GB2439298A

GB2439298A - Rear wheel axle adjustment for chain tensioning

Info

Publication number: GB2439298A
Application number: GB0612396A
Authority: GB
Inventors: Michael James Poyzer
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-06-22
Filing date: 2006-06-22
Publication date: 2007-12-27
Also published as: GB0612396D0

Abstract

A fabricated bicycle chainstay and backstay joint has a longitudinal rearward open facing slot. A block B carrying a rear wheel axle A is longitudinally adjusted by a screw extending through the joint forwardly of the slot and carrying a locknut C. The chainstay is offset from the backstay and joint towards the frame axis.

Description

Chain Tensioning System for a Single Drive Sprocket Bicycle

Field of the Invention.

This invention is concerned with the chain tensioning of bicycles which have horizontal rear facing wheel locating mounts. The invention provides a novel way of tensioning the chain in a very firm and secure way, particularly applicable in the more extreme forms of cycling like BMX and Bicycle Trials where heavy landings on the rear wheel are normal.

Background to the Invention.

Single speed bicycles ie. a bicycle with a single drive chainwheel at the front and a single fixed sprocket or freewheel at the back directly driving the rear wheel through a drive chain, traditionally require a degree of movement between the positions of the front chainwheel and the rear drive sprocket. This is because the tolerances of these mass produced parts are relatively high, which, accompanied by chain stretch in usage means that excessive sag is a very real problem with the drive chain, causing inefficiency in drive and other problems associated with loose drive chains.

The traditional method of solving the problem is to fit the rear wheel axle into a pair of parallel horizontal backwards facing slots in the rear fork of the frame. This allows the wheel axle to slide backwards and forwards. The wheel is then clamped in position using wheel nuts on washers on the axle, when the wheel is correctly aligned and the chain is correctly tensioned. This basic method of course relies entirely on the friction of the wheel nuts against the frame to hold the wheel securely in position. A further chain tensioning system was devised to add to this method and make setting easier. This consisted of a wheel nut washer to which was affixed (usually welded) a length of threaded bar. When the washer was set behind the wheel nut it was done in such a way that the threaded bar ran down the centre of the wheel axle slot protruding beyond its end. A curved "r' bar shaped piece with central hole was then fitted over this threaded bar coming to rest against the end of the frame above and below the axle slot. Finally an adjusting nut was screwed down the threaded bar up against the curved "r' bar.

Adjusting this nut towards the frame then has the effect of pulling back the washer which in turn moves the wheel axle back in the slot. When correct tension and alignment are achieved using these adjuster nuts on either side o the the frame, the wheel is then locked in position using the wheel nuts.

The wheel was thus held in position by the wheel nuts and its fixed location was maintained by this adjusting system. This system and variants of it remained in place for many years and they are still widely used today. The forces at work for normal cycling are perfectly well controlled and accomodated with this system The advent of extreme sports like BMX and bicycle trials has however shown fundamental weaknesses in the system. When a cycle is jumped a considerable distance or height and the front end rears up, the landing is made on the rear wheel with the cycle in a vertical or semi vertical position. This puts huge tensile forces on the comparatively small threaded bar and the "r' bar in the sytem and breakages and failures have become very common.

It is alternatively, an object of this invention to use modern CNC machining techniques to manufacture rear wheel location mounts at the end of the rear forks in the frame, which incorporate a chain tensioning system which utilises the considerably greater compressive strength of the steel threaded bar rather than the comparatively weak tensile strength.

This will create a system which is equally easy to use, if not easier, than the traditional method and which will handle the stresses caused by extreme sports in a much better and safer way.

Summary of the Invention.

According to the invention a bicycle achieves forward motion on the rear drive wheel through a toothed sprocket or freewheel attached centrally to that wheel. An endless drive chain passes around that toothed sprocket and also around a drive chainwheel centrally and rotationally located on the bicycle to which are oppositely attached two cranks and drive pedals on which the rider places his feet in order to rotate the chainwheel and propel the cycle forwards. The rear fork of the frame is generally but not exclusively, formed by four arms, generally but not exclusively tubular. These are a pair of topstays, usually referred to as backstays and a pair of bottom stays usually referred to as chainstays. Each backstay converges with each chainstay at a chainstay end which in the case of the invention is cnc machined from solid. These two stays are generally but not exclusively joined to the chainstay ends by fabrication using welding or brazing techniques.

