Here’s another post regarding short wheelbase recumbent bike design. This time we will focus mostly on tiller distance, and what effects it has on handling.
Tiller distance, as I and most other people define it, is the distance from your hands (when positioned in the normal spot on the handlebars) to the steering axis, as measured along a line that is perpendicular to the steering axis. Sometimes this is thought of as how far your hands are behind the steering axis when the bike is viewed from the side. While this is one obvious aspect, hand width adds to it too. Tiller length is a dimension that lives in 3D, and as a result, the true tiller distance is only observable in a 2D image when looking in line with / along the steering axis. Tiller distance is the radius of the arc your hands travel along when you steer the bike.
A word about nomenclature for a moment. Notice I am using the term ’tiller distance’ to help distinguish it from the handlebar/steering type of nearly the same name, tiller steering. This is sometimes also known as ‘preying mantis’ (PM), or even ‘hamster’ steering (but hamster is debatably slightly different). All recumbent steering types I have ever seen, short of a joystick or tank type steering, have some amount of positive tiller, unlike upright road bikes which normally have negative tiller. Beach cruisers have positive tiller. Make sense? Ok.
Tiller distance is an important factor that helps determine how a recumbent bike handles. Basically, for good handling, you don’t want the tiller distance to not be too much, and not too little, although too little is a fairly rare problem. Just to be clear, however, tiller is one factor among at least a dozen others that influences bike handling, so let’s not oversell its importance. These numerous variables are being considered one at a time, because to try to understand them all at once would be overwhelming for just about all of us.
I postulate that if tiller length is excessive, you tend to have low speed handling problems. My theory is that as you try to balance the bike and make small, precise steering corrections, your hands have to move a relatively far distance when you have a long tiller distance to affect the same balance correction. This tends to produce overshoot. And you have to then correct in the opposite direction to compensate, and you might overshoot in that direction too. Too often, you end up rapidly swinging your hands left and right repeatedly, overshooting each time, and it’s just about the most unpleasant thing you can imagine. As 2-wheel bent riders, just about all of us have experienced this. You are really not in control in such cases. Shorter tiller distances calm this down and help you make more precise steering corrections because the inertia of your hands/bars/brake+shift levers/stem are less at work. They are less at work because they only need to move a shorter distance to affect a certain steering correction, and have less chance to be accelerated to a speed that requires a challenging deceleration to be made. As a result, choosing lightweight brake levers, shift levers, grips, and handlebars can help with this issue a little bit.
Now if you go too far and you have an extremely small tiller distance, you start to have a different problem with overshoot – but this time it’s due to a lack of leverage at bars. To overcome friction at the contact patch, you might need to apply too much force to the bars – again, it’s an effort that is hard to modulate. And if there is enough leverage/tiller distance to prevent that specific problem, you still might find high speed steering to suffer (i.e. be ‘twitchy’), because even tiny movements of the hands can cause oversteering. Imagine trying to hold onto the riser/steering tube just above the headset and trying to steer from that location. Sounds impossible, right? Probably is.
The Zevo only works well with superman / open cockpit (OC) / tweener bar type steering because of how far the head tube is pushed forward in order to accommodate the particular steering geometry I wanted (steep steering angle + negative fork rake). If I had put tiller / PM /hamster steering on that bike, the tiller distance would have been huge, and this would have resulted in poor handling. Superman / OC / tweener steering is normally set up with the rider having straight arms, and this cuts down on tiller distance by about 8″ or so compared to tiller steering. Said another way, for a given bike, superman steering has roughly 8″ less tiller distance than tiller /PM / hamster steering since the bend at the rider’s elbow goes from about 80-110 deg. to about 10-20 deg.
I should note that Bacchetta stick bikes are another bike design that has a far forward head tube, and works best with OC than tiller steering, for the reasons mentioned above.
Now, I could have made tiller steering work with the Zevo steering geometry, but it would have required an indirect steering setup (like is commonly seen on USS bikes). For purposes of tiller and how it affects control, this brings the steering axis back to where the actual handlebar pivot, behind the head tube of the bike. But I didn’t go this route, because I personally prefer the feel / ergonomics of superman / OC / tweener steering anyway. Avoiding the complication of indirect steering is icing on that cake.
Long tiller distances tend to go along with another design problem that affects low speed handling. The further the center of mass is behind the steering axis, the more difficult it is for steering changes to affect balance, which also encourages overshoot / oversteering. Why is that? Well, imagine this – the rear wheel doesn’t steer, all it can do is pivot around it’s contact patch, so when a given balance correction is made by steering the front wheel, any mass located directly over the rear wheel cannot be given as much lateral acceleration by that given steering correction as mass that is located right over the front wheel. Of course, what matters is where the center of mass / center of gravity (COG) is located, and typically this ends up somewhere close to the rider’s navel, assuming the bike and luggage are reasonably light.
