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Rake - Trail - Wheelbase

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by bulby, Jan 17, 2012.

  1. Hi folks,

    I've been trawling the interwebs for information on how a bike's dimensions, namely rake, trail and wheelbase, affects its dynamics. (Courtesy of a slow day at work :p )

    I've found a few good articles that explain this in details. Like this article and several others. Problem is, the more I read the more confused I get. So I thought I'd throw my questions here. I was never the brightest student in class afterall... LOL

    So here's what I think I understand so far.
    From a very simplistic view, more rake and more trail = bike becomes more stable but less nimble? As in easier to go straight, takes more effort to corner / turn?

    Then what about wheelbase? The most obvious is of course, wheelies and stoppies (be it accidental or intentional) are more difficult with longer wheelbase. Having said that, do they have any other effect to the way the bike rides?

    I apologise in advance if this has been previously discussed to death. If that's the case, can you someone point me to the article?

  2. Pretty much as you summarise it. Couple of points though.

    Rake doesn't have much direct effect on the dynamics 'til you get to silly extremes. Its main effect is that, all other things being equal, less rake (nearer vertical) gives less trail and more rake (nearer horizontal) gives more trail. You do get to the point eventually that to go from one lock to the other requires the headstock to rise and then fall again by an appreciable amount, resulting in "chopper flop" but you've got to go quite a long way before it becomes significant.

    Thirty years ago someone (possibly Tony Foale whose website is worth a peruse) did some experiments and found that, given the correct trail, completely vertical forks gave perfectly acceptable stability. They were, however, a bit crap at resisting braking loads.

    Wheelbase has an effect on straight line stability and minimum turning circle. Long wheelbases are more stable than short ones (again, all other things being equal). However, in reality, it probably has less effect than fork geometry. As a result, apart from in the supersports bracket, wheelbase is going to be more of a function of fitting everything in and making the styling look right.
    • Like Like x 2
  3. #3 bulby, Jan 17, 2012
    Last edited by a moderator: Oct 24, 2015
    Wow. That just about answers all my questions. Thanks :D
  4. Here's a link to Mr Foale's website, which has a good article on those rake experiments amongst other excellent stuff.

  5. Interestingly Mr Foale found that a rake of 15 degrees is more stable than one of 30 degrees (all things being equal on the same bike).

    Experiments with rake and trail variations.
    © Tony Foale. March 2000. (The experiments were conducted around 1982/83.)


    The author’s ideas mentioned in other articles in relation to the angle of the steering axis (rake) were subsequently put to the test by modifying a readily available standard production machine – a BMW R75/5. There were two advantages in the choice of this machine.

    Firstly, the offset of the wheel spindle from the steering axis is divided almost equally between the offset in the yokes and that of the wheel spindle from the centre line of the fork sliders (figure A.1); the importance of this will become obvious later.

    Secondly, the BMW was large and fast enough to make the results meaningful, which might have been less so with a slow, lightweight machine such as a moped. To keep other variables to a minimum, the original frame and suspension were retained and the wheelbase remained unaltered. Two non-standard rake angles were tried, 15 and zero degrees. In each case the trail was kept to approximately the same as the standard value (i.e. 3.5” in.).

    Fig. A.1 On the BMW R75/5, the total offset (wheel spindle from the steering axis) is divided approximately equally between that in the yokes (steering axis to fork legs) and that in the sliders (fork legs to spindle. This feature made these forks ideal for the tests as explained in the text.

    The first alternative set-up tried was with a rake of approximately 15 degrees and almost nil offset. This was achieved by bolting a superstructure to the frame to support the new headstock (see photo) and reversing the yokes; since their offset is very close to that of the wheel spindle, the overall offset was reduced virtually to zero. For the second setup, the rake was close to zero (i.e., vertical steering axis). This was achieved by reversing the complete front-fork assembly, thus giving the negative offset necessary to maintain the standard trail. The new headstock was supported by an extension of the original superstructure. In both cases the handlebar was pivoted in the usual place and connected to the fork by a ball-jointed link; a side effect of this was an adjustable steering ratio – i.e., for a given angle at the fork the angle needed at the bar could be varied. With the 15-degree rake the bike had full road equipment, including lighting, so that it could be ridden under everyday conditions; indeed, five riders covered nearly 2000 miles between them, including wet and dry going, bumpy country lanes, London traffic and motorway trips. Throughout this period, no steering damper was fitted.

