Witterings

Thanks for everyone's input and help (and on my other thread about understanding tyre sizes) as I now totally get it.

With this new understanding though brings about a different discussion .... whenever anyone seems to compare a 650 and sell it's advantages relative to a 700, they always use comfort and ability to handle rougher terrain more easily BUT they always seem to do the comparison with a much narrower width 700 ie comparing a 650 x 47 with a 700 x 28 as having a "similar" rolling diameter negating any potential advantage the 700 may have of more inertia (when up to speed) and potentially less pedaling for the same distance.

Surely though if you compare like for like in terms of width i.e. a 650 x 45 with a 700 x 45, the 700 will now have the same comfort level and ability to handle the bumps (arguably more) than the 650 but with the added benefit of even greater circumference.

I get that the smaller wheel will be more agile / better for anything technical, will be lighter on climbs and will spool up more easily / quicker .... but if you're looking for something that eats miles on predominantly flat and straight roads / tracks the 700 x 45 will still do that better but offering the same comfort level??

The other argument often used is you can drop the pressure more on the wider 650 to further assist with comfort / traction but again if you're using the wider 700 the same applies.

That said at the end of the day you can talk it in theory all day long .... I think my answer is to take a leap of faith and jump in and buy one and at some stage get a 2nd set of wheels in the other size and see which "feels nicer" because that's what it's about at the end of the day especially as I'm not competing but riding for pleasure instead

And again .... thanks for everyone's help .... it's much appreciated

JonathanGennick 

This is because the subtext of these discussions is often around retrofitting an existing 700c bike to run 650b wheels, and it is the 650b wheels that allow wider tires to be fitted. The other subtext that sometimes applies is the idea that one is happy with the frame geometry the typical road wheel diameter allows, which more or less gets back to the first point. Otherwise, yes, a 47 mm wide tire in either size will provide the same comfort over gravel and other rough surfaces.

Abu Mahendra

Don't split hairs. If you are gonna ride two sizes, ride 559 and 622.

Spoonrobot 

Just as recently as 2015/2016 model year bikes were not able to take 700x45/40 tires. On most drop-bar gravel bikes 700x40 was too close for manufacturers to safely recommend as a maximum size. Thus 650b as an option since it allows a wider tire to be used as the overall diameter is so much less.

This idea was grabbed and marketed by WTB as Road Plus not only to get their own 650b tire offering on the private market but to the OEM buyers as well. This allowed individual users to convert limited clearance cyclocross bikes from almost any era as well as earlier gravel bikes to larger tires. While also allowing manufacturers to easily create and market their own 650b bikes, the rims were already there from mountain bikes and now they had tires as well. It worked pretty well, there have been dozens of bikes released in 2017/2018 model years featuring the WTB Horizon as their stock tire.

However, now that tire clearance has caught up to market/user demands, there's less reason to go 650b. It's still a good option but for a lot of users it makes more sense to stay at 700c and just use bigger tires since the frame and fork will now accept them. So the issue ends up just like mountain bikes, large 700c tires are great if they match with your physiology, for some riders they are obviously too much to pedal and they will find more success on smaller wheels with the same size tires due to weight/inertia. Rollover is another issue but that's been covered so well in the mountain bike world that there's no real need to re-tread.
Overall size difference between 700cx40 and 650bx48 is still very large. 22mm/.86 inches in diameter. Keep in mind the marginal difference in diameter between the same tire in 700c or 650b is 1.5 inches, that's a lot and is often glossed over when discussing conversions because different tire sizes are used. It's not an incremental step between wheel sizes, 650b is much closer to 26" than 700c.


It's not just aero losses, bigger tires just don't roll down as fast in the real world as all the online testing would have one believe. Pedaling around outside is not analogous to running a tire in a testing apparatus or riding around a velodrome at a statistically approved speed. No only do real world pressures almost automatically preclude larger tires from pedaling and rolling as fast as narrower higher pressure tires but the additional hysteresis losses, especially during extremely hard efforts and standing climbing, can be significant. The feel of acceleration and overall acceleration speed is also different. It takes more time to get a larger tire up to speed both in the macro sense and the micro sense.

