top of page
  • Steve

Bendy axles slow you down

Updated: Dec 20, 2023

In this blog we look at bendy axles, why they are a problem and how they create extra drag, cause all sorts of other issues and ultimately slow you down.

A lot of wheel flex comes from a flexible rear axle.

We go on to explain how the new KOM Xeno hub, with its unique design, maintains superior bearing and wheel alignment, even when the going gets really tough.

Let's start the blog with a quick summary of bearings and their requirements and then explore the workings of a legacy hub. Then, using many illustrations we'll try to explain how the Xeno hub achieves its performance improvements using its unique two independent bearing sets running on concentric shafts.

Bearings keep things rolling

There are many types of bearings but those with spherical rolling elements are used, almost exclusively, in bike hubs.

This animation shows the bearing balls rolling in a single row deep groove bearing. The cage and edges of the races are removed for clarity. Image by Plusminus taken from Wikipedia's ball bearing page.

Nowadays bike bearings are used predominantly in the form of standard cartridge bearings. However, we will not discuss the specifics of bearings in this blog. We have a separate blog that looks in detail at bearings and their alignment: Bearing Alignment: Vital for Performance.

In summary it describes how vital it is to keep bearings correctly aligned while having a quick glance at some of the published research along the way.

The one message to hold from that document is, "You need to maintain near perfect alignment to get the maximum performance and long life from your bearings."

Bearing alignment is vital for performance

To understand the reasons for KOM's unique hub design it is helpful to first see how a normal bike hub works.

Legacy rear hub

The component layout is almost identical in nearly all current rear hubs:

Most rear hubs are built in much the same way. There's the hub shell (blue) and the free hub body yellow on which the cassette (bike cassette or gears) is located.

Let's have a quick look inside

Looking at a cross section reveals the standard layout inside. Both hub shell and freehub run on a single thin axle (grey) and where the two overlap there is a ratchet (Shown as dark grey under the spoke flange where blue overlaps yellow.) In this setup 5 bearings are shown in blue though 4 might be more common.

The hub shell and freehub body both run on a thin axle usually 15 or 17mm in diameter. The middle bearing on the hub shell has to sit inboard of the ratchet which is why it ends up almost dead centre on the axle.

Stripping away most of the components to reveal just the relatively small diameter (e.g. 15mm) axle and the hub shell bearings. Riding load is applied in the centre of the tyre which applies load via the rim and spokes to the hub shell. This applies load very unevenly to the hub shell bearings (red arrows). Nearly all the load is applied at the centre, the most flexible part of the axle. (One of the worst possible bearing layouts in terms of axle bending!)

Because nearly all the riding loads are applied in the centre of the axle. It is not hard to imagine how it bends. The bending can also be simulated with FEA (Finite Element Analysis).

Finite element analysis showing the bending of a standard hub axle due to the poor bearing

positioning at the centre of the axle. (The arrowhead shows the position and direction of the load. The ends of the axle are fixed, which is an over simplification, but the final picture gives the idea.)

For the purists: a wider FE analysis showing the bending of a standard hub axle and bike rear triangle. The axle bends but also bends the bike frame at the same time. (The rear triangle does not give nearly as much support as one might imagine.)

When the other components are arranged onto the distorted axle you can see (below) how the standard hub looks under load. The larger components don't flex much themselves. (Because they are much stiffer due to their larger diameter.) However, the hub shell and free hub have to move when the axle bends. Notice how the hub shell thus tilts or twists off to one side. This angular deflection tilts the whole rear wheel off to one side. (Because the rim and spokes are attached to the hub shell.)

Standard hub components on a distorted axle. Note how the hub shell is 'twisted' so that the wheel no longer runs straight upright in the bike frame.

On a standard hub the wheel twists sideways when the hub is loaded.

This video clip shows that wheel twist is real even for super stars. These clips are taken from Leogang Elite Men's Downhill Semi-Final | LIVE DHI Racing (Time around 1:01:24.)

"The Xeno made the bike feel more stable"

Anecdotally one of the first comments our Downhill racer made after riding our hubs was "They made the whole bike feel more stable hitting bumps." It goes without saying that Tom hits things a lot harder than most of us. But we were surprised that this was the first thing he noticed.

The wheel twisting or bending sideways with a changing load is certainly not what you want as nearly all of us encounter bumps.

But I hear you ask, "How much does the Xeno hub twist when it takes a big hit?"

How much better is the Xeno hub?

Just how much difference is there between hubs? Fortunately Sheffield Hallam University's Sports Engineering Research group wrote a paper about hub flexing and hub twisting (here.) We have taken one graph from the report and copied it below.

The bar chart shows how a selection of standard hubs twist when loaded. Their angular deflection or twist is measured at the same load. The KOM Xeno hub's different design means that it hardly twists at all compared to the others.

Before returning to the main subject, for those interested, the whole wheel flex thing can be viewed in super slow motion (here). The link is to one of many Enduro racing video clips but this time in slow motion.

