guadzilla

So i recently converted my old custom steel bike into an all-road bike - got tubeless wheels for it and slapped on a pair of Gravelking 35s and took her out for a checkout spin yesterday. I decided to compare it to a previous ride i had done on the same bike, but with different wheels.

Here's how it played out:

Yesterday:
- Tires: Gravelking 35s, WAM - 37mm (new)
- Pressure: 60psi
- Tubeless
- Wheels: Velocity A23, Sapim spokes, Novatec hubs
- Out and back loop, average 30.4kph @ 185W (189W normalized)

5 weeks ago:
- Tires: Vittoria Rubino Pro 25s, WAM - 25mm (new)
- Pressure: 100psi
- Butyl tube
- Wheels: Aksium front, Planet-X rear
- Out and back loop, average 30.5kph @ 185W (190W normalized)

Differences:
- the new wheels are about 300gm lighter
- the new tires are about 50gm heavier total, once i factor in the tubeless vs tube
- the studly rider was about 1kg lighter

Obviously, the wind plays a big role - while wind conditions were not too different, they likely werent identical either. So that adds an element of error to the results. Obviously, we cant conclude that the 2 tires are identical - but it is safe to say (atleast for me, who is familiar with the route and conditions) that the differences are fairly small.

It was my first time riding fat tires at relatively low pressures - the ride definitely felt different. Maybe it is in my head, but it seemed that i could feel the additional "give" of the tires and it seemed as though they would be slow. But when i looked at my powermeter and speed, it was pretty much in the range of what I expected it to be. It's got me intrigued enough that i plan to do some more controlled testing and also play around with reducing the pressure of my RapidAir 26s (WAM 28mm) down to 70psi, from the 80s i currently run them at.


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vespasianus

Yeah, your results seem to be similar to what others have noticed. In reality, you can get a more comfortable ride with wider rims and fatter tires all while maintaining speed. So if you can get wider and fatter, do it. Note, this only applies to the bike wheels/tires, not the person!

WhyFi

60psi on 35mm tires? Jumpin' Jiminy Cricket.

genejockey 

Too late! For me, anyway.

guadzilla

Too high?

More to the point, would i gain anything by going lower assuming that I am riding on fairly good tarmac (not the smoothest of finish but good condition, no potholes)?

Bah Humbug

I'd be most interested in like-for-like (to the extent possible) so wide and narrow in the same model (23 and 32, or 28 and 40, or whatever). Rubino Pros are crummy OEM tires, and butyl tubes are markedly worse than tubeless. And part of the total package is that you can use (and get benefit out of) aero wheels with narrower tires than 37WAM, but Aksiums mask that.

That said, it's not like I think the results would be drastically different. Is ST still filled with a "narrow is aero" crowd?

genejockey 

I swapped the 25s at 110/120 for 28s at 90/95 on my Ritchey, and I keep getting PRs on the same old segments, so I'm thinking I was rattling my jimmies needlessly all these years.

Bah Humbug

Oh and that is one pretty bike!

And I remember when I got the R2 I was all excited that I could finally run 28s instead of 23s. That lasted 18 months and now I'm making flirty eyes at the ability to run 40s...

genejockey 

In my case it was going from Grand Prix 25s to Grand Prix 28s, but I made no attempt to do same route/same speed, and I don't have a power meter. And I'm not going back to the 25s for confirmation.

Bah Humbug

Me neither. Except on the tri bike, and if I were convinced tri were going to be my focus again over the next couple years I'd be tempted to go for a bike that took 28s there too.

Seattle Forrest

​​​​​​You're too skinny for this sport.

guadzilla

Oh, for sure - this wasnt intended to be a precise test nor did i control all the variables. I just wanted to get a sense of how much worse I would be. I was expecting to be 1-2kph slower, so being in the same ballpark was nice.

Thank you!

I hear you about the lure of the wide. I have actually put my R5 frameset on sale and am getting a Factor LS precisely because i am getting hooked to the idea of having atleast one bike that i can ride whereever the hell i want, without worrying too much about the road conditions.

ST seems to be slowly getting on the wider=faster bandwagon but you know the place - it's like the 41 on cocaine: saying the sun rises in the East will get atleast 5 dissenting opinions.