The tire pressure between the two is almost irrelevant and is always a red herring since sag is the main consideration. A lot of testing is worthless because tire pressure has different types of tires running at wildly different sag levels which means the contact patch is not equalized and is an additional unconstrained variable.

650bx48 was tested at 20% sag, 28 psi front/ 35 psi rear.
700cx40 was tested at 20% sag, 34 psi front/ 43 psi rear.

Approach angle gives additional benefit for the larger overall circumference system. Rolling over obstacles, handling wash boarding, rocks/roots and so forth is easier and less jarring the larger the wheel becomes. 700cx47 would be more comfortable than 650bx47. This can also make the larger wheelsize faster.

chas58

There are two completely different and distinct “advantages” to 650b as I alluded to above.

1) Riders under 5’6” will be better served on a 650b tire in order to get the proper trail (handling) and to avoid toe overlap.

2) Totally unrelated is the ability to use a fatter tire on a full size bike while keeping a similar rolling circumference.

I can use a 650b 54mm tire on the front and a 700c 40mm tire on the back and both tires will have the same diameter. Putting a big tire on the front gives me more suspension, more flotation and more traction. Some bikes will allow bigger 650b tires front and rear. These will be like a boat anchor when doing 30mph fast rides, but really have little to no disadvantage at 15mph or below (i.e. most gravel rides).

You are right that with same width tires (i.e. 650b 40mm & 700c 40mm) the 700c will be smoother and faster because of the shallower approach angle. But going above 40mm on a 700c is getting into 29er mountain bike territory.

Generally you are going to want to keep the tire diameter close to what the bike was designed for - mostly to keep your bottom bracket height.

Spoonrobot 

That's not correct, here's a handy trail calculator that also does tire measurements for wheelsizes. Bicycle Trail Calculator yojimg.net

HTupolev

I mean in terms of the total amount of tire, height and width. Based on nominal size values, the 650x48 should be about 10-15% more stuff. (Which makes it a little curious that Panaracer's catalog lists it having a nominal weight of more than 60% higher... it's a pretty large weight-to-size outlier among the GK slicks, but whatever.)

If hysteresis from pedaling forces was a problem, larger tires would be expected to fare worse in velodrome testing than rolldown testing.

chas58

Yeah, I know. I like to prove people technically wrong too, LOL.

That is a great site (I use it all the time). But I like to use the manufacturer's specs. That site's diameters (which may be "technically correct") are up to 5mm (radius) off from my manufacture’s specs, so I’m calling that within the margin of error. Not all 40mm tires are the same size.

The difference is trivial for me. There is a little fudge factor in there for the tires I quoted (5mm radius). Personally, if my ride right difference front to rear is 5mm or less, the difference is pretty subtle. Often, I’ll leave the rear alone and ride a smaller or larger front tire depending on the surface. 5mm is a trivial difference in ride height between tires on my bike. I do it all the time.

Spoonrobot 

I'm not interested in proving you wrong. I'm interested in helping you be correct.

What velodrome testing? The BQ test? Riding around on the apron of a velodrome at a set speed is a great test to confirm which tires are good at riding around a velodrome at a set speed. Otherwise it's worthless with little real world application. Riding on the road and gravel is not done at a set speed. It's moving and pedaling at wildly varying power and torque levels, putting stresses that have not been well examined onto the tires. There's a reason the tire tests are all docile and done in a statistically approved manner. Adding in something like hill sprints or timed acceleration would paint a much different picture. Hell since he was at a velodrome he could have done flying 200s on different tires and that really would have blown everyone's hair back. But he didn't because it kills the narrative that wider tires are just as fast as narrower tires. There's also been no power metered hill climbs comparing different tire sizes. The body of testing not done is much more enlightening that the tests that have been done.

As always, there is a reason no one is racing, on the road, on tires wider than about 27mm. After a certain point tire size has significantly diminishing returns. This is helpful to know for the recreation cyclist to make an informed choice. Wider tires have a lot of benefits, being as fast or faster on most paved roads is not one of them.

