Why carbon rims?

[Likes2ridefar]

Those surges are what makes the difference in the strongest riders and can often decide who wins the race…it can be trained doing interval exercises like under/overs (FTP) but they aren’t fun.

[callmeishmael]

Originally Posted by Likes2ridefar

Those surges are what makes the difference in the strongest riders and can often decide who wins the race…it can be trained doing interval exercises like under/overs (FTP) but they aren’t fun.

I know and I do... but what the good Lord did not provide, I cannot create

[MikeD]

Originally Posted by Mark McM

The quality of the spokes and the wheel build are certainly important, but in terms of wheel design the rim is often the most important parameter. Spokes fail from fatigue, and fatigue is the result of cyclic loading. A deeper, stiffer rim better distributes loads across the spokes, reducing peak loading, and reducing spoke failures. Likewise, wheels go out of true when a component (usually the rim) is deformed/damaged - since deeper rims are less likely to dent, wheels with deeper rims are less likely to go out of true.

I kinda doubt that. Jobst Brandt ran Mavic MA-2 rims, which have a shallow height and thus were not that stiff radially because of that rim profile. He claimed to have 200,000 miles on the same set of spokes without breakage.

Also, this is what he said about denting:

> What I want is a durable wheel. My CXP-30s have 36 spokes, not the
> fewer number or non-standard spokes seen these days. I am building
> up a second bike, and have the option of getting some CXP33s (32
> spoke). My sole interest in aero rims is for overall wheel
> durability. My question, rephrased: to what extent do aero rims add
> to the durability of a wheel?

None. If the tire bottoms on a road hazard it will generally dent the
bead of the rim, which is no stronger on a deep sectioned rim than on
others. Sidewall wear is also the same, sidewalls being much the same
across the field of rims, although machined rims may be slightly
thinner locally. As far as load carrying, the old socket and eyelet
Mavic MA-2 was, in my estimation, the best all around rim. I don't
have loose spokes and the rims remain true through many 1000 miles,
usually failing from worn out sidewalls, but also from dings in the
bead. I have a collection of these in my garage.

[November Dave]

Originally Posted by callmeishmael

To my mind, it's those surges at already fairly high speed that create splits in most fast rides, be they races or not, and that's also where aero gains most come into play.

This right here. When first getting into wheels, the thing I noticed was more time spent at 0 watts on a consistently hard group ride (the Rock Creek 10am) when using aero wheels. The few pedal strokes you don't have to take reattaching out of a corner, the bit of extra glide you get moving up the side of the pack.. these are subtle but they're there. Easily identified in a power file, and those brief all-out efforts are physiologically very costly. These benefits are available with well shaped, modern, wider, not-all-that-deep alloy rims of the type that if I still had a rim brake bike I'd probably use.

Crosswind stability is definitely not linear with rim depth. You'll get more stability out of a well-designed mid-depth wheel with a tire that closely matches the rim's outer width than you will with a high spoke count narrow box section wheel with a sharp transition from tire to rim. My 46mm deep x 28mm wide wheels with 28mm tires (which measure 29ish mm wide) are a dream in crosswinds.

As Mark McM points out, spokes don't get stressed from tension, they get stressed by going slack. The extra radial stability of deeper rims, combined with the decreased spoke tension drop from mounting tires, combined with improved bracing angles, all add to the structural stability (lack of truing needs/broken spokes) of a well built wheel.

[Turkle]

Originally Posted by callmeishmael

I haven't raced on the road (aside from a club 10) in a decade, but our Tuesday night chaingangs are pretty spirited, and can pull in a national standard junior, a couple of ex pros, and a current semi-pro female.

The route is lumpy, but when I get dropped - and I always do! - it's not usually on one of the two main risers: it's when people kick hard out of the corners for the nth time, establish a gap, and I just don't have the legs for another acceleration and can't get back on. Yes, positioning plays a part, but it's continually going from 35-45kph that kills me.

I also notice that if the group is pulling hard down the long, false flat downhill section, if you're careless/lazy/tired when you pull off the front and ease off too much, you get spat out the back pretty quickly, and at those speeds your odds of getting back on again (at least with my watts - ymmv) are zero, at least not without absolutely burying yourself.

To my mind, it's those surges at already fairly high speed that create splits in most fast rides, be they races or not, and that's also where aero gains most come into play.

I endorse all of this!!

[November Dave]

Originally Posted by MikeD

I kinda doubt that. Jobst Brandt ran Mavic MA-2 rims, which have a shallow height and thus were not that stiff radially because of that rim profile. He claimed to have 200,000 miles on the same set of spokes without breakage.

His claim was that the spokes did that mileage, not any wheel (which makes sense because brake tracks would wear out). No telling how many times they were trued, rebuilt, and generally futzed with.

