Features

• Price: US$375 including postage world-wide
• Weight: 450 - 465 gms. depending on type
• Range of adjustment: 160 - 183 mm infinite
• Types:
  • Shimano square and splined drive (road only);
  • Campagnolo square drive
    
  • Triple adaptors available at extra US$25
• Aerodynamically superior to conventional cranks
• tremendous strength to weight ratio and stiffness.
• Converts all energy into forward motion.
• Adjust to suit leg length and conditions in seconds

Why adjustable cranks?

Obviously you wouldn't, but most cyclists accept the cranks that came with the bike without question.

The leg has an optimum or ideal range through which it should travel during the pedalling cycle in order to attain optimum perormance.

If the crank is too long or short the leg cannot go through its optimum range and performance will suffer.

Enter the Murray TourDeForce Adjustable Cranks

• These ultra-light (450 gms) ultra-stiff carbon cranks
• have an infinite adjustment range of between 160 and 183 mms
• which can be set or changed in seconds
• without having to buy new cranks and
• remove the old ones as well as the chain rings and swop them over.

• to exactly match leg length (see table)
• to accommodate different cycling disciplines - time trial, climbing, triathlon...
• to accommodate leg length discrepancies

Here's what users of Murray TourDeForce Cranks are saying:

Bicycle Crank Geometry

It may seem as if there are very few ways to improve cycling technology; everything that can be done has been done to convert energy into forward motion. Lightweight carbon frames and wheels, STI gearshifts, titanium parts: almost every component on a bicycle has been scrutinised by engineers and aerodynamicists and improved in the most infinitesimal way to make them lighter and faster.

Although the crank has had its share of technological innovation as far as materials are concerned it remains unimproved as far as adjusting its length is concerned without impairing its strength and without imposing a weight penalty.

Cranks are at the very heart of the bicycle. This is where all your effort is concentrated in order to drive the bicycle forwards; where the reciprocating motion of the legs is changed into the rotary motion that turns the back wheel.

I want to show how important it is to choose the correct crank length so that the leg can travel through an optimum range during the pedalling cycle. So much has been written on the subject and every expert has a different theory on how to match leg to crank length and every theory contradicts the other.

For example, one expert (Kirby Palm (link to his webpage here) says that you should divide your inseam length by a factor to find the correct crank length. According to this expert, I should be using 186mm cranks and as there is only a range of 160-180mm cranks available over the counter, I would not be able to achieve optimum performance.

But how does he decide on this factor and why and why does another expert say that a 172.5 crank is good for most riders. What science lies behind the theory and how did the industry decide that a range of 160-180mm was right in the first place.

To find out, I built a test rig of a cyclist’s leg which was adjustable for femur (thigh), tibia (shin), crank length and saddle height and the knee angle calibrated in degrees to measure the included angle between the tibia and femur at the top of the pedalling stroke. Perhaps it would be a good idea here to describe leg action during the pedalling cycle. See Fig 1.

The knee pivots around a well-defined point, which is the thigh joint in the pelvis. At the top of the stroke (crank at 12 o’clock) the included angle between the tibia and femur at the knee will be at its most acute. A very important factor in the equation. More about this later.

So what I set out to find was a starting point or bench mark based on logic so that I could draw up a table of leg length vs. crank length as a guide to the cyclist when choosing a crank to suit his or her leg length and physiology.

Building the model was a humbling experience. I had the preconceived idea that femur length and crank length were the only determining factors in the equation, which would be reasonable if hip joint, ankle joint and bottom bracket were all in the same plane. But, of course, they are not and this adds to the complexity of the geometry. To retain my sanity and not to add more complexity to an already complex subject, the ankle joint and foot length are fixed.

Fig 2 and 3 illustrate the surprising difference in the length of arc of the knee for a difference in crank length of 20mm, all else being equal.

This alone is ample evidence in favour of using different crank lengths. In Fig 2 (180mm crank) the knee will travel 65mm further than in Fig 3 (160mm crank). This translates into 65mm more work per 180° of crank rotation for a change in knee angle of 2˝° at top dead centre.

This knee angle is one of the limiting factors in determining leg length. Inexperienced riders who set their saddles too low may have felt the discomfort and pain due to this incorrect set up. The same applies to using a crank which is too long. The longer the crank, the more acute the angle at the knee, at the top of the stroke. Optimum knee angle at the top of the stroke appears to be in the region of 70°.

Overstressing the knee area can lead to a number of problems such as *illiotibial band syndrome and ¶ patella tendonitis. Using my own leg as a starting point and based on my own experience of knee discomfort, I now have a reasonable benchmark without resorting to a computer and a huge data base of different leg lengths. Using my model, I can now extrapolate this information proportionately to draw up a table of crank vs leg length as a rough guide to choosing a suitable crank.

* Illiotibial Band Syndrome is an irritation of the large tendon that runs along the outside of the thigh, due to friction as the band rubs the femur. The symptom is a stabbing pain near the bony protrusion on the outside of the thigh.

¶ Patella Tendinitis is inflammation of the patella or kneecap or where it joins the tibia, caused by the knee bending too much. The symptoms are tenderness and pain during or after riding.

There are better minds than mine who will remind me that even though two riders may have the same leg length, the proportion of femur to tibia is not necessarily the same plus a whole bunch of other factors, such as physiology and foot length and movement.

At least this may engender discussion and research based on more scientific principles.

I would like to discuss the more esoteric physiology of pedalling which I have asked the good people at the Sports Science Institute to research.

How to measure your leg length to enable you to use the table.

Power.

Conclusions.

• adjust to suit any conditions in seconds
• versatile - time trials, climbing, cornering..
• can be adjusted independently of each other
• compensate for leg length discrepancies - occur in almost every rider.