HTupolev

I'm not sure if you were referencing any particular example, but that's the most obvious choice.

Tests generally try to isolate aspects of performance to some degree. Docile cruising is a mostly-controllable approach that results in a "low" amount of noise, and it's not obvious that the results of acceleration and climbing tests would be significantly different from what would be predicted by combining the docile data with basic kinematics. You've brought up hysteresis from pedaling forces as a notable additional factor, although this still leaves the results of your coastdown test puzzling.

I've been contemplating picking up some tires to do a bit of coastdown testing of my own, so I suppose I'll see.

I agree with this. I don't think that super-fat tires have benefits for smooth riding; I just don't see the costs being very significant across the widths we're discussing.

antimonysarah

Also to increase the gap between saddle and rear tire (at the same tire width) if you want to carry gear in a bikepacking/buttrocket style bag.

chas58

Well, not too long ago no one was racing (or road riding) on tires above 23mm. Somewhere around 10+ years ago I started showing up on road rides on 28mm tires and people stared at me like I was on a beach cruiser. It was a good topic starter. I road 28mm tires because I just couldn't tell the difference between 23 and 28mm, but around 32mm it seemed a little more sluggish.

Most of it is weight and aero. Double the weight of a tire and I'll feel it accelerating when racing. The 50g between 23 and 28 isn't much, but start adding 100-200g per tire, and its noticeable. And in my informal velodrome testing 40mm tires just don't cut through the wind as easily at 30+mph as a skinny tire matched to an aero wheel. Then again at 15mph a good fat tire (60mm G-One) is as fast as a skinny race tire. ;-)

JonathanGennick 

That's a good point. I'm not fast enough for the aerodynamics to matter much, but I do notice rotating weight.

Metieval

I wonder if a flappy club cut jersey vs skin tight jersey makes more difference than a tire.

Spoonrobot 

No, tires are the second most important thing after position. The difference between a very good tire and a very bad tire can be a real 20+ watts. For some riders this can be upwards of 10-15% of their FTP. The difference between cuts of jersey doesn't come close to this.

I used to ride in regular race cut kits, last year I switched to just wearing a t-shirt and cutoff shorts. Race season came around and I started getting worried so I spent two different nights playing with rolldown tests. Used a 0.8 mile, -6% hill near my apartment complex. Rolldown holding the same position wearing either t-shirt/cutoffs or race kit was within 10-15 feet for each run. I found height/angle of chest played a much larger role and sitting at the tops would knock off around to 100 feet at the end of the run. The worst tire I have is a clement strada LGG and it tends to short my best tire (Continental GP 4000s II by around 70 feet.

Actually pedaling and racing the bike seems to back up the same, my flat and overall times are about the same and my race results are actually better in a t-shirt. Take that for what you will I guess.

Metieval

now try the loose T shirt vs skin tight shirt it on a flat section of say for 3 miles pedaling in zone 4. you know where you are actually working for the time/distance.

Spoonrobot 

Difference would be even more minimal as the speed would be...much lower. I have done that too but I don't have a power meter so I only have hand-waving and (more?) conjecture to offer, wrt flat efforts.

Everyone can and should test for themselves, I mean how flappy is flappy? Some riders have a visually and functionally correct position but scoop up huge amounts of air with their chests while others only hit it with their shoulders. Some can hold a great TT position but only on paved sections and hunch up on any sort of gravel.

But really my main point is the same regardless of the detail of the question:

DarKris

So on my Old bike I could run either 700x40mm tires or 27.5x2in tires. Being a heavier rider I had bad luck with 700c/29er wheels not lasting long w/o needing to have the wheel constantly breaking spokes and falling out of true. With 27.5 I was much more lucky and had wheels that were very sturdy, and along with being able to run lower pressures with the wider tire just led me to liking that tire combo over the 700c /40mm tire combo.

Recently I got a new bike that lets me run 29er tires, and the wheels that came with it seem to be sturdier than what I was used to, so I'm going to revisit this and try out 27.5 v. 29 to see if my opinions change.