meh. people are sheep.

Nor do I care what other people spend their money on. People get rid of stuff and buy new stuff for all kinds of reasons, I stopped caring long ago.

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What I'm saying, is that both the anti-disc and the anti-rim factions can forget to revisit one's beliefs.

I don't consider the current disc brakes "state of the art", and I'm waiting to see what the next generations of disc frames and brake-systems bring. For TT use, it might take another generation or two at least before I swing over the fence, if ever. For MTB, obviously, I've long ago switched.

However, current rim brakes are also not at "peak rim-braking", either. Fitting a tyre larger than 28mm might be a problem for current systems, but that's because they were optimized for the old standard. You could design a system for wider rims/tyres. These systems exist. Likewise, rim shapes are not necessarily constrained by the braking system - we've seen proof of that in the last generations of carbon and alloy rims.

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Running that wide of a tire on a rim brake requires something different than the standard shimano center bolt brake. I have GP4K2s on Hed Ardennes blacks and the tire just barely rubs my ultegra caliper during sprints.

Obviously the 28mm GPK4S measure way wider than 28 on a 21mm inside width rim.

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But the crazy thing is, unless you weigh 300 lbs, why in the world would you even want to run a 28mm tire/rim, optimized or not, for racing (on roads)?

(Assuming again, that the goal of racing is to go as fast as possible... )

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Plus, the GP4KS are inordinately "tall" for their width due to the parabolic tread shape...

It's amazing what you can do with things like caliper mount hole placement and offset brake pad holders to allow the use of a brake nominally designed with short-reach arms and "rated" to only allow up to a 28mm wide tire.

I've got a picture I'll take later and upload showing the fork upgrade I did to my Stinner to allow the use of 30+mm wide tires (the frame was designed/built to allow up to 32mm wide tires in the rear)...I might be able to fit up to 34-36mm in the fork/front brake, actually :-)

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And "as fast as possible" depends on lots of factors, some of them judgment calls since they're hard to quantitatively measure. Aero, rolling resistance, comfort, stability, reliability. Sometimes here we get tunnel vision on aero. I have in the past - now I've backed off a tad and weight comfort a little higher than I used to.



My defense there is this was way before everyone got all smart on tire pressures. That said "too high" is context-dependent. It's absolutely not too high for running on a wood track. It's too high for chip seal, for sure (from a comfort standpoint).

I mounted 32mm Panaracer Gravel Kings just to see if they would clear. They git but only barely on my CAAD 12.

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...AND a speed standpoint.
https://silca.cc/...stance-and-impedance

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True. The comfort aspects just dominates for me since that one event is etched into my memory like PTSD. I've never been so miserable on any bike ride than the last 10 miles of that chipseal. The performance losses for were like 99% from being mentally destroyed, maybe 1% impedance/resistance. I think part of it is that my combination of pressure/bike choice, etc. found like some precise harmonic frequency that rattled my whole bike (and skeleton) like a nuclear-powered dildo.

Yes, you are absolutely right about that. And that is why ST is really a good place to learn, and to learn new things, and to also re-visit old things.

If there is evidence out there that, when designs are optimized on all sides, that wider rims and wider tires are faster than narrower rims and narrower tires for TT and Tri racing (on decent asphalt roads), then I would be very interested to see it. So far, info on this forum has shown that, when properly inflated, wider tires actually have more rolling resistance than the same also properly inflated narrower tires. And based on physics and universally accepted aerodynamic principles, lots and lots and lots and lots of info has shown that, when all other things are equal (which is the way it should be when making a comparison that focuses on a particular design element) that bodies moving through a fluid (air) that have smaller frontal areas have much lower drag than similar shapes with greater frontal areas.

That said, it there is new info out there that I am not aware of, I would love to see it. If the evidence is any good, I will toss out the conclusions above immediately. Because I have no confirmation bias, I just want to know what is really the fastest for my needs (racing triathlon on decent/typical US asphalt roads).

But for training, I don't care about my speed over the ground at all (well, as long as I can see some change in scenery during my rides), but I do care about my rear end. And my rear end says that 50mm tires at 20 to 25 psi is great !

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Ooohhhh, I'd be interested to see that as I'm thinking about picking up a Trek Domane SL in rim brake format. That frame with ~30-32mm tires at low pressures would be very comfy for training :)

To this point, here's a snippet from that Mavic technical paper I was told to Google up earlier in this thread:

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Sure, I follow that.

That said, as long the tire is not wider than the rim, this data also support the narrower is faster idea. If the tire/rim system is optimized, the narrower system will still be faster (all other things being equal of course).

You agree?

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Absolutely...the big caveat these days though is that the lowest Crr tires aren't necessarily available in the narrower sizes. For example, the Corsa Speed is available in it's smallest size as a 23C, which blows up to nearly 27mm wide on my Hed Jet+ rims. Even so, it's usually faster overall to "give up" some aerodynamics due to width matching to gain the best Crr. It may not be the most aero, but it's the fastest combo overall. See: http://bikeblather.blogspot.com/...t-3-after-party.html

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Well, the conti supersonic gets respectable Crr numbers so l am able to run a 20mm tire on the front at least.

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While I enjoyed the read, as an EE I reject the use of impedance in this ME context;)

Testing many „old“ and „actual“ configurations over the years (in a velodrone hence allmost zero yaw) a 808FC with a 20mm tire on the front is still the fastest for my fast girl.

The training TT bike is a 12 year old 11kg heavy aluminium bike with Schwalbe Marathon 30mm on cheap and heavy but bullet proof aluminium wheels with 12 years old Ultegra rim brakes. For bad weather mud gards are mounted all times. Breaking is good in all weather conditions and no tire change and no flat in the last 15000 km.

Sounds a bit familiar:

http://www.trainingandracingwithapowermeter.com/...inny-vs-fat.html?m=1

I LOVE that you mention this!!

When I first decided to call it 'Impedance' I actually emailed all the gurus I know (Anhalt, Froncioni, both Milliken's, Dave Kennedy, Damon, Gerard, Jim Manton, etc..) to ask if anybody was offended and nobody was. Although it was later pointed out that none of us are EE's so I think that there was clearly some bias in that sample group!!

Josh

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I accept an EE's condemnation of our inappropriate use of impedance.
I need to go back to school! :-)

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Engineering Manager,
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(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

OK, here's how the front looks with the 28C Specialized Turbo Cottons on Boyd Altamont rims (actually measure 28mm wide in that config).


Curious if some Challenge Strada Bianca Pro 36C tires would fit in there, I "mocked in" my disc wheel that has that tire mounted. Looks like it would work. Not bad for a brake that's only "rated" for up to a 28C tire :-)

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how are you compensating for the reduced braking force at the caliper? Or are you?

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But, if you dig deeper, it's actually NOT "inappriopriate". The term "impedance" was co-opted by EEs to describe the performance of electrical components to varying voltage/current. In a mechanical analogy sense, that's what's happening here too.

Think of it this way: By definition, electrical impedance is "the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance."

The mechanical analogy in this case is that tire rolling resistance is the analogue of "ohmic resistance", and the system losses in response to road roughness is analogous to "reactance", in that it is a response to a time-varying energy input.

Using the term "impedance" to describe this effect is actually quite apt.

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http://bikeblather.blogspot.com/

I'm not.

The brake (SRAM Hydro R) and rim (Boyd Altamont Ceramic) combination have PLENTY of power (and modulation), and if you look closely at the location of the pad with the offset holder in the arm, you can see that it really only needed a couple of millimeters to reach (not the full 10mm the holder could accommodate)...the slot in the arm was only barely not long enough.

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