The counterpoint to wanting your COG far forward are issues of weight distribution (to be covered in a separate, future post), and issues of pedal steer. Pedal steer is the tendency for the act of pedaling to create steering input. This usually presents itself as a wobbly sensation. My observation has been that the further forward the rider’s legs are relative to the steering axis, the greater the pedal steer. The act of pedaling causes your legs to primarily accelerate and decelerate in a front-to-back manner with every pedal stroke, however there is always some amount of lateral movement at work too. It should be noted that some riders have more of this lateral movement in the pedaling than others, but it’s present to some degree with everyone.
Somehow, the Zevo seems to have resolved this conflict. The COG is seemingly close enough to the steering axis that its steering is responsive and efficient for affecting balance, yet there is zero pedal steer. (I was expecting some pedal steer before I actually rode it, so that was a nice surprise.) However, upon further consideration, I think the error in my thinking is that it’s not the distance of the COG behind the steering axis per se that slows your ability to make balance corrections, but the distance behind the front tire contact patch. That’s where the forces between the bike and road are actually exerted, after all. If that concept is correct, then it helps explain how the Zevo can resolve both issues (steering-balance response and pedal steel) at once. The secret is the reverse rake. It pushes the steering axis forward (for minimizing pedal steer), but keeps the rider COG pretty close to the front contact patch for good balancing.
Getting back to the main subject – tiller distance…. I postulate that tiller distance wants to be somewhere in a sweet spot, and I would estimate it to be somewhere in the 8″ to 16″ range, generally speaking. Of course, a little less or little more can certainly “work” too, but most designs want it to be somewhere in this ballpark. To refine this further, if you have really steep steering angles (e.g. 80 deg. and higher), you can more easily tolerate a figure near the top of that range. Slack steering angles (e.g. 70 deg) tend to make the bottom of that range more appropriate. But why? Why is that the steeper the head angle, the more tiller you should have, to provide steering stability?
The steeper the head angle, the fewer the number of degrees the steering needs to be turned in order to effect a directional and balance change. (A constant wheelbase is being assumed.) For a ‘proof’ of this, try this thought experiment: If the steering angle is 90 deg, rotating the bars 45 deg. causes the front of the bike to have a new heading 45 deg. off of the previous heading. Now, imagine the other extreme – a steering angle of zero (i.e. horizontal). In that case, no matter how much you rotated the steering, you’d have no change in direction. All you’d be doing is changing the camber angle of the front wheel, not actually steering the bike into a new direction.
Practical evidence of the same is the following: My two Wishbones, each with quite unconventionally steep steering angles (about 80 deg.), have hard interference, and tons of soft interference, yet I never have any trouble with either when actually riding them. Whereas, bikes with no hard interference, and less soft interference, have made these overlaps known in practice. It doesn’t seem coincidental that those bikes had steering angles in the low 70-71 deg range, typical of most current SWB recumbent designs.
I will also note that both Wishbones have long tiller dimensions (with somewhat wide OC / tweener bars), yet it doesn’t bother me. I certainly don’t feel like I am swinging my hands and arms around a lot when steering. Their low flop geometry may be helping here too – I don’t make a lot of unintended over-steers trying to balance the bike.
So, in a way, if one were to keep the tiller distance short on a bike with a steep steering angle, then this means your hands would not move very far (arc length being short), making the steering seem “twitchy” / overly responsive. However, if you lengthen tiller, the more your hands need to move (arc length) for a given rotation angle of the steering, thus reducing the “twitchyness” of the design.
The Zevo’s tiller distance is about 14″. On the long side of average. So, while the tiller distance on the Zevo is not particularly short, it’s seemingly partially compensated for by the steep steering angle which seems to reduce the number of degrees you need to turn the bars to affect a certain balance correction.
Striking the right balance in tiller length against other design choices is probably the right path. Extremes should be avoided. Unfortunately, the interplay of the multiple independent variables associated with the overall handling characteristics of a bike makes any sort of ‘formula’ one might be tempted to develop quite complicated, and possibly underivable from a practical point of view.
In all of the above, I haven’t attempted to make any claims about which of the two steering/handlebar types I think is “best” – OC / superman / tweener vs. tiller / preying mantis / hamster. I believe handlebar / steering type selection should be mostly an issue of personal ergonomic preference. Whichever you find easier to make fine steering movements is best for you. But as was alluded to above regarding the steering type on the Zevo, you do need to consider the bike design and the resulting tiller distance before going away from the steering type the bike was originally designed around. Some bike designs work much better with one or the other, but those designs with the head tube/steering axis closer to the rider are a little more versatile and can work ok with either steering type. Examples of this are the Metabike and the Schiltter Freestyle.