How They Work

CombOvers look too small to do anything. That's the whole point — and it takes a little explaining. Warning, this page is REALLY GEEKY.

Summary

• The goal is a clean face: You only need to block the few paths mud actually takes to your eyes and mouth, so most of a normal mudguard is dead weight.
• Mud is predictable: It leaves the wheel on a tangent and follows trajectories we can plot. Only a fraction of the wheel's spray can ever reach your face.
• Two paths matter: Direct (flung UP off the back of the wheel) and Indirect (thrown UP and FORWARD off the top of the wheel, then blown back at you). CombOvers block the Direct path; the Toupee (MTB) and Toupee Gravel (rigid gravel) shorties block the Indirect one.
• How so tiny? By blocking trajectories where they pass close to your bike (and letting your frame do some of the blocking) they do the job at a fraction of the size.
• Why not closer to the tyre? The same answer from the other side. Closeness does let a guard be smaller, but it has to be the whole shield by itself. A CombOver never is: it sits against the down tube, so the frame forms the middle and each half only has to cover an edge. It gives up some closeness and gets the width of your bike in return.
• Performance: In our limited testing a ~50g CombOver and Toupee set beat a 120g moto fender, because most moto fenders leave the Direct path wide open — though that still needs independent validation.

Your face, not your bike

CombOvers aren't trying to keep your bike clean, or even most of you. They're built to keep mud out of your eyes and mouth. Once you accept that only your face truly needs protecting, you've simplified the problem enormously.

Much of the material in a conventional mudguard isn't blocking mud, either:

• It isn't on a trajectory between the tyre and your face.
• It is structural, holding another part in place.
• It is on a trajectory between the tyre and your face that is already blocked, so is redundant.

So we kept only the bits that block that path, and threw the rest away. That's why they can outperform alternative products weighing 2 to 4 times as much.

So where does mud actually go?

When you're riding, mud seems to be flying everywhere: any direction you look, you can see mud. Your face feels like it gets covered from all directions.

But let's break it down.

Mud can only reach your face from the front wheel of your bike (OK, or the rear wheel of someone else's).

The mud doesn't just expand out from a spinning wheel in all directions:

Ahoy there sailor, not like this!

It leaves the wheel on a tangent (i.e. it just carries-on in a straight line from the point it left the wheel), like this (viewed from the left side of the wheel, so it appears to rotate anti-clockwise):

Now we can take any mud particle and trace where it goes (the trajectory) after leaving the wheel.  All we need for this is to know the speed and direction of the particle as it leaves the wheel and the strength of gravity.  This allows us to plot the trajectories of particles leaving the wheel at any point on its circumference, and it looks like this at 10mph (16kmph):

All trajectories are relative to the bike - imagine panning your camera with the bike as it rides past from right to left.

As we cycle faster, the mud gets flung faster and farther. At 15mph (24kmph):

Conversely, you can see that if you cycle slowly, there is a maximum height the mud can't surpass.  At 5mph (8kmph) it barely lifts above the wheel:

Depending mostly on how tall you are, the minimum riding speed for mud to reach your face is around 10mph (16kmph) - we'll come back to this later.

The other thing to note, is that the wheel flings mud in lots of directions that aren't ever going to reach your face: all green trajectories here either never gain enough height, or will only gain that height behind you.  This applies at any speed. Here we can see it at 20mph (32kmph) - mud leaving from ~3/4 of the wheel will never hit your face.

If we were riding our bike on the moon, this would be a pretty good model.  But we aren't...

Air resistance

There are 2 parts to this:

• Mud particles travelling in any direction will be slowed by the air, until they match the air speed.
• Ideally, we're cycling forwards, so mud particles will be blown backwards relative to our own motion.

So how much air resistance does a mud particle have? We need to know so that we can adjust those trajectories. The answer is surprisingly simple: any amount! Large/dense particles (e.g. stones) have little air resistance and just move according to momentum and gravity.  Small/light particles (e.g. a very small water droplet) have high air resistance and will immediately slow to a stop and then drift with the air, i.e. mist.  Mud from a wheel contains a mixture of particle sizes, so we just use air resistance values from zero to infinity and model all possible trajectories:

Mud leaving from just below the trailing edge of the wheel at 15mph (24kmph).

• Red: no air resistance - it almost goes directly up and down
• Orange - Indigo: it is thrown upwards by the wheel, but starts to slow and get blown backwards, while gravity pulls it back to the ground.
• Violet: almost infinite air resistance - it immediately slows to ambient air speed and gets blown past the bike and rider, slowly sinking to the floor.

Combining trajectories from all points on the wheel, we get a cloud of mud that completely surrounds the rider, nicely matching experience. At 15mph (24kmph):

The two main trajectories

We can see the 2 main routes mud takes to reach your face:

• Directly up from the rear of the wheel (red and orange)
• Indirectly thrown up and forward from the top of the wheel (either in front of the fork or from behind and through the gap between the fork legs), but then blown back into your face (yellow-green)

Mountain bikes

Below ~10mph, no mud will reach your face (figure at 11mph/17kmph):

10-13mph only mud on a direct trajectory can reach your face, so CombOvers (red) on their own are sufficient (figure at 13mph/21kmph):

Above ~13mph, with suspension forks the indirect trajectory can also reach your face, so both CombOvers and a Toupee (also red) are required (figure at 16mph/25kmph):

Mud still doesn't reach your face at 40mph (64kmph):

Gravel bikes

Worth seeing how gravel bikes are different:

• Usually rigid forks (but can have short travel suspension)
• More aggressive rider position brings the head forward and lower

The maximum speed that is fine without any mudguard is a little slower due to the lower head position. At 10mph (16kmph):

CombOvers (red) on their own work to a higher speed than on a suspension bike, because a rigid fork has a far smaller gap between the fork legs for the indirect path. At 17mph (28kmph):

To close that gap completely, you'll need a Toupee Gravel (also red): a shorty designed for rigid gravel forks. A CombOver and Toupee Gravel together keep your face clean well past 50mph (80kmph), effectively any speed you'll ever ride:

Note, a road bike behaves much the same, just with less mud.

