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What effect do fenders have on speed?

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What effect do fenders have on speed?

Old 12-20-15, 02:12 PM
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woodcraft
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What effect do fenders have on speed?

- air drag between fender & tire reduces speed?

- fairing effect over tire increases speed?

- no effect?

- other?
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Old 12-20-15, 02:34 PM
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A o.3 mph reduction in average speed for each 1 mph average increase.
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Old 12-20-15, 02:36 PM
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I think with well fitting fenders it's a wash. Turbulent air coming off the back of the tires is smoothed but the slight increase in width increases drag slightly.

Crosswinds might be a different story since the yaw angle yields a greater increase in surface area. I have noticed that poorly fitted fenders can cause/contribute to shimmy on an otherwise normal bike.

I've personally noticed no significant effect during solo riding or even when I venture out in groups.
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Old 12-20-15, 02:43 PM
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They slow you down, but not by an awful lot. Like all aero effects the significance increases with speed, but when you weigh the drag involved against overall air drag, it's obvious that it's a minor, incremental difference.
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Old 12-20-15, 04:11 PM
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I ride fast in fenders, but Crud RoadRacer MkIIs, which I'd hazard to guess are the most aerodynamic fender out there. Still, I assume there is some cost, and if I was concerned with maximizing my effort, I'd not run fenders.

So, I'd say the cost-- which they all have, if only due to frontal area-- depends on the fender.
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Old 12-20-15, 04:17 PM
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It's not even winter yet !
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Old 12-20-15, 04:45 PM
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I honestly don't notice a difference in my solo riding when I break out the Jake the Snake. I seem to roll just as fast, or slow, as I would on one of my road bikes. That means holding a 17+ mph on a bad day and in the 19s on a good day. Even while using the current 700 x 30 tires. Maybe if I was faster I would see a difference.

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Old 12-20-15, 04:47 PM
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Originally Posted by Homebrew01 View Post
It's not even winter yet !
Speak for yourself! I did 20 miles today with the snow flying sideways into my face, LOL!
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Old 12-20-15, 04:52 PM
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I doubt it slows us down enough for anyone to notice a difference . I ride with fenders almost all of the time .
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Old 12-20-15, 05:04 PM
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Originally Posted by Northwestrider View Post
I doubt it slows us down enough for anyone to notice a difference . I ride with fenders almost all of the time .
Yeah, my fendered bikes are always fendered, so I don't have any direct comparisons, but, my fenderless bike holds all my PRs. That's not proof of fender penalty, but it does put the question into play.
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Old 12-20-15, 05:31 PM
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Originally Posted by Northwestrider View Post
I doubt it slows us down enough for anyone to notice a difference .
Hmmm. "Anyone?" I bet there are riders who can.
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Old 12-20-15, 05:36 PM
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Originally Posted by RChung View Post
Hmmm. "Anyone?" I bet there are riders who can.
You think if a blind test were possible, some riders could tell they were going a tiny bit slower than they otherwise would, if riding a bike with fenders ?
I really doubt it.
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Old 12-20-15, 05:37 PM
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Originally Posted by RChung View Post
Hmmm. "Anyone?" I bet there are riders who can.
Would that be an "undemanding use" of a power meter, or would it fall in the advanced user camp?
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Old 12-20-15, 05:39 PM
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However much they slow us down, it's not enough to be worth soaking wet shoes & ass.
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Old 12-20-15, 05:41 PM
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Originally Posted by Homebrew01 View Post
You think if a blind test were possible, some riders could tell they were going a tiny bit slower than they otherwise would, if riding a bike with fenders ?
I really doubt it.
I presumed he was referring to power meter testing, and yes, I think the numbers would show a difference, but as I said, obviously some setups would be worse than others; were the tires to fill out to the edge of the fender width, and were there little gap between tire and fender, would be an ideal setup, I think.
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Old 12-20-15, 05:41 PM
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air drag between fender & tire reduces speed? - possible but my instinct says no

fairing effect over tire increases speed? - the only leading edge is in the dirty air behind the seat tube. There might be some help at the top if it channels air, as opposed to twice the bike speed relative to the tire surface.

