Why deeper wheels at the rear?

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bilwit
Posts: 1185
Joined: Sun Apr 03, 2016 5:49 am
Location: Seattle, WA

by bilwit

It's pretty simple. A deeper rear is faster than a shallower rear. Period.

Yes, the front wheel affects aerodynamics much more: a deep front with shallow rear is faster than the other way around, but deep/deep is still faster. The weight offset is largely irrelevant unless you're doing something like an uphill time trial. We love to obsess about weight here and the feel of a light bike is bliss, but there is no denying at this point that in nearly every scenario--practically--aero trumps weight. "A few grams" doesn't help anything if 90% of the time you're on flat or shallow, rolling terrain.

by Weenie


TheKaiser
Posts: 636
Joined: Thu Sep 05, 2013 3:29 pm

by TheKaiser

hambini wrote: ↑
Tue Jan 22, 2019 5:36 pm
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
I was hoping for a Hambini reply! Maybe I missed it, but I didn't see if you addressed the suggestion that some others have made that a deeper rear wheel can actually increase stability, vs. a matched set. For example, 40f/60r would be better in crosswinds than 40f/40r. I had not heard that suggestion until this thread, and am undecided as to what I think of it.

Similar to what Hexsense mentioned, there are 2 (or more) ways to look at this problem, and those perspectives will lead to different answers. I have always figured that the deeper rear/shallower front setup came about from people who, for example, wanted to run 60mm f/r, but were worried about crosswinds. Dropping down to 40mm on the front was their concession to handling, and they were kind of blissfully (willfully?) ignorant of the fact that they weren't truly splitting the difference, but were in fact giving up most of the aero gainz of the 60mm. If deeper on the back truely gives better crosswind handling, then it wouldn't be as much of a farce as I'd thought. If it doesn't give better handling, then the picture becomes less clear.

TheKaiser
Posts: 636
Joined: Thu Sep 05, 2013 3:29 pm

by TheKaiser

Wookski wrote: ↑
Thu Jan 24, 2019 10:26 pm
Get an autobahn disc rear. 780g. Game over.
Image
Those things are sweet except they still seem to be sticking with their 20mm external width. They suggest 19-27mm tires, but if the 105% rule applies to discs (anyone with disc wind tunnel knowledge please chime in), you would need to be running 19mm to really make use of those things, which in the real world no one is doing these days. That is putting aside the RR vs. Aero calculation. If deeper on the back actually improves handling, as some have suggested, and if the 105% rule does apply to discs, then all LW needs to do is add 6-10mm to the width of the Autobahn for it to be a great daily driver!

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

by pocari123

TheKaiser wrote:
Wookski wrote: ↑
Thu Jan 24, 2019 10:26 pm
Get an autobahn disc rear. 780g. Game over.
Image
Those things are sweet except they still seem to be sticking with their 20mm external width. They suggest 19-27mm tires, but if the 105% rule applies to discs (anyone with disc wind tunnel knowledge please chime in), you would need to be running 19mm to really make use of those things, which in the real world no one is doing these days. That is putting aside the RR vs. Aero calculation. If deeper on the back actually improves handling, as some have suggested, and if the 105% rule does apply to discs, then all LW needs to do is add 6-10mm to the width of the Autobahn for it to be a great daily driver!

They wouldn't be getting those weight figures if they added 6-10mm. Yes it's not as aero as other discs but it's still more aero than a 50 deep rear. It is the faster alternative to OP's suggestion of 80F/50R while still keeping the weight similar


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spdntrxi
Posts: 2880
Joined: Sat Jul 20, 2013 6:11 pm

by spdntrxi

^ tub only... otherwise I would have got one. Waited a long time to sell and replace my Super9.... finally the Roval321 is the ticket.. 1003g of clincher awesomeness complete with the woosh woosh sound

TheRich
Posts: 350
Joined: Tue Jan 01, 2019 1:36 am

by TheRich

TheKaiser wrote: ↑
Mon Jan 28, 2019 1:08 am
I was hoping for a Hambini reply! Maybe I missed it, but I didn't see if you addressed the suggestion that some others have made that a deeper rear wheel can actually increase stability, vs. a matched set. For example, 40f/60r would be better in crosswinds than 40f/40r. I had not heard that suggestion until this thread, and am undecided as to what I think of it.

Similar to what Hexsense mentioned, there are 2 (or more) ways to look at this problem, and those perspectives will lead to different answers. I have always figured that the deeper rear/shallower front setup came about from people who, for example, wanted to run 60mm f/r, but were worried about crosswinds. Dropping down to 40mm on the front was their concession to handling, and they were kind of blissfully (willfully?) ignorant of the fact that they weren't truly splitting the difference, but were in fact giving up most of the aero gainz of the 60mm. If deeper on the back truely gives better crosswind handling, then it wouldn't be as much of a farce as I'd thought. If it doesn't give better handling, then the picture becomes less clear.
A disk rear just wouldn't impact handling as much, it would still be a sail, but it's low position doesn't give it much leverage so it's easier to lean into it. Even on a road bike, a gust catching your front wheel will throw you off balance and take your attention away from putting down the hurt, which makes you slower.

alcatraz
Posts: 2103
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

The graph of watts gained vs grams lost is a quite lot different on the front than the rear.

We focus so much on looks.

Lets say you want the cheapest aero upgrade without adding
major weight, just put a deeper front. It can even be narrower and give you a two-for. It would be the quickest aero gain you could do before a race.

If the speeds drop, road surface deteriorates, crosswind increases, then just stick with matched wider rims. :D

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