Rake and Trail – your Harley is not a shopping cart

The look of a Harley-Davidson may be a decisive factor in deciding a purchase, but most of us take the candidate of our dreams for a test ride. Chrome and colour suddenly becomes secondary, the right ride feeling is important. The handling of a bike is depending on dozens of factors, the colour clearly not being one of them.

If you are interested in technical details, it is important to understand the basics of front suspension geometry, as these are the major factor in how your bike will handle on a straight line, under braking and in corners, under speed or very slowly. This geometry is primarily defined by six variables, which I want to analyse more closely. At the end, it boils down to 2 headline numbers: rake and trail. Other secondary factors which I will not analyse here in detail are hight adjustments and, sorry to point it out, your very own weight.

First lets look at the primary 6 variables on a bike:

1) The Offset – This is from the centreline of top of steering neck to the centreline of top of fork tubes pic offset

2) Rake – This is the angle in degrees of the steering neck from the vertical

3) Fork Length – This is the distance between the top of the fork tubes to the centreline of the axle.

4) Diameter of the front (and back) tire – this is… the Diameter of the front and back  tire

5) Trail – the most important number, a result of other factos, its the distance defined by vertical line from axle to ground and intersect of centreline of steering neck and ground

6) Raked Triple Trees (not shown above) – Making things complex, and in order to bring trail figures back into line, triple trees with raked steering stems can be used. Expressed in degrees, usually adjustable in 3, 5, and 7 degrees of rake, this number should be added to the Steering Head rake.

Uhh, new technical term, and no, its not Xmas, this is what is called a Triple Tree:

You could consider the Triple Tree as 2 plates, to which the steering neck and the front fork are attached. As shown above, the Offset is defined for the upper plate. If the lover plate has the same offset, both steering neck lne and fork line are parralel. If however the offset is increased on the lower plate of the triple tree, additional rake is introduced, increasing the total rake.

Now lets look at some Harley-Davidson data, collected from the official website (*) and other internet sources (**), not sure they are all correct, as sometimes marketing managers tweak data for a more pleasant look,  and not everything published on the internet is certified to be correct.

Lots of remaining question marks, lots of other influencing factors, but the trail is clearly the number to watch. For example the big difference in trail for the 883R and 883L (low rider) comes mainly from different wheel diameter. The Road King however gets its long trail primarily from a negative offset.

From experience, I ride a Road King and a 883R regularly, above numbers are butt tested and prove to me that a bike with lots of trail will be directionally more stable. It will tend to go straight and be easy to ride hands-off (never do that!!), it will not have its direction changed by every tiny bump in the road. But such a bike will take more physical effort to steer than a bike with less trail, that is also why bikes with long trail have wider handlebars. A bike with lesser trail like sport bikes will be livelier, it will take little effort to change its direction, whether that effort comes from your hands on the bars or (bad!) from a bump in the road. However, too much or too little trail will make your bike undrivable, and chopper builders go to great length to control rake and trail, check out this front special axle, designed not just to please the eye, but also to reduce the trail.

Trail causes the front wheel to act like a caster, and the greater the trail, the more forcefully the wheel tries to align itself with the direction of travel. But only few wants a bike so stable it can not be turned but prefer a bike that can be ridden no-handed for at least a few seconds without going out of control at the tiniest bump. Twisted City Roads vs a straight highway to the horizon, these extremes define your trail needs.

The above numbers however also show one thing very clearly: there is no golden solution, the mix is important, as well as drivers preference. Other external factors also play a role, as the weight of the driver influences the trail (heavy dude lowers the bike and reduces the trail), and the trail actually changes constantly, because under breaking the fork length reduces. Thus entering a corner under breaking (or not) changes the behaviour of the bike, and for a 20-stone dude its again different then for the slim lady on the same bike.

We do not need a PhD in geometry to evaluate a bike, our butt will do that, but to understand the influencing factors will help to formulate preferences. Harley-Davidson website only provides some geometrical data, but rake and trail is disclosed, giving a first hint on how the bike could handle. Then you can more consciously test ride different bikes and customise your own with more then just chrome covers for the back footrests, but maybe a new Raked Triple Tree for Xmas. And you may understand that using raked triple trees on a stock Harley front may not be the best idea, as it may shorten the trail too much and lead at instability at higher speeds. You do not want your bike to handle like a TESCO shopping cart.

Disclaimer: above are my personal research and thoughts. This is just to explain the principles, not to serve as instructions or recommendation, and worse of all, it could be wrong. Happy to receive your comments and corrections.

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2 Responses to “Rake and Trail – your Harley is not a shopping cart”

  1. Great and very useful article – thanks. I am in phase of purchasing a bike (Sportster Roadster 883 is one of the candidates) and till now I did not realize, this Rails is an important parameter.
    Also the the whole RoadKing blog is very valuable.

  2. Graham Manchester Says:

    if I was too worry about the handling of a Harley based on this information I would never buy one. The Motor Company does not have a clue when it comes to bike design for road handling BUT under 90 mph who worries.

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