    The standard BMW R75/5 used as a basis for the experiments in rake and trail, maker’s figures were 27 degrees rake and 3.5” ground trail.

    Although the results of these tests are essentially subjective and might be expected to depend on experience, personal preferences and preconceived ideas, there was in fact no divergence of opinion between the various riders. The initial testing was done on a bumpy, rutted country lane at speeds up to 50 mph. Here the most noticeable effect was the total insensitivity of the steering to ruts and bumps. Not only could the bike be ridden hands-off but at the same time it could be weaved from side to side across the ruts with little effort and no detectable deflection of the steering. In corners, bumps had little effect, which was contrary to the behaviour of this particular machine before conversion, when it had a strong tendency, with no steering damper, to shake its head (sometimes violently) on bumpy corners. This lack of disturbance by longitudinal ruts was also confirmed on smoother roads at higher speeds, when the machine was ridden deliberately on the edge of painted white lines. Though unforeseen, this benefit is easily explained by reference to figure A.2. If we visualize a 90-degree rake (i.e. horizontal steering axis) we can see that the side of the rut gives rise to a moment about the steering axis that tends to steer the wheel back into the rut. With a vertical steering axis, however (zero rake), there is no effect on the steering; instead, the disturbance tends to cause the complete machine to lean into the rut. In this case, though, since the inertia of the whole bike is much higher than that of the front wheel alone, the effect on directional stability is considerably smaller and the rider is less aware of the rut. Thus the steeper the steering axis the smaller the effect.

    Fig. A.2 The effect of ruts on steering increases with rake angle, as shown in this exaggerated case. A vertical steering axis reduces the effect.

    In the foregoing chapter, we suggested that balance might be enhanced, particularly at low speeds, by steepening the head angle. To verify this, much riding was done at very low speeds and balance was indeed improved by the modifications. The machine could easily be ridden much more slowly than when in standard trim before the rider had to put a foot down. (Of course, champion trials riders can balance a stationary machine indefinitely; but that is exceptional and most of us need to be moving slightly to maintain balance.) In heavy traffic, it was noticeably easier to trickle along slowly on the modified BMW, making it less tiring to ride from one side of London to the other. When, without prior briefing, a novice was asked to try the machine, he commented on the surprising ease of moving off from rest; there was less wobbling than usually seen with a learner and his feet were quickly on the rests. It has been suggested that an unusually steep steering axis might induce wobbles at high speed. Nevertheless, with both the experimental rake angles on the BMW (15 degrees and zero) this was not noticed. With the handlebar released, the machine was ridden from approximately 100 mph down to a walking pace and at no time was there any tendency to wobble or weave. With confidence built by several such runs, the handlebar was knocked to try to initiate a wobble. Whatever the speed, though, the disturbance was damped out within less than one cycle. In standard trim (27-degree rake) this particular machine could develop a pronounced wobble when ridden no-hands at 30 to 40 mph, though it was easily damped out by grasping the handlebar. Directional stability was always excellent and tremendous confidence was instilled in the rider at an early stage.

    A further advantage of the steeper head angles was increased sensitivity of the front fork to small bumps. This results from reduced ’stiction’ in the fork sliders as a consequence of the decrease in side loading. (The normal side-load component is approximately halved by reducing rake to 15 degrees and practically eliminated at zero rake.) In addition, this reduction in the side-load component is accompanied by an increase in the spring-load component as the fork is steepened – which gives the same effect as a lower spring rate. The effective rate varies little between zero and 15 degrees rake but is approximately ten per cent higher at 27 degrees. Similarly, the spring-load component of the braking force is reduced as the fork angle is steepened. Moreover, since this spring force acts in concert with weight transfer to compress the fork, the reduction means less nose-diving. For this reason, the effective drop in spring rate was not detrimental and ride comfort was appreciably improved.

    The top photo shows the superstructure which gave an unaltered trail with zero offset and rake angle of 15 deg.

    The second shows the extended structure, with the reversed forks, giving equal trail with near zero rake angle.

    It was under braking, however, that a disadvantage was noticed, in the form of severe shuddering in the fork as the braking force tried to bend it backward. Naturally, this was more severe with the steering axis upright, even though the reversal of the forks converted the twin-leading-shoe brake to much less effective twin trailing shoes. Such juddering was entirely absent with the standard rake, though quite bad at 15 degrees.

