The first time, many years back1, that I read Neil Carver’s article on speed wobbles (published in a 2011 issue of Concrete Wave) it had seemed to me to check out. But I read it again recently with more attention. I realized that the article is totally, embarrassingly wrong about what trail is. It doesn’t define trail with words, but with the sketch on the third page; which makes the mistake even more glaring. It makes me wonder what sort of environment these people work in. Does Neil have any colleague around him with some understanding of vehicles? Why didn’t anyone say something about his blunder in the article? Before I explain what’s wrong with Neil’s article, please remember that he is most probably not an exception, but rather an ordinary, moderately successful skateboard truck maker.
Look at his diagram on the right, as published in Concrete Wave. He defines trail as the distance between the kingpin and the wheel contact patch. What on earth does the kingpin’s direction have to do with the trail? No, no, no, Neil! Trail is defined not by the kingpin, but by the steering axis, i.e. the line that connects the pivot cup with the hanger hole. If this axis intersects with the ground on a point ahead of the point where the wheels touch the ground, then we can say the trail is positive (that’s elementary, Neil! Were you mislead by the C7, which you yourself designed, whose kingpin coincides with its pivot axis? The CX is not like that, Neil, it’s a conventional truck, hadn’t you noticed?)
So, it doesn’t really matter whether a truck is RKP or TKP, but where exactly the axle is (for more on this, see article on TKP/RKP trucks).
(I also have a problem with the author’s (implicit) claim that all skateboard trucks, by definition, have a negative caster, but that’s another story. I’m not even convinced that caster matters for stability as it does on supermarket carts, so I’ll leave that for another time, or for people with more specialized education than mine.)
This undergrad paper gets it right, I guess, but then it goes on to say “the reverse kingpin truck is notable because the pivot axis and kingpin are 90 degrees to one another. This allows for less binding on the kingpin [note: not sure what is meant by that] and a linear turn to lean ratio.” The latter is an obvious but widespread misconception, seeing as several brands, such as Sabre, market trucks with slightly different rakes precisely because, as the latter write, “less rake gives a linear turn” (link). It must be taxing for one’s imagination to visualize an axle with rake not turning at the same rate as one without rake for the same deck tilt! For more on this, check this piece.
Anyway, back to Neil Carver. The orientation of the KP does not define the steering (or pivot) axis. [the latter is defined by the pivot and the intersection of the KP with the hanger – on conventional trucks]
The intersection of the steering axis with the ground and the point of contact of the wheels with the ground define the trail. That’s what’s taken to be trail on every other vehicle that has been theorized properly; it’s also simply the logical thing. Elementary, my dear Neil…
Neil Carver may be the first manufacturer to make trucks explicitly for pumping. He – or more probably someone in his team – played with KP angles to harmonize the trucks’ resistance to turning while using the same bushing hardness between rear and front trucks. He also believes that backwards rake makes skateboard motion resemble surfboard motion and that’s extra points for creativity. But he lacks geometrical understanding of his own creations and the article is testament to that. It has a huge, glaring mistake that needs revision.
1. This article is based on a post I wrote in a forum in early 2017.
4 thoughts on “Trail: Carver’s article “The science of the speed wobble””
For bicycle and motorcycle, the front wheels almost always have positive trail. How does positive or negative trail affect the skateboard & surfskate trucks? A traditional TKP front truck usually has negative trail, but a swing-armed surfskate truck usually has positive trail. Does the positive trail give you the feeling that when turning you engage the rail and follow that turning line? Does negative trail has any negative impact for the ride?
Beats me, Wang! 🙂 Considering that usually rear skateboard trucks have negative trail, but nevertheless don’t have any noticeable stability issues (in comparison to the front ones for, let’s say, the same pivot axis angle), I’d say the jury is out on this one. As a matter of fact, the lower the pivot axis angle of the rear truck (=> the more the negative trail) the more stable it is at speed. With this thought experiment, I would conclude that there’s something else going on here, and trail is not relevant for skateboards in the same way it’s for bicycles etc. But I am not going to try to make a strong case about that either. 😅
But a traditional TKP as the rear truck actually has positive trail, doesn’t it? As illustrated in the post #8 by ‚Ianmeenywoah‘ here https://www.mediafire.com/convkey/512a/50yqr43qpfue02azg.jpg
I think the mechanical principles still apply, the question is to what extent. For motorcycles, the choppers usually have huge amount of positive trail, that’s probably because the rider sits back and the front wheel doesn’t have a lot of pressure, whereas for a racing motorcycle, the rider’s weight is more evenly distributed, so you don’t need as much the trail (also quick handling required by racing). For skateboards, the front truck bears as much, if not more, pressure as the rear, so the actual difference caused by the trail might be quite small in most scenarios. Anyway, just my hypothesis…
Oh, yes, of course you’re right: rear trucks generally have positive trail. I had done that thought experiment (at some point in the past and didn’t now remember the important details, sorry about that) with a rear truck with a negative pivot axis angle (like, when you reverse it, or trucks like Gbomb’s torsion tails). In that case, the truck becomes super stable at speed, while the trail gets vastly negative. Think about it.
Your thought experiment (with choppers vs racing bikes) sounds solid to me, thanks! I subscribe to it. 🙂 Thanks for the useful content Wang!