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ffiaux 07-25-11 08:54 AM

Mixing deep section rims
 
Hello,

I was wondering about mixing deep rims for some specific racing events, like crits, even with crosswinds and a lot of sprinting. Some crits around here are in very short circuit, but the master-amateur championship is a 25km circuit on road.

I mean having a higher rim on the rear wheel. For example, I've got a pair of carbon reynolds with 32mm depth. What about getting another reynolds rear with 46mm or even 66mm?

(Let's not discuss about brands, ok, reynolds is an example)

Is there any (real) gain? Or maybe I would even feel it?

Sometimes we see in competitions like TDF some guys doing that. When it's a good idea to do it? When it's short, or long, or few crosswinds, forget about it when climbing... what's your opinion and experience?

Thanks.

datlas 07-25-11 09:06 AM

Yes it's a good idea to have something shallower in the front and deeper in the back.

This assumes you are racing, especially in a time trial.

If you are a recreational rider it probably is not going to make a big difference.

3kmi 07-25-11 09:06 AM

I've been running a Zipp Firecrest 606 (404 front 808 rear) this season, and at 150ish lbs, I haven't had much issue with crosswinds. I'll tell you, though, they are stupid fast. I've taken them on windy rides specifically to see how they perform in crosswinds, and I definitely felt the wind, but it was totally manageable. I've been very happy with the combo.

datlas 07-25-11 09:15 AM


Originally Posted by 3kmi (Post 12982720)
I've been running a Zipp Firecrest 606 (404 front 808 rear) this season, and at 150ish lbs, I haven't had much issue with crosswinds. I'll tell you, though, they are stupid fast. I've taken them on windy rides specifically to see how they perform in crosswinds, and I definitely felt the wind, but it was totally manageable. I've been very happy with the combo.

Please define "stuipid fast."

I suspect anything more than say 0.1 or 0.2 MPH faster is placebo effect.

3kmi 07-25-11 09:25 AM

While descending, I pass people on "regular" wheels who are pedaling hard just by tucking. They also hold speed on flats really well, pacelines at 24mph are easy to maintain.

laserfj 07-25-11 09:29 AM


Originally Posted by datlas (Post 12982716)
Yes it's a good idea to have something shallower in the front and deeper in the back.

Why?

ffiaux 07-25-11 09:34 AM


Originally Posted by laserfj (Post 12982858)
Why?

I've read that there's a lot of drag resistance on the rear wheel, and the junction between seat tube, chainstay (the triangle)

ffiaux 07-25-11 09:35 AM


Originally Posted by datlas (Post 12982716)
Yes it's a good idea to have something shallower in the front and deeper in the back..


How shallower and how deep?

laserfj 07-25-11 10:00 AM


Originally Posted by ffiaux (Post 12982888)
I've read that there's a lot of drag resistance on the rear wheel, and the junction between seat tube, chainstay (the triangle)

I'm not saying that's wrong, just wondering why. Seems to me like the front wheel would be more important, since it is fully exposed to the wind.

The back wheel is already in the slipstream of your front wheel and your bike, so it's basically "drafting." I would think that would cause it to have less drag from air resistance.

Anyone know why people always put deeper in the back? I'm sure there is a reason.

carpediemracing 07-25-11 10:14 AM

Front wheel contributes to instability. It decreases wind resistance.

Rear wheel contributes to stability. It decreases wind resistance by about 1/3 of what the front wheel does.

Select front wheel based on stability thoughts, like if you're going over 50 or 60 mph on a descent, if it's really gusty (30 mph gusts for example), etc. My two choices are non-aero (Ardennes rim) and aero (60 mm). The 60 is good until about 30-35 mph gusts in a crit, about 50 mph on a somewhat windy/gusty descent. The box is totally stable at 50-55 mph, which is about as fast as I've gone on them.

