Wide rims, narrow tires, and tread width

[Mark McM]

As has often been discussed, mounting a tire on a wider rim will increase the inflated width of a tire. This has many advantages, including being able to run lower pressure to improve compliance (while still maintaining pinch flat resistance), and possibly improving rolling resistance. But what affect does this have on the relative width of the tread, and the effects that tread width has on performance?

Tires are usually designed so that when mounted on a standard width rim, the tread will cover about 180 degrees of the circumference of the casing. This guarantees that there is sufficient tread between the casing and the ground (for traction and puncture protection) even when the bike is leaned over into a corner. (presumably the width of a puncture protection layer is similarly designed based on the standard inflation width of the tire). But when you install a tire on a wider rim, the tire will inflate to a wider width, and the tread (and puncture protection layer) will cover a smaller arc of the now wider circumference. This would seem to expose more of the casing to the ground, especially when leaned over into a corner. (If you leaned over far enough, you could potentially shift the ground contact area off the edge of the tread.)

Has anyone seen or heard of any problems caused by insufficient tread width when a narrow tire is mounted to a wide rim?

[simonov]

I guess it's theoretically possible at the extreme ends of the spectrum, like a narrow 23 (or a 21) mounted on an uber-wide ride, but in practice I don't think there's much to worry about. I've put tons of miles on Conti GP4000S 23s on wider rims and the "tread" part of the tire still wraps well down the sides.

[HTupolev]

Sidewall exposure can be an issue if you're running skinny tires on loose chunky gravel, sidewall cuts and whatnot. In reasonable combinations, it's not really a problem for paved road riding.

[Andy sti]

It's all about the dish, tuck and stretch!

[ergott]

I don't think any road tire tread is worth anything to bother. Compound effects grip far more than tread and you don't need grooves to shed water like a car tire. Never heard of a road bike hydroplaning.

[Mark McM]

Originally Posted by ergott

I don't think any road tire tread is worth anything to bother. Compound effects grip far more than tread and you don't need grooves to shed water like a car tire. Never heard of a road bike hydroplaning.

I wasn't asking about pattern of the tread. The term "tread" doesn't refer to the pattern, but to the strip of rubber bonded to the casing. A tread may be smooth (slick), or it can have a pattern.

The reason I'm asking is because I have a racing frame that has limited tire clearance. With a set of HED Jet Plus wheels (internal width 21mm, external width 25mm), the widest tire that will fit the frame is a 700x23C (which inflates to a true width of 27mm). When inflated, the tread of a 700x23C Continental GP 4000s extends about 120 degrees around the tire cross-section circumference. The lean angle at the maximum traction limit of a bicycle tire is about 45 degrees. When I press the tires onto a flat surface with the wheel at a 45 degree angle, it appears that the ground contact patch extends all the way to the edge of the tread, and depending on surface conditions, in some situations it may go a little past the edge.

Not many people corner this hard. I've never actually measured my maximum lean angle, but I tend to corner harder than most others. When in the heat of a race, I'd prefer not to have in the back of my mind the idea that I might lean past the edge of the tread.

[ergott]

When I google tire tread... but I digress.

Sounds like you already know your answer then.

[HTupolev]

Originally Posted by ergott

When I google tire tread... but I digress.

The first tire-related definition that comes up when I google "tread":

tread
/tred/

noun

the thick molded part of a vehicle tire that grips the road.

People sometimes do use "tread" to refer to the particular pattern molded into a tread, but the word also refers to the part of a tire intended to make contact with the surface the tire is rolling on.

Same usage applies to stuff like shoes.

[bicycletricycle]

This might happen in some extreme cases. I wonder how much reduced lean angle a rim width change would net in common cases.

[m4rk540]

23s or 25s, any brand, on early era, 23mm external width rims.

Conti 4000IIs 25 or 28s on new era, ultra wide, 27.5-29mm carbon rims.

What was the question?

[bfd]

Originally Posted by Mark McM

I wasn't asking about pattern of the tread. The term "tread" doesn't refer to the pattern, but to the strip of rubber bonded to the casing. A tread may be smooth (slick), or it can have a pattern.

The reason I'm asking is because I have a racing frame that has limited tire clearance. With a set of HED Jet Plus wheels (internal width 21mm, external width 25mm), the widest tire that will fit the frame is a 700x23C (which inflates to a true width of 27mm). When inflated, the tread of a 700x23C Continental GP 4000s extends about 120 degrees around the tire cross-section circumference. The lean angle at the maximum traction limit of a bicycle tire is about 45 degrees. When I press the tires onto a flat surface with the wheel at a 45 degree angle, it appears that the ground contact patch extends all the way to the edge of the tread, and depending on surface conditions, in some situations it may go a little past the edge.