    Fig. A.3 Steepening the steering head reduces the stiction in telescopic fork sliders, so improving sensitivity to small bumps. Nose diving under braking was also reduced.

    This effect apart, one of the most interesting results (mentioned by all the riders) was the surprisingly normal feel of the modified machine, with the steering pleasantly light at low speeds but always totally stable. No special riding technique was required and cornering was accomplished normally. The variable steering ratio mentioned earlier was tried from 1: 1 (equivalent to conventional direct steering) to 1: 2 (steering angle doubled from handlebar to fork). In normal riding (dry roads) it was impossible to detect the difference, only when maneuvering at a standstill, using large steering angles, was the heavier feel of the 1:2 ratio noticeable.

    However, since the steeper rakes made the steering lighter anyway, even the effort required with the 1:2 ratio was similar to that with the standard machine. Indeed, the reduced handlebar swing could be a bonus when designing a non-steerable fairing, as handlebar clearance usually results in a bulky shape if steering lock is not to be restricted.

    In both experiments the handlebar was connected to the fork by a rose-jointed link providing for steering ratios anywhere between 1:1 (direct) and 2:1 (geared up). This was accomplished by sliding the bar atop the forks toward greater or lesser radii.

    With the modified BMW, Tony Foale demonstrates the stability achieved with the 15 degree rake angle.

    The scope of our experiments was limited by time and money. Nevertheless, the results indicate a need for more exhaustive and quantitative testing. We hope that one of the large manufacturers may appreciate the potential benefits and divert some of their resources to further investigation. These tests indicate that currently favoured geometry may be far from “optimum”.


    From our experiments it seems there is nothing magical in the conventional rake angle of 27 to 28 degrees. Indeed, balance, stability and lightness of steering were all enhanced by steepening the angle. The greater improvement came from the first change (from standard to 15 degrees), the subsequent move to near zero rake producing only minor differences. Many effects of castor angle are approximately dependent on the cosine of that angle, the cosine of 15 degrees is 0.97 which is little different from the value of 1.00 for a vertical steering head. At 27 degrees the cosine reduces to 0.89, a more significant difference.

    The only drawback noticed – juddering under braking – is a consequence of the poor structural integrity of the telescopic fork as a type. It is not suggested that machines should be built with a steep steering axis, using a headstock mounted fork, whether telescopic or leading/trailing link type, because the consequent high, forward location of the headstock causes structural and styling problems. Much better to consider some form of hub-centre steering or other wishbone layout, such as that used in the trail experiments.


    Apart from the need to avoid the critical situation mentioned, there seemed no obvious optimum value. Results were satisfactory throughout the full test range, so making personal preference the decisive factor.
  6. Given that, it makes sense that less rake makes a bike more stable - it is the steering of the front wheel that determines stability rather than the rake (though the steering is a function of the rake) (there is a page in msgroup about it) so I don't know what you mean by stability.
  7. Tony is da man!

    Bulby, go back to the msgroup site and trawl the forums... you will find a world of information... and pain... what ever you do, don't ask stupid questions and definitely do not show up JRD as he doesn't take it very well... but it's hard to find someone as accessible with as good a level of motorcycle physics knowledge.
  8. Pat any idea does wheelbase also have an effect on braking? I am sure I read somewher once that all things being equal (which of corurse they never are) a longer wheelbased bike should outbrake a shorter wheelbased bike. though I can't really follow why.
  9. Tony Foale is required reading. The article on wikipedia on bicycle and motorcycle dynamics is also very good.

    A couple of points based on what's been said above:

    Trail is in part a function of rake. Less trail means less self-centring of the steering, which makes for quicker, lighter steering. It will help make the bike agile and light to steer at higher speeds, and generally cause fewer odd habits in the steering at low speed, but it comes at the cost of stability and feel. Oh - and it also helps the bike to not stand up so sharply if you must apply front brake while leaned over.

    A shorter, more compact frame will flex less, and be lighter. If you have to extend the steering head a long way forward, you need to add a lot of weight to the frame to do it in a way that offers the same stiffness. You also make an odd stretched out riding position, unless you do very odd things with the handlebars.

    I find it interesting that race bikes for short circuit / road-race have had trail figures as low as 80mm at times, but the bikes they race on real roads at the NW200 and the Isle of Mann and so forth, usually have between 100 & 110mm of trail. Historically, what works on a race bike on real roads works pretty good on real road bikes too. It makes them a bit heavier and slower in the steering, but stability and control are more important.

