prevent free movement of the sliders. If you have the facilities then changing from the usual 15 or 17mm. diameter wheel spindle to a more rigid 20mm. one (like those on some Italian machines) can be quite effective.

Now just a word of caution.
Frame stiffening as discussed will in most cases significantly reduce the stress levels in frame members as well as stiffening the whole structure, but there are occasions where the stiffening of one part of the frame may lead to increased risk of failure in another unstiffened area. A flexible frame acts as a spring and can absorb and reduce the effects of some types of loading, if only parts of the frame are stiffened then we may pass more load through to the unstiffened areas which may deform locally more than before even though the whole frame deforms less. Well, now you have a rigid frame to work from, but that's all it is at the moment. Handling will probably have improved somewhat already but to get first class results you must start the fine tuning process. That is, selecting spring rates, matching tyres, changing geometry by moving the fork sliders in their yokes, etc. the list is endless. But that is all another story. _ _