Not many people corner this hard. I've never actually measured my maximum lean angle, but I tend to corner harder than most others. When in the heat of a race, I'd prefer not to have in the back of my mind the idea that I might lean past the edge of the tread.

Jobst Brandt wrote a FAQ on smooth thread, but you need to corner like him:



link: http://www.sheldonbrown.com/brandt/slicks.html

Good Luck!

[Mark McM]

Originally Posted by bfd

Jobst Brandt wrote a FAQ on smooth thread, but you need to corner like him:



link: http://www.sheldonbrown.com/brandt/slicks.html

Good Luck!

This discussion isn't about tread pattern, its about whether a tread can be too narrow for the width of the tire (so that when you lean over, the contact point goes past the edge of the tread).

As far as cornering like Brandt ... Well, I do sometimes corner as hard as my teammate, Brooke O'Connor (below). We both taught cornering at our club's racing clinic. But previously, I've used used standard (narrow) rims, so the tire inflated to the spec'ed width and the tread had good coverage across the tire cross-section. Now that I've got a set of narrow tires on wide rims, I'm wondering if the tread has wide enough coverage for high lean cornering now that the tire is expanded to a wider width.

Brooke Cornering4x3.jpg

[rustychisel]

Well, to your specific question. It is possible, and I think I've seen it done.

Back in the day, like late 1970s and early 1980s
- we were all racer boys on limited budgets
- choices of rims and tyres were severely limited
- we weren't too smart (oh, we still aren't)

Wheels were 27in x 1 1/4 in rims, I know I had a set of Araya, for example; and we used to run stupid silly thin tyres on those wheels because as we all 'knew', skinny was real racer.

Avocet 1in tyres were popular, for a couple of summer seasons I ran my favourites of the time, Wolber 3/4in on those rims, back and front. They were genuinely low volume skinny clincher tyres, probably 19 ~ 20mm in current terms, and they sat well inside the rim profile.

We used to come down Norton Summit, about 6km of descending, taking it to the limit as 18 year olds can... more than once my mate slid out his front wheel and dumped it, and I had a set of wheels with contact scrapes on the upper outside edge of the brake track where I leaned to the point of 'almost' no return.

But the were 'fast' and they looked the part. It would be my contention that you are highly unlikely to reach the outer edge of the tread (and break adhesion) before you reached the natural adhesive limits of a tyre in any case. I cannot think of any tyre which has a narrow enough running strip to make that a possibility, not to mention dynamic distortion of the tyre carcass itself.

Slightly OT: trackies worry about this on the boards, when there's a possibility of running on the tyre label on polished high angle timber. Some I know used to reverse their tyres so the label was on the other (non-drive) side, and nearly all used to scuff their tyres with fine sandpaper to try and obviate what they saw as an issue. Regardless of all this, I never saw anyone lose traction on a tyre label.

[AngryScientist]

Originally Posted by Mark McM

This discussion isn't about tread pattern, its about whether a tread can be too narrow...

valid question.

it's more easily visualized on a skinwall tire where the intended road contact area is glued to the casing, like the vittoria corsas, pictured below. obviously, only the black part is ever supposed to see the road, not the gumwall.

this should be easy enough to test for though, right? if you can estimate your lean angle in the most extreme case, then replicate that angle with the wheel/tire mounted off the bike, you can get an idea of if the tread is on the road or if you are approaching the sidewall area?

[Mark McM]

Originally Posted by AngryScientist

this should be easy enough to test for though, right? if you can estimate your lean angle in the most extreme case, then replicate that angle with the wheel/tire mounted off the bike, you can get an idea of if the tread is on the road or if you are approaching the sidewall area?

That's what started my query. When I pressed a flat object against the inflated tire at about the maximum lean angle (45 degrees), the contact area seemed to extend to the edge of the tread. But that's for the static case in perfect conditions - real conditions have a lot more variables (rough pavement, bumps, varying road cambers, etc.). So I thought I'd ask to see if anyone had any real-world experience with cornering at the limits on narrow tires mounted to wide rims.

Based on the experiences related above, I think I may chose to find some wheels with more traditional width rims for this bike, just to be on the safe side. At least when I do technical criteriums, anyway.