no effect? - I think there is net drag

other? - There is more increase in cross sectional area to the wind at the fender sides which makes me think there is more increase in drag with any cross-wind.
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Old 12-20-15, 05:42 PM
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Originally Posted by Bob Ross View Post
However much they slow us down, it's not enough to be worth soaking wet shoes & ass.
Hear, hear!
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Old 12-20-15, 06:30 PM
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Originally Posted by chaadster View Post
Yeah, my fendered bikes are always fendered, so I don't have any direct comparisons, but, my fenderless bike holds all my PRs. That's not proof of fender penalty, but it does put the question into play.
Not for me, my fendered bikes are a CX bike and a MB. I don't expect them to set any speed records. I just find it comforting that the CX bike can still keep me within my annual average pace.
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Old 12-20-15, 06:32 PM
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Someone has to mention Jan Heine! He did a wind tunnel test on a rando bike with and without fenders and found no difference. By memory they were 38mm fenders on 32mm tires. The article speculated that the increased frontal area was canceled by the smoother air flow. These were well fitted, permanently mounted, full coverage aluminum fenders. Poor fitting, clip on, open on the sides fenders may be totally different.
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Old 12-20-15, 06:42 PM
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And what about all the extra weight too !!
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Old 12-20-15, 06:59 PM
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Originally Posted by Bob Ross View Post
However much they slow us down, it's not enough to be worth soaking wet shoes & ass.
This.
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Old 12-20-15, 07:32 PM
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Originally Posted by jamesdak View Post
Not for me, my fendered bikes are a CX bike and a MB. I don't expect them to set any speed records. I just find it comforting that the CX bike can still keep me within my annual average pace.
I don't understand. Are you saying your fendered bikes hold PRs on rides you also do on a fenderless bike?

Otherwise, I think it's essentially the same thing, namely, that you don't have any basis for direct comparison between the same bike, fendered and fenderless, right?
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Old 12-20-15, 07:43 PM
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Originally Posted by catgita View Post
Someone has to mention Jan Heine! He did a wind tunnel test on a rando bike with and without fenders and found no difference. By memory they were 38mm fenders on 32mm tires. The article speculated that the increased frontal area was canceled by the smoother air flow. These were well fitted, permanently mounted, full coverage aluminum fenders. Poor fitting, clip on, open on the sides fenders may be totally different.
BQ posted the following summary of the tests on their blog:

"Well-mounted metal fenders do not affect the bike’s aerodynamics. The front section of each fender shields the tire and reduces the wind resistance, while the rear fender increases the aerodynamic drag. The two effects cancel each other."

While I understand the words, there is so much information left out of the summary that it really sounds like, "it depends" is what they're saying. But, I've never seen the test methodology, results, or findings, so the degree to which "it depends" is unknown to me, but I don't see how effects can cancel each other out all the time, in all setups, in all conditions, so it's a suspicious claim.
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Old 12-20-15, 07:43 PM
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Originally Posted by woodcraft View Post
-- other?
On wet streets I go as hard as conditions allow on my winter/wet bike w/o a thought of the sluicing of toxic road spooge the mudguards keep off.
In a paceline on wet roads fender equipped riders can follow safely/comfortably closer than fenderless to share pace more efficiently.

My winter/wet bike is faster in those conditions than my light fender less road bike(s).

-Bandera
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Old 12-20-15, 07:46 PM
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Originally Posted by chaadster View Post
I don't understand. Are you saying your fendered bikes hold PRs on rides you also do on a fenderless bike?

Otherwise, I think it's essentially the same thing, namely, that you don't have any basis for direct comparison between the same bike, fendered and fenderless, right?
No, no.... What I'm trying to say is that my bikes with fenders are different than the "race" bikes I ride for the PRs I track. Fastest century, fastest descent top speed, fastest 10 mile, fastest 20 mile, fastest time up any given MT, etc. etc. Most PRs, except for the climbing ones, are held my my beat up old 2000 Lemond Zurich.

My point is that it's not fair to compare my CX and MB to any of my road bikes since the road bikes are built for speed and not fenders while the CX and MB have a different purpose. If I could get some will fitting fenders on the Zurich I would not expect to see any measurable effect on the bike.
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