Why deeper wheels at the rear?

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AndreLM
Posts: 351
Joined: Tue Feb 05, 2013 11:53 pm

by AndreLM

To the aero experts... If the rear wheel has a much smaller impact on overall drag, why should it be deeper than the front (such as in Enve wheelsets)?

Wouldn't it make more sense to use a shallower (but still aero) rear wheel to save a few grams with minimum impact on drag?

Something like F60/R45, or even a R35 (which I admit would look quite strange...)

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by Weenie


alcatraz
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by alcatraz

Hehe, I rode an 88mm front and a 58mm rear in my last race. Worked fine and I was pleased with my resultm

If I had an 88mm rear I'd probably have used it. (You don't really need more than 50-60 if you're drafting.)

I agree it doesn't make that much sense. The same goes for running the same tire width in front as in the rear. It's not ideal for aero.

Possibly even rim width would be better with narrower in front and wider in the rear. Imagine running a narrower and deeper wheel in front (given that crosswind conditions allow it) and a shallower but wider rim in the rear. Then you could run the same tire as the wider rim will impact the width of the tire giving you more support where your weight is, in the rear.

Lets say you want to have an aero bike that also climbs very well, it makes sense to save weight where it doesn't cost many watts. The rear costs less watts than in front.

Weight vs aero is a complicated issue. One simply has to pick a level somewhere inbetween. There is no way of maximizing both.

pocari123
Posts: 36
Joined: Mon Oct 16, 2017 5:14 am

by pocari123

Stability. When your front wheel catches the side winds it’ll twist your handlebars. A deeper front means your front end will be very twitchy in high wind situations. Some with a deeper front will say “my setup wasn’t twitchy at all” but it could be even more stable had they gone with a deeper rear.

That’s why they only use front disc wheels in the velodrome where there’s no wind. It’d be very dangerous using a front disc outdoors


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alcatraz
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by alcatraz

The question of crosswinds didn't come up.

The OP tries to make a point that for whatever depth wheels you deem to be optimum aero for a situation (crosswinds taken into account), then what do you really have to lose by downsizing the rear a step or two?

The price of reduced watts per gram of added weight might prove to be quite expensive if lets say a 38mm rear wheel is 200gr lighter (extreme hypothetical case) than a 58mm. Wouldn't then the saving of maybe 1 watt for the added weight of 200gr be a price too high?

On a tt bike people have to decide between using an 88mm or a disc wheel in the rear. That is a very hard choice if you are averaging relatively low speeds at 35km/h.

/a

pocari123
Posts: 36
Joined: Mon Oct 16, 2017 5:14 am

by pocari123

I think OP is just wondering why no one uses a deeper front when there is a weight advantage and minimal aero penalty as OP said. And to the best of my knowledge, the answer is crosswinds.
alcatraz wrote: The OP tries to make a point that for whatever depth wheels you deem to be optimum aero for a situation (crosswinds taken into account), then what do you really have to lose by downsizing the rear a step or two?

/a
Stability. I feel like if stability isn’t an issue because the speeds are too low, it’ll be faster saving some weight and going with a shallow front too. Feel free to correct me on this, I haven’t done the math


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AndreLM
Posts: 351
Joined: Tue Feb 05, 2013 11:53 pm

by AndreLM


alcatraz wrote:

The OP tries to make a point that for whatever depth wheels you deem to be optimum aero for a situation (crosswinds taken into account), then what do you really have to lose by downsizing the rear a step or two?

/a
Exactly my point. I understand 200g might be too much, but not sure what would be the aero penalty of downsizing the rear rim.

I would probably take 200g over 1W not only because of overall weight, but also because this is rotational weight and might play an important role in acceleration. Additionally, overall crosswind stability should increase as well (even if it is more affected by the front than the rear wheel).

