Pedal bike and e-bike stability, and the need for a steering damper

Sport and off-road motorcycles often have a hydraulic steering damper to inhibit uncontrolled displacement or oscillation of the handlebars. The need for such a device arises because of a terrifying situation that sometimes occurs with high-performance machines. This is known as a 'Tank Slapper' or 'Speed Wobble'. It involves an uncontrollable side to side movement of the steering, driven by the transfer of energy from forward motion.  The hydraulic damper limits the rate at which the steering can change direction, essentially acting as a low-pass filter to inhibit the troublesome energy transfer process.
     What is perhaps less obvious, is that light 2-wheeled vehicles, such as ordinary pedal bikes and e-bikes, can also have severe stability problems in a variety of situations.  Those situations can be outlined as follows:

1) Needing to let go of the handlebars, e.g., to make hand signals (particularly on poorly maintained roads in traffic).
     Many cyclists seem to solve this problem by not giving signals, but the 'keep 'em guessing' approach is not necessarily an optimal survival strategy. For an e-bike with its large battery, the problem can be addressed by providing motorcycle-type lighting equipment, particulaly traffic indicators, a brake light, and a headlamp flasher (see e-bike lighting and electrics), but given that few will adopt such an approach, matters can be improved by making the steering less-easily deflected.

2) Being forced into pot-holed or rutted parts of the road by inconsiderate car and van drivers.

3) When riding with heavily loaded panniers, bags, or a basket.

4) Transient loss of front wheel grip (wheelies and skids, too much weight over the back axle).

5) Going fast (25 - 40 mph, 40 - 64 km/h) and unexpectedly encountering rough surfaces, stones and rubbish.

6) Going off road.

It is desirable to solve these problems or, in the immortal words of Bob Dylan:
"One day you'll be in the ditch, flies buzzing around your eyes, blood on your saddle."

     In addition to the problem of external forces suddenly displacing the handlebars, bikes have a serious ergonomic flaw, which is the way in which the steering flops from side to side when the rider attempts to park or manoevre.  A full ±90º motion of the bars is desirable when turning the bike in confined spaces while dismounted, but the tendancy of the steering to go straight to one extreme or the other when released is problematic.  Particularly, the simple act of leaning the bike against a wall can result in it folding-up and falling-over as soon as the hands are removed from the grips.

     There is at least one true hydraulic steering damper for pedal cycles, the hopey, but it costs around $200 + shipping. The hopey damper fits inside the steering column. There are also devices, referred-to as 'steering dampers' or 'limiters', that provide a return spring intended to bias the steering towards the centre position. Accessories of the latter type are the Hebie limiter spring 695 (helical stainless steel) and the Hebie handlebar limiter 696 (synthetic rubber). 

     The Hebie limiter system introduces the idea of fitting a stabiliser on the underside of the frame, underneath the tube from the steering column to the bottom bracket, leaving the top tube free. An independently engineered version of such a damper might include both a hydraulic piston and a spring, but it has the disadvantage that many spring systems require a mudguard or caliper-brake mounting-hole for attachment of a lug. In the event that no such hole is available, an expanding nut inserted into the bottom of the steering column is a potential solution, but it can be difficult to get a sufficiently secure attachment. Certainly, there are poor quality expanding nuts that simply do not work. Hebie however make a Clamping Cone (Klemconus), with a lug for a spring, to fit 20.5 to 25.8 mm i.d. steering columns, and this works well. The Klemconus is designed to anchor a spring on its own, but it might also accommodate a hydraulic damper if provided with an additional linkage.

Hebie Klemconus

Klemconus parts.

The screw is M6, with a thread length of 45mm. Note that, in the mounting instructions, the ribbed split-cylinder is called the 'adapter to 25.7mm.' The smaller split cone that engages with it is for 20.5mm ID tubes or slightly larger.

     For my own return-spring experiments, I purchased a Klemconus and a Hebie 108mm 'replacement' damper spring from Duch Bike Bits.  I did not use Hebie's own frame-tube clamp and adjuster system because I wanted the option to try out some kind of hydraulic damper at a later date. I therefore made up an attachment out of a length of 8×16mm aluminium bar, with recessed 5mm screw holes so that it could be fixed to the under-frame bottle-holder mounting sockets. The adjuster is a length of M8 A4 stud, with one end ground down and drilled with an eyelet hole for the spring. An M8 handwheel nut and washer, with plenty of silicone grease, controls the spring tension. A length of translucent 10mm OD, 8mm ID silicone tubing covers the stud and the spring.


     Parking with a stand, or by leaning the bike against a wall, is greatly facilitated by using the Hebie limiter. The picture below shows the attitude adopted when using a side stand. The steering naturally turns slightly towards the direction of lean, but there is no longer any need to take care in positioning the handlebars.


     The particular concern in fitting a limiter spring is, of course, that it might adversely affect the handling; but this is not the case. When riding at speed, large deflections of the handlebars are not required, and the return force is not great enough to be noticieable when both hands are taken off the grips. At low speeds moreover, the centering force improves stability when giving hand signals. It is fair to say, that on some badly maintained streets in the author's home village, hand signals while in motion were almost impossible before the damper was fitted. There was always the feeling that the bike would fall over and be crushed by any vehicle following it. With the damper installed, good long signals can be given with ease. I am drawn to the conclusin that a spring damper should be considered basic equipment.


Future work
I am interested in the possibility of fitting a hydraulic steering damper, such as the type shown below:



This is a 330mm linear damper intended for use on motorcycles, with stiffness adjustable by means of the knurled knob on the shaft end.
     These dampers are usually fitted asymmetrically, so that they give about the same damping effect for any position of the steering. I plan to use one symmetrically however.  This means that it will barely change length for small deflections of the steering, and will apply progressively more damping as the deflection increases.
     It should be possible to fashion a new end-fitting for the Klemconus, with an M8 socket, or perhaps a choice of sockets at different radii, for the damper end bearing. Actually, this new fitting will be better described as a bracket, since it will need to be cranked up towards the bottom frame tube and then bent back parallel, so that the damper will not get in the way of the front suspension.  Slippage of this bracket can be prevented by having four threaded holes, two in front of the fork arch, and two behind. Screws can then be put into these holes, and the threads covered with silicone tubing to form anti-rotation pins.
     I would also like to retain the steering return spring. This should be straightforward if I put a pin in roughly the position of the original spring lug, and have the adjuster on a clamp around the body of the damper.
     One difficulty I envisage is that of getting the Klenconus out of the steering tube so that the modifications can be carried out. The device is not as vicious as a star-nut, but it seems unlikely that it will give up its tenure easily. It should be reasonably straightforward however to tap the 20.5mm cone upwards from below and thereby realease it from the 25.7mm adapter. It should then be possible to tap the 25.7mm adapter out of the tube by passing a thin rod down through the star nut at the top of the column.
     To prevent it ever being necessary to remove the Klemconus again, I plan to replace the M6 screw with a length of A4 studding and a nut. Bottom attachments can then be changed at will without the nightmare of having to recapture the topmost threaded cone.

. . . . TBC

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More inforrmation:
 Speed wobble - Wikipedia.
Shimmy or speed wobble - Harris Cyclery.


Suppliers:
 hopey steering damper.
Dutch Bike Bits (Hebie steering damper kits and parts).
Holland bike shop (Hebie steering stabilizer parts and kits).
Frank (Search for 'Hebie'.)
Hydraulic damper 330mm (KK-Match)
Hydraulic damper 330mm (Ting Ting Car & Motor Store).