# Learn me about fenders



## locobaylor (Mar 11, 2008)

I recently purchased this fender for my junk commuter bike:
Origami Fender
I thought it would do the trick of keeping my behind and backpack dry. But I arrived at my destination tonight and found dirty water drops on my backpack. I know that a full-length fender will have more coverage, but shouldn't this one do the trick also? And even if there is spray, shouldn't I be riding away from what isn't blocked by the fender?? Thanks.


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## InfiniteLoop (Mar 20, 2010)

The water in your tire is traveling in a circular trajectory. When it is thrown off it it will continue that (though it's orbit increases). Looking from the drive side - a water droplet leaving the tire at 10 o'clock will hit your back. SO, you pretty much need a fender down to almost 9 o'clock. Something going to about 11 o'clock can work if it has a really larger kicker that goes out far enough to block those droplets leaving the tire at between 9 and 10 o'clock


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## Lotophage (Feb 19, 2011)

Yup, full length fenders are pretty much what you need, plus mudflaps. What you bought isn't really a fender, it's a splash guard. 

Velo Orange sells metal full length fenders for about the same price as plastic fenders. They are a minor PITA to install but once they're on, they're on.


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## JCavilia (Sep 12, 2005)

your conclusion is correct, IL, but your physics is wrong


> The water in your tire is traveling in a circular trajectory. When it is thrown off it it will continue that (though it's orbit increases).


The water on the tire travels along a circular path as long is it is in contact with the circular tire, but when it leaves it does not go into "orbit" unless your wheel is made of something dense enough to create a gravitational field of sufficient strength -- something like a black hole or a neutron star, I'd estimate.

When the water droplet leaves, it travels in a nearly straight path, tangent to the wheel circumference, curving down slightly in a parabolic path in response to the earth's gravity. But it doesn't curve much in that short path. You can observe this if you ride next to a fenderless rider in the rain. 

The real answer to the OP's supposition that he should "outrun" the drops is that the top of the wheel is going twice as fast as the he and the bike are moving. So there's plenty of momentum to overcome your speed.


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## InfiniteLoop (Mar 20, 2010)

Ran this by a couple of physicists. They said the droplets will not proceed in a straight line coming off a wheel, but didn't fully agree on why that is. Gravity plays a part but they also said that built-in trajectory and rotation of the droplet as it leaves the tire both play a role.


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## bikerjulio (Jan 19, 2010)

"Mythbusters" already tackled this extremely important topic, although in their case it was moving pistols.

They determined that the bullet from a moving gun still travelled in a straight line.


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## Lotophage (Feb 19, 2011)

InfiniteLoop said:


> Ran this by a couple of physicists. They said the droplets will not proceed in a straight line coming off a wheel, but didn't fully agree on why that is. Gravity plays a part but they also said that built-in trajectory and rotation of the droplet as it leaves the tire both play a role.


Suffice it to say, short fenders=wet ass.


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## InfiniteLoop (Mar 20, 2010)

I wasn't able to find it. Do you know the episode number or a youtube link?

A bullet fired from a gun will still curve towards earth due to gravity. I think though that the two key issues the physicists argued about would not have been present with a bullet - that the droplet had built-in circular travel and thus when freed from the tire would have continued this somewhat and that each droplet likely rolls off the tire and thus has induced spin that also causes a curved path. This is all well beyond my limited knowledge of physics, but still rather fascinating.


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## locobaylor (Mar 11, 2008)

Thanks for all the info, I might try buying a longer fender as my bike isn't conducive for full-length fenders. One last question: I currently have my fender clamped to the bottom of my seatpost. Would moving it to the top help catch any of that water? It seems like that would change the angle and how much water it catches at least a little. Thanks again for all the help.


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## LePatron (Jan 5, 2011)

If your commuter bike has eyelets for mounting I suggest you look into a set of SKS longboard fenders. These are the best at keeping the hydro off you and your riding partners based on my experience.


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## wooglin (Feb 22, 2002)

locobaylor said:


> Thanks for all the info, I might try buying a longer fender as my bike isn't conducive for full-length fenders. One last question: I currently have my fender clamped to the bottom of my seatpost. Would moving it to the top help catch any of that water? It seems like that would change the angle and how much water it catches at least a little. Thanks again for all the help.


Probably let water under the fender to hit the back of your legs. I'd rather have it hitting my pack. 

Have you looked at raceblades, which are designed for bikes without eyelets? 

