Rim width vs tire width
 

I’m currently running Specialized Roubaix Pro 25/28 tires on 622x14c rims, and have to partially deflate them to clear the brakes when removing the wheel. I’m ok with this but I’m planning on upgrading my wheels and want to know what effect a wider rim will have on tire width? I haven’t decided on my new wheelset but let’s use 17mm wheel width for sake of argument. Going 3mm wider on the rim width will do what to my tires? Make them wider? Taller? Frame is a 2012 Specialized FACT 8R carbon frame. I don’t want to have to change tires but already don’t have much clearance remaining on the frame. As it stands I only have about 2mm clearance between the tire and the chainstays and the top of the front tire is starting to get close to the bottom of the fork. Is there a rule of thumb as to
“x”mm increase in wheel width = “y”mm increase in tire width? And will tire height change with a change in wheel width? Thanks.

A very basic geometry problem.

Given the info, you're adding 3mm to the cross section circumference of the tire/rim assembly.

Divide by Pi to get roughly a 1mm increase to the diameter, aka width.

Since the inside wheel diameter can't change, the entire 1mm will be added to the height.

This isn't super precise but it's more than close enough for the purpose.

Yes it will be wider and taller. And 2mm is too close. Done it. Picked up sticky rocks from a freshly paved road. They get wedged and wear a groove in the tire tread.

I don’t quite understand. Doesn’t a 622x14 rim have the same circumference as a 622 x17 rim? They both have the same 622mm diameter, only wheel width has changed.

As they sit the 25/28 Roubaix Pro’s have an inflated width of 27.5mm on the 14c rim. I really want a slightly wider modern rim, and the ability to go tubeless should I choose. If I change rims should I be looking at something like a 700x25c tire? Really don’t want to go back to 23’s.

I went back and edited to try and clarify

All dimensions I referenced are about the cross section, not the overall wheel diameter

I dropped my rear wheel and measured width between the chainstays where it line up with the tire and got 34mm, so around 3mm clearance between tire a chainstay. Regardless of my measurements my current tire has to be shoehorned in and it sounds like it would only get worse with a wider wheel. It’s looking like if I want new wheels I’ll have to also get new tires, and it looks like 700x25’s are going to be as wide as I should go.

Shop around because actual tire width varies compared to the published size.

That's partly because the maker has to guess what rim the tire will be on.

So, what you might do is lay your current tire flat and measure bead to bead.

Since you're adding 3mm to the rim, you want to buy a tire that's 3mm less bead to bead.

The chainstay clearance is what is critical. 3mm on each side is about the bare minimum you should have there. Any closer and the tire will rub the chainstays on climbs as the wheel and frame flex.

A wider internal measurement on a rim will indeed change your tire's effective width when mounted and inflated. Take a piece of paper and curl it into a tube still open at the bottom. Now open the space on the bottom a little more. That effectively increases the size of the paper tube, correct? I found when I went from 15mm rims to 17mm rims, my tire width effectively increased by 1mm. The height will increase slightly too, so if you are already close to the bottom of the fork, that could be problematic.

Are you flipping the little lever on the brake calipers to open them up? I only had 25mm tires on my bike with rim brakes. But it seemed that the 105 5800 brakes opened up more than enough to let 28mm tires get in when the lever was thrown.

Regarding the brakes, yes I’m flipping the lever to open them up. I guess it ultimately comes down to whether or not I want to spring for new tires along with new wheels, plus going down in tire width. The Roubaix Pro 25/28’s are almost new and they REALLY improved the ride from the stock 23’s that came with the bike. The only wheelset that I’ve been looking at that would still allow me to run the Roubaix’s is the Shimano WH-RS500 “Ultegra” at 15mm internal. I’m glad I asked here before rushing out and buying something that ultimately might not fit.

The differences in the tire profile itself aren't going to be significant if you change rim width. However changing rim width to wider might help with the issue of the pads not opening enough and touching the tire when changing the wheel.

Just don't put too wide a rim on for that. Sheldon and Continental say about a 19mm wide rim (internal) is the max for a 28mm tire. Just make sure your brake calipers will open far enough to handle wider. Likely they will.

And then there is the question that hasn't been asked yet which is what are you looking for in a new set of wheels that your old ones aren't providing?

I was waiting for that question. When I’m on these wheels I’m always comparing them to the Ultegra’s on my old Tarmac. Current wheels are some generic Specialized 460v 622x14 entry level wheels. Main reason? Probably the same reason people put fancy wheels on their vehicles - vanity.

This is probably true. I am one of those people who doesn't like a brake that engages with very little lever travel. I went from 700 x 25 to 700 x 28 tires on my Look 595 last year and noticed a big improvement in ride quality on some of the worst roads around Montreal. I have plenty of frame and fork clearance so no problem there. However, even when I fully open my brakes, the new tires hang up on the brake pads during wheel installation. My wheels are 10 year old Campagnolo Shamals with an internal rim width of 15 mm, somewhat like the OP's. A wider rim would probably deal with this minor problem, and I believe that I would still have sufficient clearance. I measured my tire width and my Hutchinson Fusion5 all season tubeless tires measure at almost exactly 28 mm wide. A wider rim effectively increases tire volume, so a narrower tire will pretty much have a larger volume on a wider rim

I highly doubt you'll have a problem. In any case you can always buy new tires if or when it becomes necessary.