The machining of the chainstay end is done in such a way that there is a step inwards beyond the front end of the slot on the chainstay portion. In the case of a normal conventional frame of this type with a plate chainstay end, when viewed from above the backstays and chainstays are in direct alignment and the backstay obscures the chainstay.

in this case however when viewed from above the chainstay is visible to the inside of the backstay. The resultant inward step in the machining of the component leaves a space for a hole to be drilled and threaded from a point forward of the wheel slot through the chainstay end running through into the wheel slot in a direction perfectly parallel and in alignment with that slot. A long headed bolt is then screwed from a forward position into the hole and right through, along the horizontal length of the slot. A locking nut is screwed onto the bolt before it is fitted so that it can be locked in any position to suit requirements. A small metal block with one side curved in the shape of the wheel axle is then fitted in the slot at the end of the bolt. This curved portion butts up against the wheel axle and when the screw is turned it moves the wheel axle up the slot in a rearward direction. When correct chain tension is achieved and the wheel is correctly aligned in the frame the wheel nuts and washers are tightened to secure the wheel in the cycle.

When that cycle is jumped in such a way that a landing is made in a vertical or semi vertical stance the upwards forces on the axle push along the adjusting bolts central axis with a compressive force, in which the steel bolt has much greater strength than it had in tension with the traditional system and is far less likely to fail. It is thus also less likely to cause injury with such an accidental breakage.

Brief Description of the Drawings.

FIGURE 1 is a detailed drawing of a typical embodiment of the traditional method of chain tensioning.

FIGURE 2 is a detailed drawing of a CNC machined component which forms the basis of a typical embodiment of the invention.

FiGURE 3 is an alternative rear view of the same component.

FIGURE 4 is a comparison top view of a frame with the traditional method of chain tensioning and a frame with an embodiment of the new method of chain tensioning.

FiGURE 5 is a detailed drawing of a fully assembled typical embodiment of the invention.

Detailed Description of the Drawings.

FIGURE 1.

This shows a typical embodiment of the traditional method of chain tensioning. The backstay, chainstay and chainstay end are fabricated together to form one side of the rear fork of the bicycle frame. When the bicycle is assembled, the rear wheel axle (A) fits down the slot of the chainstay end and is secured by the wheel nut (B). To allow adjustment backwards and forwards which facilitates tensioning of the chain, washer (C) is fitted behind the wheel nut. Washer (C) has a piece of threaded bar attached to it at point (D) (usually welded) . This piece of threaded bar locates along the rear exposed section of the wheel slot protruding beyond it at the rear. A machined, shaped "r' bar (F) with a central hole is then located over the end of that threaded bar, butting up to the upper and lower end faces of the chainstay end. Finally a locking nut (0) is screwed down the bar up to the "T" bar. As this nut is then screwed further it has the effect of pulling the axle backwards, thus tensioning the chain.

FIGURE 2 This is a drawing of the CNC machined chainstay end component which forms the basis of the invention. On it you can clearly see the way that the spigot to which the chainstay is attached is clearly set back from the axis through which the backstay spigot passes.

FIGURE 3 This is alternatively a view of the same component as figure 2 but from the rear. It clearly shows the way the lower spigot is set inward from the upper spigot.

FIGURE 4 This is a comparison view from above of(A) a frame using the traditional method of chain tensioning and (B) a frame using an embodiment of the invention for chain tensioning. With (A) because the stays are aligned the view of chainstay is obscured by back stay. With (B) the chainstay is clearly visible because it is set in from the backstay.

FIGURE 5 This is a full assembly drawing of an embodiment of the invention. Please note that the wheel axle nut and washer have been omitted from the drawing for clarity. The chainstay, backstay and chainstay end are shown as fabricated. The chain adjustment bolt (D) is shown in position with its locknut (C). This screws through the chainstay end and into the wheel axle slot. It then has a small block (B) fitted on its end which has one side relieved to follow the contour of the wheel axle. This butts against the axle and when the screw is screwed into the hole the wheel axle is forced backwards, thus tensioning the chain.

Abstract.

Bicycle rear wheel adjustment system for tensioning the chain on bicycles with horizontal wheel axle slots and a single sprocket drive. A novel method of rear wheel axle adjustment for chain tensioning on cycles with horizontal rear facing wheel axle slots and a single drive sprocket, either fixed or freewheeling.

Claims

Claims Claim I A rear wheel axle adjustment system for tensioning the

chain on bicycles whose frames have a horizontal rear facing rear wheel slot and a single sprocket or freewheel drive.

Claim 2 A system as claim I using a bolt adjuster screwing backwards from the direction of the centre of the frame adjusting the rear wheel in a rearwards direction.

Claim 3 A system as claim 2 with a frame using a CNC machined rear chainstay end stepped in such a way that the chainstay is fixed closer to the centre line of the frame allowing placement of an adjusting screw through the chainstay end and into the wheel axle slot.

Claim 4 A system as claim 3 where the adjusting screw is an alien head bolt which can be adjusted using an alien key.

Claim 5 A system as claim 3 where the adjusting screw has a screw head which can be adjusted using a screwdriver.

Claim 6 A system as claim 3 where the adjusting screw has a hexagon bead which can be adjusted with a spanner.

Claim 7 A system as claim 4, 5 and 6 where a locking nut is fitted onto the adjuster screw prior to insertion which can be locked against the frame component to securely hold it in position once the correct drive chain tension has been achieved.