    Small race bikes have always had pretty rapid steering, but bigger race bikes - 500s, motoGP, superbikes, tend to have geometry that's pretty similar to a litre class sports road bike. In fact, the race bikes (depending on which track) are sometimes less agile than the road-going sports bikes.

    A longer wheelbase reduces weight transfer, as has been mentioned. That's not a bad thing under power or brakes, (bike upright) but it's a bad thing for trail braking into the corner, and it's a bad thing for power down off the corner - (Bike leaned over.) A longer wheelbase also results in a bike that has to lean further to make the same corner at the same speed, or a bike that will run wide at the same speed and lean angle, or a bike that simply can't carry the same mid corner speed on the same tyres. It would be nice if this was entirely predictable, but sometimes it isn't. I have a ZX14, which is l-o-n-g and low. It's got all the cornering clearance you need, it steers and handles well enough, and on some corners it turns just like a good one, and on others it just goes straight ahead. (I don't mean it stands up and goes straight - I mean it's fully leaned over and turning - it's just running a much wider line than all the blokes on 600s I'm trying to keep up with.) As a very basic rule, it runs wide on slow corners and works more like a sports or race bike on fast corners, but there's a degree of inconsistency about it. A longer wheelbase does help high speed stability and composure. Again, the IoM racers sometimes lengthen the swing-arms on bikes where this is allowed. Even with a standard swing-arm, you can add one or sometimes two links to the chain and run the back axle right at the back end of its adjustment. That's another 10 ~ 12mm of wheelbase...

    Racing fans will know this, but for the newer people...

    A common methodology for setting up a bike at a track you've not gone to before is to first set the sag, set the damping where you think seems right, then do a few laps. If you're not getting stability issues, you steepen and sharpen the bike to turn in quicker and better. This is done by raising the rear ride height and lowering the front. You keep doing that until you start to get weave and slap issues and then you back off a fraction. Along the way, you identify any other issues or specific problems and alter your set-up to address those specific issues. (Bottoming out under brakes, topping out, chatter, knocking tyres around...) On a short circuit / race track, you will generally go quickest and best with a bike that is bordering on too twitchy and nervous. On real roads, you need something that's a bit tamer and more stable than that.
  10. Good question. Very simple answer.

    Find a pic of a long low bike, not a chopper, a standard or touring bike. ZZR1100, GPZ1100, K100 BMW ... FJR1300 - that sort of bike. Now find a pic of something that's short and high. Any hard-core sports bike will do, but the Buell V-twins are perhaps the best example.

    Same size, same scale, side on. Mark the front & rear contact patches, and the combined centre of gravity point of the bike and rider. Connect the dots and draw a triangle. Measure the angle between the front contact patch and the CoG. If it's 45 deg, then any more than 1 g of braking and that bike will stoppie - and face plant the rider if he's not paying attention. If it's 60 deg from vertical, the bike can stop at 2g before it does a forward somersault. But if the line between the front contact and the CoG is only 30 from the vertical, then any more than 0.5 g will see the bike trip over its own front wheel.

    The exact same principle applies in reverse when you're talking about acceleration.

    It's pretty obvious why a long and low bike will accelerate and brake better while it's straight up and down. But what happens when you start to lean over? Now things change. Short tall bikes allow the rider to fine tune the weight distribution with the throttle - far more so than long low ones. Modern sporty tyres can and will find more grip if you can find more load to put on them. A powerful bike with a rearward weight distribution, like an 1198 Duc, can get pretty much all its weight on the rear wheel while still leaned 35+ deg in a 1st or 2nd gear corner, and the rider can get lots of throttle on and drive off the corner. Hell - the bike can actually accelerate harder like that than it can straight up and down - the centripetal force holds the front down for you - you just open the the throttle and go. Rear grip under power is (almost) not an issue. Compare that to my ZX14 which will spin up and slide the rear (in 1st or 2nd) at 35 deg or so with about 20% throttle. That bike has to be up to about 20 ~ 15 deg before you can get full throttle. Ducati - gorn! Elvisimo has left the building. [edit] captain Francesco Schettino has abandoned ship. [/edit] Good thing I've got 200hp to hunt him down.
  11. Thanks kd