But now back to the other issues with bendy axles and hub flexibility.

The root of hub flexibility problems

Thin tubes are flexible especially when loaded in the middle.

Loading the axle in the middle is the weakest point so it's bound to flex. The problems have got worse in some hubs as their axles have recently been reduced in diameter to try and fit new cassette types or to squeeze in slightly stronger bearings. Over recent years bike dropout widths have also increased. First to 142mm, then to Boost 148mm, and now to Super Boost 157mm all making axles longer and thus, even more flexible when loaded in the middle.

So why is bendy bad?

Well, we've seen how wheels twist on bumps. Some riders notice it as an awkward 'unstable' feeling on landing. Also, we know that excessive bending could also result in axles snapping but, other than that, why does a flexible axle matter?

An important issue is how the axle supports and interacts with the bearings and the rest of the hub components.

Bearings like to be aligned correctly

In order to operate efficiently bearings need to be correctly aligned. That is having their inner and outer races concentric but also 'straight' or parallel to one another. Correct alignment allows longer life and low friction. If bearings are out of alignment by even a small angle then their life is dramatically reduced. This is well known amongst bearing experts. (We do a deep dive into bearing alignment in another blog that can be found here.)

This graph, from NSK, shows the importance of correctly aligned bearings.

The graph above is published by NSK but there are similar graphs published by other bearing manufacturers. The point is that bearing life starts to plummet once bearing misalignment exceeds about 0.005 radians. The graph shows a loss of 85% of bearing life at a misalignment angle of just 0.007 radians. What this graph does not show is the corresponding increase in friction but it is just as frightening.

A major increase in drag

With some hubs this misalignment and wear can get so bad that the bearings actually bind up completely and, rather than rolling, the bearings spin on the axle when pedalling. This massively increases pedalling drag.

The still image is taken from a 5 minute video made by Wheelworks. This image shows where bearings have bound up enough to spin and leave marks on the axle.

On a legacy hub all the bearings turn

None of us want our precious pedalling effort lost to friction. When pedalling you have to turn all the bearings on a legacy axle. It's bad enough having to turn all the extra bearings when they are aligned correctly but far worse when they are misaligned or showing signs of wear. For example, if your freehub bearings are a bit worn and creating extra drag they still have to be turned every pedal stroke sapping energy and speeding failure of the already damaged bearings. The KOM Xeno is completely different in this respect as free wheel and pedal bearings automatically switch role as you change from pedalling to free wheel with the other set resting and causing no drag. But more of that below:

A major hub redesign was required to overcome all these issues.

When designing KOM's Xeno hubs, bearing alignment was one of the foremost considerations. We know that larger diameter tubes are stiffer which is why larger diameter axles were chosen for the Xeno. We also know that loading an axle at the edges is far stronger than at the centre so we were very keen to move the bearings as far apart as possible.

In this image the large diameter tube is used to move the load to the edge of the thin tube. This is essentially how the layout in the Xeno rear hub works: A larger diameter shaft transferring the load to the edges rather than the middle of the axle.

Small diameter tubes are especially flexible when loaded in the middle. However, when loaded at the edges they bend far less. That is why moving the bearings to the edge of the axle results in a far stiffer set up. A slightly larger diameter axle is also a lot stiffer. (It's not a linear relationship between axle diameter and stiffness.)

In this image the component parts of the hub are 'exploded' along the axis of the hub. Note the bearings are positioned at the extreme edges of a larger19mm diameter axle.

Top: Full Xeno hub assembly. (Note silver ratchet on left, disc side, of hub body.)

Middle: Drive shaft. This acts as full width freehub body transfering loads to the outer edges of the inner axle. (Also it transfers drive to the disc side of the wheel.)

Bottom: Inner 19mm Diameter axle. Note bearings on outer edges of the axle for maximum strength and stiffness.

It's a massive redesign but it keeps the bearings far better aligned and enormously increases their working life. You could say it's KOM secret sauce:

Xeno secret sauce

In the Xeno the bearings are separated into two distinct groups that work completely independently:

  1. 'Pedal bearings' and

  2. 'Freewheel bearings'

Easy names: Pedal bearing work when pedalling and freewheel bearings when freewheeling. This has many rider benefits including, reduced friction, reduced bearing wear and improved performance.

This clip shows the bearings are rolling when you can see the bearing balls moving and black when the balls are stationary in their races. The pedal bearings are the smaller diameter bearings outermost on their axle.

Pedal bearings

These pedal bearings are positioned at the ends of the inner axle for even load distribution, to maintain good bearing alignment and maximum stability. When pedalling only these bearings are active.

The Xeno uses a larger 19mm diameter inner axle with its bearings located at the edges. The result is evenly loaded bearings on an axle that bends less keeping the bearings correctly aligned. Also importantly, when pedalling only these bearings turn. (The other bearings on the larger diameter drive shaft are only active when freewheeling.)

Free wheel bearings

The free wheel bearings on the far larger diameter driveshaft are only active when freewheeling. (The pedal bearings take a break and stop rolling when the freewheeling takes over.)