SKS RaceBlade Clip-on Fenders | Urban Velo


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## locobaylor (Mar 11, 2008)

wooglin said:


> Probably let water under the fender to hit the back of your legs. I'd rather have it hitting my pack.
> 
> Have you looked at raceblades, which are designed for bikes without eyelets?
> 
> SKS RaceBlade Clip-on Fenders | Urban Velo


I haven't looked at those yet; they seem to be my best option for the rig I have now.


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## rmsmith (Feb 15, 2007)

InfiniteLoop said:


> Ran this by a couple of physicists. They said the droplets will not proceed in a straight line coming off a wheel, but didn't fully agree on why that is. Gravity plays a part but they also said that built-in trajectory and rotation of the droplet as it leaves the tire both play a role.


Once the water droplet breaks free from the hydraulic surface tension it will travel in a straight line according to relativity theory. However, real world gravity and atmospheric friction will quickly dissipate linearity and velocity.


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## RotatingShifts (Nov 12, 2006)

"Have you looked at raceblades, which are designed for bikes without eyelets?"

I don't know how these raceblades continue to survive in the free market. The short front unit allows all of the filthy water from the front tire to assault your feet and ankles, the front de'r, chain, chainrings, pedals, and associated cabling. The rear unit may keep the filthy water off of your backside, but everything mentioned above gets hit from the back.

Not worth a cent, IMHO.


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## UrbanPrimitive (Jun 14, 2009)

RotatingShifts said:


> I don't know how these raceblades continue to survive in the free market.


Because product quality has little to do with product sustainability?


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## Mr Evil (Aug 12, 2011)

I have something similar on my bike, and it works pretty well. It looks like it's a bit longer than the one you have, and it also follows the curve of the wheel somewhat, so it isn't too far away from the tyre at the far end (unlike the pic on Zefal's site, which has it angled rather high).


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## AndreyT (Dec 1, 2011)

InfiniteLoop said:


> A bullet fired from a gun will still curve towards earth due to gravity. I think though that the two key issues the physicists argued about would not have been present with a bullet - that the droplet had built-in circular travel and thus when freed from the tire would have continued this somewhat and that each droplet likely rolls off the tire and thus has induced spin that also causes a curved path.


Both of your "physicists" apparently bought their diploma.

Firstly, on a stationary bike, with its wet rear wheel spinning, the droplets will begin their travel in a _perfectly straight_ tangential path after leaving the wheel. The path will, of course, curve parabolically due to gravity (unless the droplet was shot straight up).

Secondly, a droplet leaving the wheel will not have any substantial spin. I.e. whatever "spin" it might have, it will not be sufficient to alter its trajectory in any noticeable way.

Thirdly, the leading factor that determines the trajectory of the droplet is not about the droplets and the wheel. The trajectory is determined predominantly by the turbulent air currents that always exist behind the cyclist's back as he rides through the air. _This_ is actually the primary factor that determines the amount of water droplets he'll catch on his back.

And this is the reason short fenders don't work: the sector "shadowed" by the fender in question is not sufficiently large. The droplets that shoot up right by the tail end of that fender are still so close to his back as to get picked up and carried onto his back by the air currents. The short fender in question can substantially reduce the number of droplets, but it won't eliminate them all.

Note also that the "sucking" effect of the turbulence is much greater when riding upright than when riding in the drops. This fender will work fairly well for a rider in a well tucked-in position. But for a typical much more relaxed commuter posture it is insufficient.


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## sli4 (Mar 26, 2012)

I love that this turned into a physics question. without going into the depths of physics lingo... the water would be carried up by the tire and pushed forward as if flung in a straight line. of course different parts of this water traveling with the rotation of the tire will be released from the tire at different points depending on the speed of the wheel and the amount of water.. that is to say if you are going VERY slow/walking your bike and its just a little wet out.. you wont see much water being flung about from the top of your wheel (towards your back) but if you start cranking up the speed the water will be released at hire points (as well as lower points) along the wheels rotation. 
so it's safe to say, as someone else did, if you're riding your bike at a normal riding pace, your butt will get wet. the only way to stop this would be a longer fender that extends around the curve. BUT I'm sure a shorter fender would still help more than not having one at all.


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## davidka (Dec 12, 2001)

locobaylor said:


> Would moving it to the top help catch any of that water? It seems like that would change the angle and how much water it catches at least a little. Thanks again for all the help.


Nope, the farther from the wheel, the less spray it will catch. 