Rear wheel you select based on availability (i.e. do you own it) and weight. A taller wheel always makes your bike more stable. I used a disk wheel for a while, and I weighed 130-145 lbs. I have three heights rear wheel, box (Ardennes), 60mm, and 90mm. The 90 is a clincher and too heavy (at least 300g heavier without including the heavier tire/tube/strip combo) so I only use it for training. The 60 is my default race wheel so I use it a lot. I use the box section in rainy races as I've been too lazy to swap pads for wet weather for my 60.

asgelle 07-25-11 10:17 AM


Originally Posted by laserfj (Post 12983043)
Anyone know why people always put deeper in the back? I'm sure there is a reason.

Stability. 1) not being movable, the rear wheel affects bike handling less than a front in crosswinds so however much instability a rider can handle with a certain depth front rim, they will feel the same effect with a deeper rear. 2) By increasing area at the rear of the bike, a deep rear rim moves the center of pressure back and makes the same depth front wheel more stable than with a shallower rear.

Data from Zipp shows the reduction in drag from a deeper rear rim is about 75% of what is seen making the same change in front.

laserfj 07-25-11 10:21 AM


Originally Posted by carpediemracing (Post 12983145)
Front wheel contributes to instability. It decreases wind resistance.


Originally Posted by asgelle (Post 12983158)
Stability. ....Data from Zipp shows the reduction in drag from a deeper rear rim is about 75% of what is seen making the same change in front.


Cool--I knew there must be a reason. That makes sense.

AdelaaR 07-25-11 10:29 AM


Originally Posted by ffiaux (Post 12982894)
How shallower and how deep?

As always ... it depends.
For doing perfectly flat courses ... a disc wheel or zipp 1080 in the rear is the best option.
Can't go wrong with zipp 1080 or 808 in the front either depending on crosswinds.

But: these things are heavier than shallow rimmed wheels.
If you need to do a lot of hills the difference will be made there.

The pros let their specific wheels depend on the course they will ride and other factors.
There is no simple "what is the fastest wheel" answer.

ffiaux 07-25-11 11:38 AM


Originally Posted by AdelaaR (Post 12983244)
As always ... it depends.
For doing perfectly flat courses ... a disc wheel or zipp 1080 in the rear is the best option.
Can't go wrong with zipp 1080 or 808 in the front either depending on crosswinds.

But: these things are heavier than shallow rimmed wheels.

Some of you guys are mentioning disc wheel. But I'm into road racing, so I discard it. Although it is good to learn about it. Thanks.


Originally Posted by AdelaaR (Post 12983244)
If you need to do a lot of hills the difference will be made there.

Yes, when going up a lot of hills i'll keep my both 32mm, i really like them.


Originally Posted by AdelaaR (Post 12983244)
The pros let their specific wheels depend on the course they will ride and other factors.

That's the point, I'm not a pro but I'm trying to learn when to use the deeper rear wheel, and also finding myself a reason to buy one :lol:.


Originally Posted by AdelaaR (Post 12983244)
There is no simple "what is the fastest wheel" answer.

Yes sure, I agree. What I'm trying to get here is the benefits of a deeper rear wheel. What about accelerating, is it better or not? When sprinting, how does it feel?

carpediemracing 07-25-11 01:14 PM

With a rear wheel you gain some stability. It's a stabilizing effect. I've raced in quite windy conditions with a rear disk, crits at a beachside ex-airfield, so dead flat and lots of wind.

Also, although rear wheels only help a little in head on or slightly head on winds, they really help in cross tailwinds. A cross-tailwind will really boost your speed if you have taller rims, like a disk rear or a 90mm or 100mm or TriSpoke/HED3. I've held 60 mph for a mile with a massive tailwind (Hurricane George in Florida, just before the "real" storm hit), limited only by my gearing. If I know a race is ending on a straight with a cross tailwind I'll use the biggest rear wheel I have, taking weight into account. I used to go to races with a slew of wheels in the car - Zipp 440s (pre-404s), Zipp 340s (pre-303s), sometimes a disk, box front 28H, TriSpokes (now HED3), all tubulars.

In general I try and use the tallest rear wheel I have. I actually want to get a Stinger9 (90mm) for the rear, using it mainly with a Stinger6 front, but not sure if that can happen. I thought I also wanted a Stinger4 front (40mm) for gusty races but after thinking it over I can use the Stinger6 in virtually all wind conditions I see in CT area.


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