Only these bearings turn when freewheeling. The Xeno driveshaft is a much larger diameter shaft that runs over the inner axle enclosing the pedal bearings within. Moving the ratchet to the disc side allows the ratchet to be larger and stronger. It also frees up space for the bearing on the cassette side of the hub shell to be moved outboard taking the place of the ratchet. This rearrangement allows for more evenly loaded hub shell bearings (red arrows). (Compare the drive shaft with legacy hub shell bearings image, where one bearing in the middle of the axle takes almost all the rider loads.)

The Xeno driveshaft can be imagined a bit like a free hub body that has been stretched and extended the full width of the rear axle. The bearings within are thus moved much further apart. That makes the driveshaft far more stable on its axle than a short free hub body. But that is only part of the story. The freewheel bearings are also much larger diameter bearings.

Different jobs for different bearings

The larger diameter freewheel bearings run on the outside of the driveshaft supporting the hub shell. In the image above the arrows give an impression of how the normal riding loads are distributed. It might lead you to wonder why the bearing on the left is such a large diameter bearing, by far the largest of all the bearings, when it carries a smaller proportion of the total riding load. Well, the reason for this is that this bearing has to do another key job; that of holding the driveshaft and pawl carrier central in the ratchet. The loads in this area can get very large especially if only one pawl is taking all the drive torque. On most hubs this happens infrequently, which is just as well as it tends to damage and ultimately destroy free hub and adjacent bearings.

Use a shaft up to the job

Some of the reasons for such a large diameter drive shaft have already been explained. One of its main requirements is to transmit torque drive from the cassette side through the wheel hub to the brake disc side. Larger diameter shafts are far better and transmitting torque with minimum distortion. They are also far stiffer in bending so good at transmitting riding loads from the free wheel bearings to the inner axle and ultimately the bike frame.

But as mentioned above a super strong drive shaft is required in this area is so the hub can handle high torques even when applied unevenly for example if just one pawl engages.

Single pawl engagement

When only a single pawl engages it has to take all the load that would normally be shared with its fellow pawls. So, if there are normally three pawls engaged when only one pawl drops it has to take 3 times the load.

One way to increase the strength of the pawl for high load situations is to make it larger with more teeth. (Legacy pawl on left KOM pawl on the right.)

The Xeno is designed to take these occasional high loads. Increasing the diameter of the ratchet reduces the load in its teeth as does increasing the number of teeth engaged.

The root of many hub failures

Single pawl engagement is the bane of many hubs and can often be traced back as the root cause of most hub, ratchet and axle failures. It is a massive subject and for those interested in a really deep technical dive and explanation it is covered in a whole separate blog.

What happens to cause the damage?

High torque pedalling, combined single pawl engagement results in a very large uneven load perpendicular to the axle that has to be resisted by axle and bearings. It is for this reason that the drive shaft and bearing are so large in the Xeno (and why the two groups of bearings, pedal and freewheel, operate totally independently.) In the Xeno the pedal bearings are protected from these high loads by the drive shaft and its bearings so that they can continue running silky smoothly.

Xeno super strong shaft

The Xeno uses a far larger drive shaft and bearing to safely handle these loads.

The difference is striking when you compare the axle size (shown below) and wall thickness of the axles between the Xeno and legacy hub.

On the left is the KOM Xeno drive shaft that the large bearing runs on. On the right is a typical axle from a legacy hub (15mm OD 12mm ID.) The thin little axle only has about 4% of the stiffness of the Xeno drive shaft. Or to put it another way the Xeno dimensions on the left make the axle 25 times stiffer and of course many times stronger.

It's a similar story with the bearings too.

On the left is the KOM Xeno single bearing whose job is to keep the pawl carrier and drive shaft central in the ratchet even when the going gets tough. On the right is a typical bearing from a legacy free hub struggling to do a similar job.

Xeno: Performs better and lasts longer.

So that's a look at bendy axles. I hope it gives an insight into why the Xeno performs better, its axles don't break and its bearings last many times longer.

We intend to do an anniversary blog, looking at the first KOM prototype hub that was actually fitted to a bike, almost exactly 6 years ago. The bike used to do a lot off road, took me over the Alps and Dolomites to Venice (Italy) and is still used, normally several times a day as a commuting bike. The hub is still running on its original bearings so it should be interesting to have a look at how things are wearing. (All the other bearings on the bike have been replaced or serviced.)

We have a couple of other interesting stories in the pipeline hopefully out soon or early next year :

  • Christmas brake? We are nearly always testing brakes and have lots of interesting news to update you on.

  • Does it sometimes feel like you left the parking brake on when you arrive at the top of a climb? We will show you why you might be right.

If you or friends would like to be informed when the next blog comes out or to receive our newsletter (once or twice a month at best) or both then please sign up here (or below) and you'll be the first to know. (You can of course unsubscribe at any time.)

170 views0 comments


bottom of page