When I roll over wet pavement with my scooter it apparently flicks up onto my back. However, the angle between the wheel and the rear fender is -3 degrees. How is it possible for the water to be propelled by the wheel and go up and around the fender to reach my back coat? Not only does the water go up around the fender, but as I'm moving, how is it possible for the stagnant water to go faster than I am to reach my back?

scooter fender angle

muddy water spots on back of coat

My back does not extend past the rear wheel. I am not going up or down steep hills.

I fixed it by designing and 3D printing an extension and epoxying it to the end of the fender.


scooter fender extension

  • $\begingroup$ Would be neat to see a video of it. But one possibility is that some drops are striking the edge of the fender and getting pulled forward. $\endgroup$
    – BowlOfRed
    Mar 21, 2017 at 7:02

2 Answers 2


This is just a suggestion since I cannot do the experiment on your bicycle, but I would guess turbulence is pulling the spray of water forward onto your back.

Since humans aren't terribly aerodynamic when we pass through air at any significant speed we generate turbulence and especially vortices behind us. A quick Google found this example of the air flow over a cylinder:

enter image description here

(image from the Rensselaer web site)

If water is being sprayed upwards at an angle near the vertical, and 3° is pretty close to vertical, then the water may be entering the vortices and being pulled forward onto your back.

If so then the only solution is to use a mudguard that goes back farther, or of course to cycle more slowly.

  • $\begingroup$ Some nice wind tunnel pictures in this website cobbcycling.com/john-cobbs-advanced-positioning-system $\endgroup$
    – Farcher
    Mar 21, 2017 at 9:25
  • $\begingroup$ Oh! My back is creating a low pressure environment which sucks the water drops in! I never thought about that! Since I am standing on the scooter going about 18mph, it probably looks exactly like the cylinder air flow from the top down. $\endgroup$
    – Chloe
    Mar 22, 2017 at 1:58

I believe the correct explanation is given by the answer of Rob Jeffries here :

The center of your wheels are traveling at the same speed as you. However the top of the wheel must be moving forward faster than this in order for the wheel to be rotating.

Thus it is possible that some of the muddy bits which get dislodged from the wheel, ( when the centrifugal overcomes the adhesion force,) leave off on a tangent and some of those bits catch up with your back as they leave tangentially with a higher velocity than the velocity of your back. Turbulence as described in John's answer will enhance the effect.

The rough argument is that the most backward part of the wheel is traveling with the velocity of the bicycle and your back. As the wheel turns if a bit of dirt dislodges due to loss of adherence it will have a larger tangential velocity than your motion with the bicycle and may hit your back. No laws are broken The spots in the picture are from muddy stuff. I agree that the water at three degrees will not turn and hit your back, unless with the turbulence mechanism described by John. Water itself does not have a high adherence and takes off before the angle is such that the tangential velocity hits your back.Mud has higher adherence.

Since the spots appear even though there is a narrow mud guard the tiny spots must be those bits of mud scattering off the circular edge of the mud guard some of it ending on your back, after all the spots are not so many, and mud breaks up easily so a tiny bit could hit your back.

  • 1
    $\begingroup$ OK I can buy that the tangential wheel velocity is much greater than the horizontal speed, but the water drops still shoot up at -3 degrees, and without a force acting on them after they leave the wheel, will continue in that same direction. If you shoot a bullet at a corner, will it go around the corner just because it's going faster? $\endgroup$
    – Chloe
    Apr 4, 2017 at 20:40
  • $\begingroup$ I agree with Chloe on this. The mud stop is there to prevent that, there is no path to travel between the two that actually works. $\endgroup$
    – JMac
    Apr 4, 2017 at 23:17
  • $\begingroup$ Chloe I am not arguing against the three degree water you see . The spots in the picture are tiny spots of mud and do not come from the three degree tangential. Water yes, will come with the mechanism described by John. Mud adheres on the wheel and a bit of it dislodges because adherence is lost and if it is towards the middle it is at the right angle to hit your back. The rough argument is that the most backward part of the wheel is traveling with the velocity of the bicycle and your back. As the wheel turns if a bit of dirt dislodges due to loss of adherence it will have a $\endgroup$
    – anna v
    Apr 5, 2017 at 3:29

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