jyl

My road bikes have 23-25 mm 700C tires, typically at 100-120 psi, good quality tires (Michelin Pro4 clinchers, Tufo tubulars, etc)

Suppose I built a road bike with 42 mm 650B tires, at whatever psi one uses with such things, using good quality tires (Grand Bois?)

How much faster will I be able to descend a steep, twisty road?

catgita

Can't say. Many years ago I would do runs going over 60mph on 23mm tires, but today 32mm tires feel way too skittish to go very fast at all. I don't ride Compass 42s for ultimate speed anyway, but for comfort and higher average speed.

Slash5

I'd say you will not see any meaningful different - top speed on a steep twisty road would be more about technique and risk taking more than traction.

Andrew R Stewart 

In theory the wider tires will have a better footprint for cornering. In the real world I read in the morning paper, every day, of car accidents involving cars with wider tires then I use (on my car). Just proving Slash5's suggestion that equipment isn't the limiting factor. Andy.

gregf83 

I wouldn't feel comfortable pushing wider tires. They might work OK but with lower pressure they also move around a lot more. 23mm tires have enough traction to lift the rear wheels under heavy braking which is what you'll want to be doing when descending. Not many people have the nerve to test cornering limits on a steep mountain descent but good luck!

habilis

The more important limiting factor may be the road, not the tire. At 40+ mph, even a familiar piece of road can have surprises, like a newly opened pot hole (had that happen). In the fall, there will be wet leaves, maybe even ice (had that happen). In the spring, there will be all of the above plus sand from the spreader trucks (had that happen). Hate to sound like such an old man, but I'm also a survivor - so far.

Bike Gremlin

First let's define conditions:

Both tyres use the same compound and profile.
Both are inflated at appropriate (optimal) pressure and put on a rim of optimal width for the tyre.
The test is done on paved roads.
There is no snow, or ice.

With all the above conditions met: yes, wider tyre will corner as well, or better. It will more easily compensate for pavement irregularities. They will also have better spread of load - over a wider surface, making it possible for the tyre compound to be both grippy (and soft) and hold a lot of cornering force without being torn (skidding, leaving black marks on pavement). This is not possible with a narrow tyre.


On snow and ice - narrower is sometimes better. For example: a thin layer of snow over concrete, or ice road with studded tyres.

Similar goes for off road: sometimes wider is not better - one needs to find an optimal width, thread profile etc.


Wider tyres are also heavier, and take more space. Those are tradeoffs even for pavement riding, on which wider is better for traction and cornering. I believe 42mm width is a point over which there is very little gain (on pavement) - even on rough roads. 28mm is the sweet spot for me and my riding style, conditions.

FastJake

The only way to really find out is to perform a test. I suspect it may have as much to do with the two specific tires or your inflation pressure as it does with the width.

Setup a marked corner in a clean empty parking lot. You will have to paint a line on the ground or put cones out. Look at your speedometer and enter the corner at a higher speed each time until you wash out. Then repeat with the other tire. You'll want to wear pads for this...

With enough runs this will also give you an excellent idea of at-the-limit handling. Similar to learning how much front brake to apply before going over the bars.

SpikedLemon

I'm not convinced bicycle tire profiles are limited in traction by width alone as they are in cars or, even, motorcycles.

Wide motorcycle tires are used for additional traction but it's often due to narrower tires traction profile at the edge that plays more a part than simply "width" as they're not round but intentionally profiled. Bicycle tires are, however, less rigid, more round shaped and don't seem to be designed to run out of traction at certain lean angles.

I'm certain there's a point that width would provide more traction before you start to see other limits of bicycle tires based on their current designs.

And unless your trial-and-error testing involves taking it past the limit (i.e. crashing): I'm sure that comfort level will limit you well before your traction would.

SquidPuppet

If the rubber compound and inflated profile (properly sized rims being used) are the same, the 42s would offer significantly greater cornering grip. The contact patch is more than 65% larger than with the 25s. The 42s would also handle surface imperfections better, leading to even better grip.

CliffordK

Are you talking pavement or gravel?

I've wondered if some of the pro crashes are caused by too high of tire inflation (so the issue may not be wide tires, but rather over inflation).

A little lower pressure and larger contact patch may help. Perhaps the benefits are non-linear. 28mm? 32mm? 70 psi?

One issue that has been mentioned is that a wide tire mounted on a narrow rim will tend to flex to the side a bit, which may be problematic, so one should choose a decent matching rim and tire.

rowebr

A few years ago I converted my steel Bianchi road bike from 700x28mm to 650Bx38mm. I have more fun with the 650B version in all respects, but especially when descending. Its actually hard for me to explain how big the difference is without sounding like I'm shilling for 650B. Just to take one example among many, for small cracks and bumps in the pavement that you would want to avoid with a 25mm tire, you can just ride right over without even noticing with 650Bx38mm or 42mm.

Earl Grey

Isn't the benefit really the latter, rather than the former? The contact patch is larger but the force per unit area lower in equal proportion.

fietsbob

Seems there was a more triangular profile so contact patch larger when leaned over ..

FIM Motorcycles have devoted more study on this .. a grippy soft compound may require a wheel change mid race, so there are limits.




You want to take bets on the difference then test it and report back ? and pay the one who hits the right Number

SpikedLemon

If you could proportionately increase the rigidity of the 42's sidewall to match that of the 25: i'd buy your logic. Taller tire would contribute to more movement of the tire.

ThermionicScott 

I would expect the impact to be mostly psychological, so highly subjective and individual. Some people have no fear descending the steepest roads at top speed on 23mm tires (or narrower). OTOH, I know I feel a lot more secure descending and cornering on wider tires.

hueyhoolihan

i'm still up in the air WRT wider tires and rims.

but i'm thinking of investing in a set, if the chatter is correct, that they also do light house-cleaning and windows...

FastJake

Gosh darn you all. Now I want to build up a fat tire (38-50mm) bike and try this out for myself.

ThermionicScott 

You mean you don't have one already?

60_tooth_ring

Another factor worth considering: the type of rubber in the tires. I have a hot rod car that has soft rubber tires, 335's in fact, and I don't get 10,000 miles to a tire change. But I'd take a 90 degree turn at high speed and feel safe (ok, maybe not 90 degrees...)

Harder rubber lasts longer but does not grip as well. Soft tires do, but they also wear out faster.

squirtdad

My question would be....if wider are really faster on down hills (and I think that is a big if) why wouldn't pro's use them?

Could be as simple as the inefficiency going uphill is greater than down hill gains .

Or simply that wide tires are not faster

Andrew R Stewart 

Well, there is a movement among pro teams to not use just 23s for all but cobbles. But more to the reason is that pros are paid to ride by companies who have a investment in their product's market placement. In other words the pros are paid to ride what their sponsors want then to. Or at least look like it in the photos and press. Sure, some top tier riders have clauses or exclusions in their contracts but these are the exceptions. Another aspect of riding as a pro is that it is a job. Both in wanting to keep the job as well as not wanting to spend one's own money to work the job. A bike, to many pros, is a tool. Sometimes your work requires you to use certain tools, like an OS system or a brand of drug for 'scripts. Last thing is that racing bikes can be so very psychological. Perceptions often over rules measurements. This last point is slowly changing from what I can tell but still has a lot of momentum. Andy.

caloso

Good point. I came here to make this point. I definitely feel the difference going from 25mm to 36mm.

SquidPuppet

I'm not following you here.

Plimogz

Are the 650B wheels wider rimmed to go along with the wider tires?