There is a long version of the racer blade that work like full length fenders, which are what you want. Ride in the wet enough and the part of the fenders you come to appreciate the most are the bottom of the front and a rear that goes all the way past the BB.


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## franzt (Feb 24, 2013)

Greetings from Tbilisi!
First off, I've only just recently bought a used bike for commuting, and was looking at a set of SKS Chromoplastics to keep the water off my backside. Unfortunately for today I got throughly soaked :/ Just wondering if these are comprable to the longboards?

Seconly, water droplets would exit a revolving tyre following a path tangental to the wheel, the Earth will then pull it to ground level. Not necessarily parabolic trajectory, imagine a H2O mass exiting the far side of the rear wheel, it will go up, then strait down. JCavilia was correct. 

Anyway, I commute 60km daily (to and from work), just wondering about these Chromoplastics for rain cycling.


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## bmor_62 (Mar 7, 2012)

You can run full length metal fenders on a frame without eyelets by using P-Clamps. That's what I do. 
Can anyone recommend a mud flap that can be mounted on existing fenders?


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## Andy M-S (Feb 3, 2004)

That's not a fender. Now _this_ is a fender.
View attachment 277100


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## profpenguin (Mar 1, 2012)

bmor_62 said:


> You can run full length metal fenders on a frame without eyelets by using P-Clamps. That's what I do.
> Can anyone recommend a mud flap that can be mounted on existing fenders?


If you are a craftsman or craftswoman, you can hand fabricate some yourself using an old water bottle or dish soap bottle, or liquid laundry soap bottle...


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## Love Commander (Aug 20, 2009)

locobaylor said:


> I haven't looked at those yet; they seem to be my best option for the rig I have now.


I'm assuming you're on a road bike that doesn't have much clearance for fenders, yes? These clip-on fenders are also worth considering.


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## Love Commander (Aug 20, 2009)

RotatingShifts said:


> I don't know how these raceblades continue to survive in the free market. The short front unit allows all of the filthy water from the front tire to assault your feet and ankles, the front de'r, chain, chainrings, pedals, and associated cabling. The rear unit may keep the filthy water off of your backside, but everything mentioned above gets hit from the back.
> 
> Not worth a cent, IMHO.


Depends on their intended use. I'll throw on a rear raceblade when I'm headed out for a road ride and it's raining or recently rained. It keeps mud and road grime off of my back and off my leather saddle, I don't care about the rest, since everything else can easily be cleaned after I'm home. And since we have +225 days of sunshine here, a set of full fenders is a lot of noisy dead weight to be hauling around.

Worth every penny, IMHO.


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## Andy M-S (Feb 3, 2004)

bmor_62 said:


> You can run full length metal fenders on a frame without eyelets by using P-Clamps. That's what I do.
> Can anyone recommend a mud flap that can be mounted on existing fenders?


Planet Bike makes an excellent mudflap for their Cascadia fenders. Since the fenders come in several widths, so does the mudflap, and they sell them for $5/pair with free shipping. On my bike (see photo) I use SKS fenders because I like the shape, but I've fitted them with Planet Bike mudflaps--one to extend the length in front, and one on the rear of each fender. They make a huge difference in staying dry, and one of their nicest features is that because of how they're shaped, they hold that shape even in the wind.


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## lgh (Feb 21, 2006)

Be aware that the long board SKS fenders have very little ground clearance, like on the order of 2 inches per grant p. That might be an issue for storage or if you have to drag your steed up a steep hill or stairs.

Larry


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## Allthunbs (Mar 5, 2013)

Andy M-S said:


> That's not a fender. Now _this_ is a fender.
> View attachment 277100


I have to agree with this idea of a fender. My tourers are equipped with plastic full fenders. When it's wet on the ground and the air is cool, there's nothing worse than a wet back. However, I prefer the plastic fenders. Invariably, the fenders take a beating from door frames, porch railings etc. The metal bend permanently but the plastic flex and bounce back.


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## davidka (Dec 12, 2001)

lgh said:


> Be aware that the long board SKS fenders have very little ground clearance, like on the order of 2 inches per grant p. That might be an issue for storage or if you have to drag your steed up a steep hill or stairs.
> 
> Larry


Excellent point, they may also interfere with a fork mount roof rack. I've been using the Bontrager Interchange Ultimate Commuter fenders. They have a nice long flap that folds under and allows me to carry the bike in a fork mount. Great stuff.


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