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.

TL;DR

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 through the fork, then blown back at you). CombOvers block the Direct path; the Toupee and Wig shorties blocks the Indirect one.
It's speed-dependent: below ~10mph no mud reaches your face; 10–13mph CombOvers on their own are enough; above ~13mph you want both; but mud will get past the set eventually (40+mph).
Gravel & road: a CombOver alone helps at lower speeds; pair it with a Wig for a clean face at any speed.
Why it's 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 weight.
Performance: a ~50g CombOver and Toupee set out-performs a 120g moto fender in testing, because the moto fender leaves the direct path wide open.

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 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):

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 wheels fling mud in lots of directions that aren't ever going to reach your face: all trajectories coloured green 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 (the green trajectories) 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 traveling 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 by the wheel (through the gap between the fork legs: this only really applies to suspension forks), but then blown back into your face (yellow-green)

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):

Eventually, when you are fast enough, the indirect trajectory CAN still reach your face around the CombOver/Toupee combination. Mud still doesn't reach your face at 40mph (64kmph):

Gravel and road 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 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 Wig: a gravel/road-specific shorty. A CombOver and Wig together keep your face clean well past 50mph (80kmph), effectively any speed you'll ever ride:

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 a 6" high rock would have a vertical shove of ~0.4x bike speed and an 8" rock gives a ~0.6x shove.

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

Riding at 20mph (32kmph) with 6" rocks (notice it is the mud leaving the wheel in front of the fork that is most affected):

Riding at 20mph (32kmph) with 8" rocks:

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 modeled 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 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 traveling 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, see Figures 3, 4.

Above ~10mph, you steer mostly by leaning, keeping the front wheel fairly straight. CombOvers are sized to match how much the wheel actually turns at speed (when mounted on the bike, they are ~130mm wide - significantly wider than other down tube mounted mudguards).

So on the occasions mud does get around the side, you are riding slowly enough that it won't reach your face anyway.

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, 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.

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. Note, the model was constructed after riding prototypes as a sanity check (and yes, it needed a few bugs fixing to match reality, but now it is pretty close).

On real rides, in side-by-side 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, around 95%.
The speed thresholds (Figures 9, 10, 11) match when mud actually reaches your face.

But why not just use a moto-style mudguard?

Moto-style mudguards can work well, and they have a particular look.

Fork-mounted moto-style 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), so your face still gets muddy. The model shows exactly why, for all three 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. It's small and completely blocks the direct path, and the combination of CombOvers and Toupee (50g) out-performs a 120g moto fender at under half the weight.

Got questions this didn't answer? Email us: info@combover.co.uk

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