A bump in the road

Another test we can do with our model, is to imagine the front wheel going over a bump. This gives it (and any mud it releases) an instantaneous upward shove. We need to make some assumptions about the elasticity of the wheel and tyre, but a reasonable back of the envelope calculation suggests a wheel hitting an 8" high rock would get a vertical shove of ~0.6x bike speed.

Here we can see the mud cloud riding at 20mph (32kmph) on a smooth trail:

Riding at 20mph (32kmph) with 8" rocks (notice it is the mud leaving the wheel in front of the fork that is most affected):

Riding at even bigger rocks or faster will give you a muddy face: but this is getting pretty extreme - well done!

Why slats? Won't mud go through the gaps?

So far we've modelled both CombOvers and Toupees as solid shields.  However, CombOvers are made of many small slats to reduce weight. Let's do some high-school geometry...

If trying to create a shield, to make it as small as possible, you want it to be at right angles to the thing you're shielding from.  Any other angle and it has to be bigger.  If it is at 45deg, it needs to be 40% bigger to have the same effect.

So the CombOver slats can be smaller than the gaps between them because the mud arrives at an angle:

Using the mud trajectory modelling above, the slats are carefully positioned and angled so that mud always meets the face of a slat, never a gap. From many viewing angles it looks like mud could slip straight through, but those angles aren't the trajectories the mud is actually travelling on.

Won't mud go round the side when I steer?

In practice, no — for two reasons.

Below ~10mph, mud leaving the wheel can't travel fast enough to reach your face before gravity pulls it back down. It simply doesn't get there:

Above ~10mph, you steer mostly by leaning, keeping the front wheel fairly straight. How straight you keep the wheel is dependent on riding style, terrain, etc. Certainly, you should try to watch your own style while riding (but don't forget to look ahead too ;-).

CombOvers are sized to match how much you actually steer at speed. When mounted on the bike, they are ~130mm wide (depending on the down tube width) - significantly wider than other down tube mounted mudguards. With a 60mm wide down tube (fairly typical for a carbon frame), CombOvers will fully shield a 29x2.35" wheel steered up to 7deg either side:

This is what that looks like for real, riding at ~15mph (24kmph):

Shouldn't a mudguard sit as close to the wheel as possible?

All else being equal, being close to the tyre means the mud can be shielded before it has a chance to spread-out: it removes the influence of air resistance and bike movement on where the mud goes.

However, being close to the wheel has several drawbacks:

• The mudguard protrudes further from the bike structure (it can't attach to the wheel), requiring more purely structural material. The result is heavier and more fragile (and can be inconvenient if you remove the front wheel to transport your bike).
• When mounted on the fork as unsprung mass, the mudguard reduces suspension performance, and needs heavier construction to handle the suspension movement without rubbing the wheel
• When mounted on the fork the mudguard also increases swing weight, making steering slower (subtle and subjective whether this is good or bad)
• The mudguard is more likely to clog with mud.

The proximity argument also leaves out width. What covers the wheel is the CombOver pair PLUS the frame between them: ~130mm across with a typical down tube, far wider than other guards, but much of that width is free because the frame provides it. Distance is more than paid for by the width of the frame.

It is a tradeoff, rather than an absolute. Using an understanding of where the mud flies and a different mounting position, CombOvers occupy a 3rd sweet-spot to provide improved performance for far lower weight.

It is a tradeoff, rather than an absolute. Using an understanding of where the mud flies and a different mounting position, CombOvers occupy a sweet-spot to provide improved performance for far lower weight.

The opposite extreme: goggles and a face mask, also works. But then you're cleaning your goggles instead.

And for real?

That's all very good in theory, but does it hold up on the trail?

Yes.

The model was built after riding prototypes, and tuned until it reproduced what we saw on those rides (it took a few bug-fixes to get there). The real test came afterwards: it went on to correctly predict results from rides we hadn't used to build it.

On real rides, in comparison testing:

• CombOvers on their own subjectively block roughly half the mud that would otherwise reach your face; a Toupee shorty alone, also about half; together, subjectively around 95% (there's a small overlap between them).
• The speed thresholds match when mud actually reaches your face.

But why not just use a moto fender? 

Moto fenders can work well, and they have a particular look.

However, fork-mounted moto fenders trade off coverage against how much they get in the way. They come in several sizes, or even versions you can adjust via clip-on extensions, so you can choose your own compromise on the day.

Testing with a middle size version: surprisingly it leaves the Direct path (mud flung straight up off the back of the wheel) wide open (you can see the tyre when riding), so your face still gets muddy. The model reproduces this, and shows how the exposure changes across the sizes.

Small (13mph/21kmph):

Medium (13mph/21kmph):

Large (13mph/21kmph):

The large size does close the Direct path — but only by increasing the bulk, toe overlap and transport hassle. There's no size that's both small and effective, because the location itself is stuck with that compromise.

CombOvers sidestep the compromise by mounting where the bike is already close to the mud trajectories and letting your frame do some of the work, so they can be small and yet completely block the direct path. In our (limited) testing, the CombOver and Toupee combination (50g) beat a 120g moto fender at under half the weight — which fits the theory, since most motos leave the direct path open. This needs proper validation though (and a large moto fender may catch a little more of the side-spray thrown out as you steer — how much depends on your riding style and terrain), so we encourage you to try it for yourself.