Why deeper wheels at the rear?

Everything about building wheels, glueing tubs, etc.
AndreLM
Posts: 354
Joined: Tue Feb 05, 2013 11:53 pm

by AndreLM

RussellS wrote: For the very simple reason that a deeper rear wheel is FASTER than a shallower rear wheel.
Chill down... I think you misread my post, so take a breath and do it again.
We all know a deeper wheel is faster.
I know some people worship light weight.
:noidea:
So... you are in weightweenies for what?
But anyone with any brains at all, will happily sacrifice weight to get more speed.
And anyone with any brains at all, will take a lighter part if it doesn`t sacrifice aero (hint: read my first post again)
If my 100 pound bike is faster than your 10 pound bike, then I WIN.
Wrong. If YOU are faster than me, you win. If I am faster than you, I win. The bike would probably not change that
Weight, and aero too, is irrelevant. All that matters is speed. How you get faster is irrelevant.
:noidea:
So... you are in weightweenies for what?
Do you really think pros care how much their bike weighs?
No, and I don't care what they care. Do you?
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.
Nothing guarantees you are the first across the line. Their best shot is to train harder, and use the best equipment they can.
They would laugh at you for your light weight BS if they won every day.
Any of them would probably beat me (and you) regardless of bike... or not

Seriously... you are in weightweenies for what?

alcatraz
Posts: 2208
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Lets say you were faced with the impossible mission to build a bike that would produce the best combined time on two courses.

1. 100km climbing 3500m, grades up to 30%

2. 200km flat.

The winner would be whoever has the lowest time of course 1 plus course 2. See it as a separate time trial event, no drafting allowed. No swapping of bike equipment would be allowed. Very little crosswinds.

Would you pick an 80mm front and a 50mm rear?

Would you really go 50mm front for 200km flat?

Would you carry two 80mm wheels up 3500m?

(Yes it's a ridiculous hypothetical case, but please humor me. You want to win this challenge!)

by Weenie


IchDien
Posts: 526
Joined: Mon Nov 05, 2012 12:23 am
Location: Veneto

by IchDien

is that 100km of slow 3500m ascent or 50km of 3500m ascent with 50km of descent? If the latter still 80mm yeah...
Last edited by IchDien on Mon Jan 21, 2019 8:42 am, edited 1 time in total.

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

by pocari123

Are you two looking for confirmation of your theory or an actual discussion?

Side winds. Twitchy bike.

I don’t care if you can make it work. Kudos to you if you can. But this is one of the biggest reasons why this setup is never used.

The other being it looks weird as hell.


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AndreLM
Posts: 354
Joined: Tue Feb 05, 2013 11:53 pm

by AndreLM

Not trying to confirm anything... just wanted to check with those that knows aerodynamics more than me if it would make sense or not. I don`t have mismatched wheels, and not planning to buy any (not even a deeper rear wheel).

You brought a very good point that a deeper rear wheel might make the bike MORE stable by moving the center of pressure out of the front, and I fully agree that a shallower rear wheel would look weird.

The idea was to have some discussions with those that care about aero AND weight, if that "aero penalty" was worth paying or not. Even taking looks out of the equation, it seems the answer is no.

alcatraz
Posts: 2208
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

IchDien wrote:
Mon Jan 21, 2019 8:42 am
is that 100km of slow 3500m ascent or 50km of 3500m ascent with 50km of descent? If the latter still 80mm yeah...
Mountain top finish :D

IchDien
Posts: 526
Joined: Mon Nov 05, 2012 12:23 am
Location: Veneto

by IchDien

OK. 3500m is quite hard to achieve in 100km; that'd be an average gradient of 3.5% so aero would still trump weight over the course.

Geoff
Posts: 5259
Joined: Sat Dec 13, 2003 2:25 am
Location: Canada

by Geoff

Also, the deeper rims tend to be stiffer. Assuming that the wheel has enough spokes to handle the rim, then the deeper rim could also result in a stiffer wheel.

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Lelandjt
Posts: 525
Joined: Tue Jan 19, 2016 7:10 am

by Lelandjt

Where I live it's too windy for a 50mm front wheel. I've tried it and turned back from rides due to not having fun and feeling sketchy, so my last bike had 38mm rims. For my new bike I was going to do 38 at each end but saw that a 50mm rear would only add 50-60g. It's more aero and stiffer so I decided to take that weight penalty. So I'm on 20 spokes and 38mm up front (light, fairly aero) and 24 spokes and 50mm in the rear (lightish, more aero, stiff). The wheelset weight is like 1350g so pretty light for a cheap aero wheelset. I like this balance. The more spokes you have, the more aero improvement you should see by replacing a section or those spokes with a carbon fairing.

So my answer for why I run a deeper rear rim? Stiffer and more aero, even if those are only slight increases, at only a small weight penalty and no noticable impact on handling in wind.

alcatraz
Posts: 2208
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

What wheel combo would result in the fastest time for a brutal mountain top finish, and a 200km flat ride, combined? All else being equal (i know impossible)

Would you bet on a smaller front and larger rear?

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

by pocari123

Well assuming wind isn’t a factor, just run front and rear discs lol

lightweight autobahns at the rear and one of their track specific disc fronts. They’re both around 800g so you won’t give up much in terms of weight and you’ll be flying on the flats.

If wind is a factor, then a deeper rear would be my choice because of handling issues (as mentioned in the numerous posts beforehand)


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AJS914
Posts: 3479
Joined: Tue Jan 28, 2014 6:52 pm

by AJS914

alcatraz wrote:
Tue Jan 22, 2019 12:15 pm
What wheel combo would result in the fastest time for a brutal mountain top finish, and a 200km flat ride, combined? All else being equal (i know impossible)

Would you bet on a smaller front and larger rear?
Why don't you go put this in a calculator and figure it out and then come and tell us the result? My guess is that the 80mm wheels over the 200km would net a lot more time savings then the time they would lose climbing 3000 feet because of their extra weight.

If we were talking UCI rules, then it would be possible to build a 6.8kg bike with 80mm wheels.

hambini
Posts: 493
Joined: Tue Nov 15, 2016 8:13 am
Location: Cologne, Germany

by hambini

The main reason is for stability as others have said,

Most of the rider weight is on the back wheel and has no flexible linkage so you can treat it as a rigid body.

The front wheel is on a flexible linkage and will be affected by airflow over it. You will generally get low pressure on one side and high pressure on the other side. The difference causes a turning moment that tries to turn the wheel. Generally you resist this movement with your arms and is quite controllable.

The issue is usually when the air speed is fluctuating and the turning moment fluctuates with it, you will find yourself over correcting and this causes a perception of instability.

For ultimate aerodynamics, you would need a front and rear disc. Air has a tendency to develop vortices because of internal friction, when there is an area of "nothingness" like the centre of the wheel, it is uncontrolled and generates a lot of drag. If you have a disc, there is a physical boundary and air cannot penetrate that. It has to wait to get to the end of the wheel/tyre before the ensuing vortex shedding starts.

Hope that helps

Hambini
Hambini Aeronautical Engineer, Polluting YouTube since 2016 - views expressed are my own...
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AndreLM
Posts: 354
Joined: Tue Feb 05, 2013 11:53 pm

by AndreLM

Thanks @pocari123 & @hambini for the valuable insights

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by Weenie


Wookski
Posts: 1099
Joined: Tue Feb 23, 2016 5:51 am

by Wookski

Get an autobahn disc rear. 780g. Game over.
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