You currently have 3mm per side clearance. Using a rim that's 3mm wider will increase overall width by about 1mm, or about 0.5mm per side, reducing that clearance to 2.5mm or so. The concerns flex when climbing are exaggerated so that shouldn't be an issue. However you will have less forgiveness should the wheel get bent far from home, though you've done so OK so far.

In your shoes, I'd buy the wheels I want, and wait and see about the tire. I'd then revisit the clearance issue when it's time to buy the replacement.

FWIW - I don't normally carry a spoke wrench except for multi day tours. But one of my road bikes has very minimal tire clearance (2mm or so). I keep a spoke wrench with that bike because I hate walking, and because of Murphy's Law.

This depends on the strength and weight of the rider as well as the stiffness of the wheel. A low spoke wheel paired with a strong rider is a recipe for frame rubbing. Even with 3mm of clearance, I was getting some frame rub on climbs with low spoke wheels. That problem went away after building a set of wheels with a 32 spoke rear.

What happens if you break a spoke. Without a spoke wrench, you will be walking or calling Uber.

Yes, I said, "exaggerated" not non-existent. I based my estimate on the fact that the OP hasn't had issues so far, and so the 0.5mm change alone wouldn't change that much.


Sadly it would be the former, since I don't carry a cell phone on day rides. OTOH - I don't worry about this because I don't break spokes. In well over 100k lifetime miles I've broken 1 spoke. That was on my touring bike over 10,000 miles after an overshift notched all the outside DS spokes.

Overall I think about my bikes' road worthyness the same way a mariner thinks about his boat's sea worthyness. I simply refuse to ride bikes I cannot trust to get me home from a 100+ mile ride, with plenty of margin for error. But I also apply different rules based on conditions, ie. OK for a coastal trip vs. a trans-oceanic crossing.

Point taken here. i just think for as light and small as a spoke wrench is, I may as well have it in my pack. I've actually broken 2 spokes in about 40K miles. One was 30 miles from home. Granted they were on cheap factory wheels before I got into wheel building and now I only ride my own wheels. The spoke wrench is still in my pack and I was sure glad to have it when I needed it

My multi-tool has a couple built-in spoke wrenches. They may not be as convenient to use as a dedicated tool, but I think they'd do the trick, in a pinch. I'm glad to have them.

Another thing to consider is that your tools are perhaps not just for you. There are other riders out there, and sometimes they need help. Heck, I once stopped in the middle of a long gravel race to help another rider with a mechanical. I wasn't going to win the race, but I did make a new friend that day.

Currently it’s an absolute b*tch to install the current tires on the 14c wide rims, will going to a wider rim have any effect on installation ease? I hope that’s the case because as it stands I have to use a tire jack to have any chance of getting the tires on and I’d rather not have to carry the jack when I’m riding.

It might. The wider rimmed wheels of the mountain bikes my kids had were always super easy to mount tires on.

The biggest reason of what I think is the issue is that the first side of the tire you already got on the rim along with the tube is competing for that valuable part of the rim where the diameter is the least. So the bead being installed doesn't get in to that part without either encouragement of the tire bead jack or you simply pushing the other bead up onto it's bead seat where it's out of the way.

For a long time I simply thought pinching the two sides together would get them both in that area of least rim diameter (the spoke channel). But I was wrong. However when I discovered that I should only push on the side of the tire that I was installing, it was a game changer for me. Hard became easy and tire can even be removed without levers if I want to show off.

Maybe, but not because they're wider. Mounting a tire on a rim with the OD greater than the bead diameter (describes all rims and tires) is only possible because the tire can go deeper into the rim on one side giving the slack to allow the opposite side to reach over the rim. So the key to tire mounting ease or difficulty is the rim depth at the center.

If you increase the rim width by 2 mm you must also add 2 mm to the width of the tyre casing in order for the sidewall to move out 1 mm on each side. So if you fit your current tyres to wider rims the effective tyre width will increase by only half the increase in rim width.

Just for fun, I fired up AutoCAD last night and drew two rims with 14mm and 19mm inside widths. On the 14mm rim I drew a tire of 25mm width that I will call the 25C. Also a 28mm width tire (28C).

Now, I assumed that the distance around the cross section of the tire from the inside of rim to inside of rim for the various width rims does not change for a given tire. Also that the tire takes the shape of an arc of a circle. (Ignoring possibly varying depth of thread and any forced molded shape. So what a quality high TPI tire with no tread at all would do. I also ignored the varying wrap around the top of the rim hook.

Looking at the 28C. Width for the 14mm rim is the spec'd 28mm. Height of that tire is 26.1mm.

Width for the same tire on the 19mm is 29.9mm or 1.9mm wider or 6.8% wider.
Height of the tire is 26.5mm or 0.4mm higher than the same tire on the 14mm rim. (1.9% higher.)

Now, running that 25C tire on the 19mm rim:

Width = 27.1mm or 0.9mm narrower than the 28C on the narrow rim.
Height = 23.1mm or a full 3.0mm lower than the 28C on the narrow rim


So, boosting rim width 5mm increases tire width ~2mm and height ~1/2mm. But dropping the tire one size decreases the width ~1mm and height a big 3mm.
(I don't claim this numbers represent any tire out there. But the trends should work for any manufacturer that is consistent in it's specs and QC.)