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AndreLM
Posts: 351
Joined: Tue Feb 05, 2013 11:53 pm

by AndreLM


pocari123 wrote: I think OP is just wondering why no one uses a deeper front when there is a weight advantage and minimal aero penalty as OP said. And to the best of my knowledge, the answer is crosswinds.
@alcatraz got it right. Why not use the deepest you are comfortable with as your front wheel, and downsize the rear to save weight? Why should the rear be deeper than the front if it plays a less important role in overall drag?


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pocari123
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Joined: Mon Oct 16, 2017 5:14 am

by pocari123

I’m a bit out of my depth here but it has to do with something called center of pressure. Heard it from the flo guys on a podcast somewhere.

Basically a deeper front shifts the center of pressure from side winds to the front where your handlebars are which will make your bike twitchy.


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pocari123
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by pocari123

No idea if this is legit. But it’s one of the first hits from google and seems to explain this concept better than I

https://sites.google.com/a/mpstraining. ... gstability


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AndreLM
Posts: 351
Joined: Tue Feb 05, 2013 11:53 pm

by AndreLM

Thanks for the link. Reasonable analysis... but it seems it is focusing on deeper wheels (discs, 80+mm, etc) in TT/Tri scenarios.

Would that have the same effect when talking about 35-60mm range in more road racing application? Would stability be compromised by a deeper front wheel?

none
Posts: 129
Joined: Mon Dec 17, 2018 11:29 pm

by none

AFAIK, front wheel is what breaks the wind, by the time the airflow reach the rear wheel, there is already significant amount of turbulance generated from the front end of the bike. The deeper section rear wheel is likely to less batter up the turbulance, smooth out air turbulance instead of creating more as airflow pass the bike/rider.

akafinto
Posts: 5
Joined: Fri Nov 06, 2015 1:43 pm

by akafinto

Just to add to the crosswind discussion, there's also an argument that running a deeper rear wheel (especially a disc wheel) moves the centre of pressure further towards the rear of the bike. By shifting further from the free-moving steering axis at the front wheel, it also improves stability.

Would be interested to see if any research has been done on this, but anecdotally I've known people almost counter-intuitively run a rear disc to improve handling in windy conditions.

AJS914
Posts: 3339
Joined: Tue Jan 28, 2014 6:52 pm

by AJS914

We all know the real reason is looks. :D

A deeper rear wheel looks better. A deeper front and shallow rear just looks dorky.

Hexsense
Posts: 859
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

Optimizing problem needs a defined goal.

Is it most aero for given weight
or
most aero possible for the riding condition?

Your original question looks in the first way, how to be as aero as possible for a given low weight.
But a whole lot more people simply want most aero that is still ridable regardless of weight.
- People can ride very deep rear wheel with no problem even in very gusty condition. Deep rear is more aero than shallow rear. Regardless if next statement is true or not.
- From pocari123's link, Deeper rear wheel can even make whole bike more stable than shallower rear, thus it is possible to increase depth of front wheel further without having it too unstable. So deeper rear wheel allow more depth up front to be used too. Hence even more aero.

See ? , when you take weight out of the equation but simply want the most aero possible for a given wind condition, rear deeper than front make sense.

RussellS
Posts: 860
Joined: Wed Feb 03, 2010 1:31 am

by RussellS

AndreLM wrote:
Sun Jan 20, 2019 2:37 pm
@alcatraz got it right. Why not use the deepest you are comfortable with as your front wheel, and downsize the rear to save weight?
For the very simple reason that a deeper rear wheel is FASTER than a shallower rear wheel. I know some people worship light weight. But anyone with any brains at all, will happily sacrifice weight to get more speed. If my 100 pound bike is faster than your 10 pound bike, then I WIN. Weight, and aero too, is irrelevant. All that matters is speed. How you get faster is irrelevant. Do you really think pros care how much their bike weighs? They would happily ride grandmas 3 speed bike with a basket on front if it guarantees they are first across the line every day of the Tour, Giro, and Vuelta. They would laugh at you for your light weight BS if they won every day.

by Weenie


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