He also rode 6 and 7 speed drivetrains, which had much less rear wheel dish. You could build wheels for that dish at 100kgf driveside and have more NDS tension than a 10/11 speed wheel with driveside laced to 120kgf. Makes a huge difference. He was also big on using aggressively butted spokes, which are more durable than straight spokes.

[MikeD]

Originally Posted by November Dave

His claim was that the spokes did that mileage, not any wheel (which makes sense because brake tracks would wear out). No telling how many times they were trued, rebuilt, and generally futzed with.

He also rode 6 and 7 speed drivetrains, which had much less rear wheel dish. You could build wheels for that dish at 100kgf driveside and have more NDS tension than a 10/11 speed wheel with driveside laced to 120kgf. Makes a huge difference. He was also big on using aggressively butted spokes, which are more durable than straight spokes.

I edited my post to provide further clarification. He claims to not have fussed with the wheels much (i.e., truing). I assume he also had 36 spokes as well. I hear you on the wheel dish, but didn't offset spoke bed rims correct most of this? He also didn't use thread locker on the spoke threads but used a lot of spoke tension (probably too much), which kept the nipples from unscrewing. But, he rode his bike off-road a lot too.

[Mark McM]

Originally Posted by MikeD

I kinda doubt that. Jobst Brandt ran Mavic MA-2 rims, which have a shallow height and thus were not that stiff radially because of that rim profile. He claimed to have 200,000 miles on the same set of spokes without breakage.

Also, this is what he said about denting:

> What I want is a durable wheel. My CXP-30s have 36 spokes, not the
> fewer number or non-standard spokes seen these days. I am building
> up a second bike, and have the option of getting some CXP33s (32
> spoke). My sole interest in aero rims is for overall wheel
> durability. My question, rephrased: to what extent do aero rims add
> to the durability of a wheel?

None. If the tire bottoms on a road hazard it will generally dent the
bead of the rim, which is no stronger on a deep sectioned rim than on
others. Sidewall wear is also the same, sidewalls being much the same
across the field of rims, although machined rims may be slightly
thinner locally. As far as load carrying, the old socket and eyelet
Mavic MA-2 was, in my estimation, the best all around rim. I don't
have loose spokes and the rims remain true through many 1000 miles,
usually failing from worn out sidewalls, but also from dings in the
bead. I have a collection of these in my garage.

I don't think Jobst had much experience with using deep rims. His finite element analysis shows that the majority of the weight bearing wheel load is distributed to just a few spokes at the bottom of the wheel, and this is due to the radial bending of the rim. Had he run the same analysis with a deeper, stiffer rim (as others have since done), he would have found that the deeper rims bent inward less under load, thereby distribute the load over a wider area, and thus reduced peak spoke loading (and increasing fatigue life). In Brandt's day, spoke breakages were common, even when there were 32 or 36 spokes. Today spoke breakages are much less common, even with 20 and 24 spoke wheels. Sure, spokes are better today, but there can be no doubt that today's deeper rims have also contributed to spoke longevity.

As far as rim denting, Brandt's comments are mostly about denting the bead. But wheels go out of true when the overall cross section of the rim is dented. I've done many miles on multiple sets of MA-2 rims, and I also have many miles on deep sections rims, including a pair of Velocity DeepV rims that I built up contemporarily with my MA-2 wheels. I usually retired an MA-2 rear rim when it was so dented that it could no longer be trued adequately, and all my MA-2 wheels have long been retired now. But the set of DeepV wheels are still straight and true after more than 25 years, and are now only being retired due to sidewall brake wear. I've had similar experience with the robustness of other deep aluminum rims.

[Mark McM]

Originally Posted by MikeD

I edited my post to provide further clarification. He claims to not have fussed with the wheels much (i.e., truing). I assume he also had 36 spokes as well. I hear you on the wheel dish, but didn't offset rim bed rims correct most of this? He also didn't use thread locker on the spoke threads but used a lot of spoke tension (probably too much), which kept the nipples from unscrewing. But, he rode his bike off-road a lot too.

Like November Dave said, Brandt got 200,000 miles on a set of spokes (and possibly hubs), not on the rims. If you read Brandt's book, he talks a lot about how spokes fail from fatigue, and supplies several methods to reduce stress concentrations in spokes and increase their fatigue life. And yes, his wheels had 36 spokes.

Offset spoke rims help some, but even with offset rims today's 11/12spd wheels still have far more dish than the 5/6 spd wheels Brandt used. Brandt did use spoke tensions that were considered high for his day, but modern wheels with deep section rims and reduced numbers of spokes use higher tensions than Brandt's wheels did. While Brandt's wheels might have spoke tensions of 100 - 110 kgf, some of today's deep section rims may have spoke tensions of 150 kg or so.