Honestly, even i am not convinced about the math behind it. Generally, the argument, as I understand it:
- RR reduces with tire size but for the same pressure - lowering the pressure increases RR, although not significantly (1W or so for 10psi lower)
- RR also reduces at higher tire pressures - but up to a point. After this, vibration losses increases significantly and you lose a lot more watts (5-10W for 10psi more)
- So it is better to be a little too low than a little too high

So far so good.

My 2 issues with this:

1) the sweet spot for, say, a 25c tire, is around 90-100psi as per Silca's blog. Based on info that i got from emailing Zipp about the vibration loss curve, the sweet spot for a 28c tire should also be in approx the same pressure range, albeit with a lower overall number (ie, better efficiency). So i dont understand where this whole "run them at 70psi" math is coming from, that Zipp and Enve seem to be fobbing on us. The sweet spot for max speed seems to be around 80-100psi, depending on the quality of your tarmac.

- I have yet to see any numbers that compare aero for an optimized 28mm tire on a matching rim vs an optimized 23mm tire on a matching rim. Going to a wider tire saves you a bit on RR. But what does it ocost you in aerodynamics? No data on this.

In short, I admit to being a bit skeptical about the degree to which the pendulum is swinging towards wide tires, atleast from maximizing performance.

WhyFi

On good tarmac? Probably not, but I would really doubt that it would be a detriment, either. As soon as stuff get a little more choppy, lower would be likely be much better. I know that I mentioned elsewhere, but I'm running 28s at 60psi at a similar weight.

WhyFi

Oh, and the other thing I'll say is this - while I don't think they're (much) slower, big, wide tires still feel sluggish to me. For a bike primarily intended to be run on pavement, I'd maybe run a 32 nominal at the largest, but only if it was a bike where steady power delivery was the norm. Anything that's expected to change speed and/or direction quickly is going to be a 28 nominal.

Bah Humbug

I thought the "wider tires lower RR, but only at the same pressure" comes form BRR, which made me skeptical of their pressure findings (as opposed to overall, which I suspect they do a great job of ranking tires). Zipp's statement to you counteracts their chart of decreasing pressure reducing RR until an inflection point. And that's separate from what I notice, which is that handling worsens under 70psi with good tarmac; it's most clearly visible on COTA in the descent from Turn1. But being able to run 80psi for a tri would be great on certain courses.

And yes, the whole "fully optimized for each situation" is the test we never seem to get for anything.

Bah Humbug

How's the pavement quality up there? Half my roads are the most brutal chipseal, but I haven't tried 32s.

guadzilla

For tarmac, I have 28mm WAMs on both my road bikes, but will replace one with 30mm WAM (likely GP5ks) - and for my roads, that works plenty well. This bike is entirely for solo rides where speed doesnt matter - so I would be happy to trade off a little bit of speed on the pavement for better handling on dirt trails and some loose-over-hardpack. Now that you mention it, I do remember you saying 60psi for 28mm: so 40-45s for trails? I admit i am paranoid about running too low a pressure - years of ingrained habits to overcome, I guess.

Agree with you about the tires "feeling" sluggish. I suspect it is the lack of bounciness. On a similar vein, my Lynskey always felt a little flexy, even though the speeds didnt reflect it.

WhyFi

Some chipseal, but it's not horrible, at least in my experience. The main thing is cracks and potholes (you know - the whole freeze/thaw thing in the shoulder seasons).

vespasianus

Actually, I am working on that experiment right now!

WhyFi

40-45psi for trails for you? Or are you asking what I run for trails? I run 35mm tires at 35-40psi.

I'll do short sections of gravel on the 28s, but that's about it - I wouldn't plan a ride with several miles of gravel on those tires, but I know that many in ye olde times did that and lived to procreate just fine. *shrug* It's just too skittish for me and my brand of gravel.

To me, it's the increased weight/moment of inertia/gyroscope effect; mostly when moving the bike side-to-side. Again, I don't think that it translates meaningfully to the stopwatch, but the handling feel is significant to me, and if I want a bike that handles, I want a bike that handles (not that I'm a crit-boy, but still).

Bah Humbug

Right, when I moved down here I was looking forward to pristine pavement. There are few potholes or heaves, of course, but the surface is nasty and slow.

guadzilla

I seem to recall seeing in a few places that "wider = lower RR for a given pressure" and "lower pressure = higher RR" (although this is for a perfectly smooth surface - the vibration losses account for the roughness, I guess).

So here is what I was trying to say. If you take a 25mm tire and plot its RR and vibration loss, you get 2 curves - the line in blue showing the RR and the line in green, showing the vibration loss for a given surface. The point at which the sum of the two is minimum is the inflection point. This is the whole Silca graph (https://blog.silca.cc/part-4b-rollin...-and-impedance). Assuming for a second that the "wider = lower RR", then for any given pressure, a wider tire will have a lower RR than a thinner tire. So the RR curve for a 28c tire will look like the line in red. My question for Zipp was whether the vibration loss curvel (for a given surface) changed for a tire width or was it dependent entirely on pressure - their reply was that it was more or less dependent entirely on tire pressure (again, for a given surface).



If so, then the inflection point for a wider tire is going to be fairly close to the inflection point for a narrower tire (given the the RR curve for the 28mm tire and the RR curve for a 25c tire are separated by less than a watt - source: https://www.bicyclerollingresistance...000-comparison) .

If you look at the Silca graph on the link above for optimal inflection point for asphalt, it is around 90-110psi for a 25c tire. Based on this, the inflection point for a 28c tire or even a 32c tire should be well within 5-10psi of the same pressure. So 80-100psi. Where the heck are Zipp and Enve coming up with 60-70psi? Admittedly, using the Silca graph again, this means we are looking at a 2W reduction in efficiency - so it isnt huge. But for a product aimed for maximizing speed where even fraction of watts saved were touted as a Big Thing (that's why we are buying $2k wheelsets, right? For that little marginal gain), it seems weird to move to what appears to be a slower product.

And I havent even gotten into the aero side yet. Scroll down a bit to the section on Rolling Resistance vs Aero on the DT Swiss site (https://www.roadrevolution18.dtswiss.com/endurance/) and you find this chart:



So the wider tire has an aero disadvantage. And that disadvantage is worsened at speed. So above 35kph, you dont really want too wide a tire after all, it seems. Atleast as per DT Swiss and SwissSide.

Yet the narrative from Zipp and Enve is pushing us all to wider tires (which have a very small improvement RR for a given pressure, but a significant reduction in aero at higher speeds) and lower pressures (which appear to be far lower than what would be indicated by Silca's numbers, atleast).

Possible reasons I can think of:
1) Industry is latching on to a new trend to sell more wheels (which is a bit overly simplistic - I dont think the industry would just make stuff up across various companies)
2) Silca's science is correct but the numbers are off, and that in reality, the inflection point is at a lot lower pressures
3) There is another aspect - perhaps rider comfort/fatigue - that we havent considered. But if the wheel brands had factored it in, they would talk about it

WhyFi

I think that the other thing that needs to be considered, in terms of optimization, is the average Schmoe on the bike is a lot ******g heavier than the guys getting paid to play bikes. To me, that moves the break point considerably - moving from 23/25 at ~100 psi to 28/30 at 65-70 immediately translated to being fresher, less beat up and faster on anything longer than an hour or so.

HTupolev

Which is really the key thing. Most of the "we did gravel just fine on 25mm racing tires back in the day" people make false assumptions about what different folks' gravel is like.

I really need to get around to finishing my Black Mountain Road+ build. It'll end up with a trail slightly lower than typical road racing bikes, and my use of 650b rims should reduce the gyroscopic impact of the 48mm tires. It might end up feeling a lot like a road racing bike.

guadzilla

Ah, that's a really good point - I didnt factor weight into account as a variable.

Would a heavier rider shift the break point to the left or the right, compared to a lighter rider? On one hand, heavier could mean less bouncing and so less vibration loss. OTOH, heavier could also mean a greater downward force, resulting in a greater upward force (and so more vibration loss). I dont know the answer to that.

The Silca tests were done with 190lb rider+bike weight, FWIW.