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Tyre width vs wheel width question ??

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Tyre width vs wheel width question ??

Old 02-04-16, 10:11 PM
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Tyre width vs wheel width question ??

i have a set of boyd altamonts, and at their widest they are 24mm i was wondering which tyre width would be better in terms of aero vs cornering vs comfort which combination would be better, 23mm tyres, or 25mm tyres, which would be better. i'm thinking of getting vittoria open corsa cx tyres but not sure which tyre combination to try. boyd says the altamonts were designed around a 25mm tyre but doesn't that make the tyre 1mm wider than the rim and kinda ruin the aero part, aren't they supposed to match??

i just found out they make vittoria open corsa's sl in 24mm would that be a better match???

Last edited by allen254; 02-04-16 at 10:55 PM.
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Old 02-05-16, 03:05 AM
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From a purely aerodynamic perspective, I think you are correct that having a smooth sidewall to brake track transition by having them equal width is better, as well as you are correct that a 25c tire would bulge out past the brake track than a 23c tire on the Altamonts, and so a 23c would "match" better.

Below is a graphic from Hed showing how rim inner width and casing size relate. Altamonts are 18.1mm internal:



Whether the variance is, in your real-life application, appreciably substantial given the other factors affecting wheel performance, e.g. rider weight, road conditions, inlation pressure, riding speed, etc., I don't know. My thinking is that unless you're particularly heavy (like north of 230lbs, maybe), go 23c tire.

In any case, tires wear out, especially hi-performance ones, so you'll be able to cHange out sizes in short time, and nothing beats trying out the options for yourself.
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Old 02-05-16, 03:12 AM
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Originally Posted by chaadster
From a purely aerodynamic perspective, I think you are correct that having a smooth sidewall to brake track transition by having them equal width is better, as well as you are correct that a 25c tire would bulge out past the brake track than a 23c tire on the Altamonts, and so a 23c would "match" better.

Below is a graphic from Hed showing how rim inner width and casing size relate. Altamonts are 18.1mm internal:



Whether the variance is, in your real-life application, appreciably substantial given the other factors affecting wheel performance, e.g. rider weight, road conditions, inlation pressure, riding speed, etc., I don't know. My thinking is that unless you're particularly heavy (like north of 230lbs, maybe), go 23c tire.

In any case, tires wear out, especially hi-performance ones, so you'll be able to cHange out sizes in short time, and nothing beats trying out the options for yourself.
thank you that was very help full and yeah i've been using 25mm tyres and well i love the comfort and fast rolling i can corner real on the 25's, but they do stick out quite a bit and im thinking its killing the aero advantage but i think i may be trying some 23mm tyres next time i replace them im around 175 lbs so i don't think the 23mm tyres will suffer so much .
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Old 02-05-16, 05:47 AM
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The wider the tire, the more likely the width will more closely match your down and seatube diameters.
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Old 02-05-16, 06:25 AM
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Originally Posted by allen254
i have a set of boyd altamonts, and at their widest they are 24mm i was wondering which tyre width would be better in terms of aero vs cornering vs comfort which combination would be better, 23mm tyres, or 25mm tyres, which would be better. i'm thinking of getting vittoria open corsa cx tyres but not sure which tyre combination to try. boyd says the altamonts were designed around a 25mm tyre but doesn't that make the tyre 1mm wider than the rim and kinda ruin the aero part, aren't they supposed to match??

i just found out they make vittoria open corsa's sl in 24mm would that be a better match???
Not necessarily "match" - note how aero things often get broader before narrowing? The ideal aero situation for an Altamont would probably be a 23mm GP4KSii or 22mm Attack to make the overall shape of the tire + rim be that narrow-wide-narrow shape. Of course, this is all relative, and something like a 25mm GP4KSii will be more aero than the 24mm Open Corsa, because the Contis are super aero and the Vittorias are rather famously not.

One thing I learned in my last big tri, though, was that chasing pure aero can kill your speed if everything else isn't perfect. Specifically, I ran high-pressure Attacks on my November Rail 52s. On the first half of the course it was beautiful and fast and glorious, on smooth tarmac. On the second half of the course, which was narsty chip-seal, I had to slow down a lot because the vibration was so bad I thought my bike was going to fall apart beneath me and my feet were hurting. I'd have been faster overall with low-pressure 25s that would have absorbed a lot of that, even though I'd have given up a bit of speed on the first half of the loop. I have no idea what the roads are like where you are, but it's something to think about.
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Old 02-05-16, 06:55 AM
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If you have the newer Altamonts, then the internal width has gone up to just under 20mm.

In regards to aerodynamics, the 23mm tire is more aero than the 25 at all wind angles, but you really start to see a difference in higher crosswind situations. This is also why running a 23mm tire will give you better crosswind handling in those high wind days.

Unfortunately I do not have wind tunnel data on the Altamont comparing a 23mm and 25mm tire. It is something we have planned this year at some point.
However, there is a trend when comparing 23mm tires to 25mm tires in aero situations.
Take a look at the following wind tunnel graph


On the left is grams of drag (lower is more aero). On the bottom is yaw angle (the angle at which the wind is hitting you).
This is taken from our 44mm clinchers, but I have tested a lot of wheels in the tunnel with 23mm and 25mm tires and have seen this trend on almost every single wheel.

Until about 12.5 degrees yaw angle, the lines take the same shape but the grams of drag are different. In this case they stay about 25-30 grams of drag apart (at 30mph).

Once you hit 12.5 degrees of yaw angle, this is where the back side of the rim comes into play and how the air flows over the rim onto the tire. This is why older V shape rims tend to get less aero in crosswinds, whereas the newer shaped rims do better in crosswinds (because there are two air foil shapes). This is also what affects crosswind handling as that air flow on the backside of the rim can push the wheel around if you don't have a good shape on the back side of the rim.

So, back to 23 vs. 25mm
In heavy crosswinds is where you'll see both the better aerodynamics and the better crosswind stability. With a wheel like the Altamont (30mm deep), crosswind stability isn't that much of a consideration like it would be on a 60mm deep carbon wheel. There are other things at play besides aerodynamics as well. With a 25mm tire, your rolling resistance goes down due to a wider but shorter contact patch with the asphalt.
When you are riding, your rolling resistance forces are a constant. They are always there and speed does not affect them (road surface does). Your aero forces exponentially rise the faster you go. So, if you are going 12 mph you don't have a lot of aero forces going against you but the rolling resistance forces are a higher percentage of your overall forces. If you are going 30mph, then aero forces are much higher and the rolling resistance becomes a much lower percentage of overall forces.

The 25mm tire has better rolling resistance, handling, and vibration dampening. For myself I like to ride in the mountains with slower climbing and fast descending while leaning the bike over. I also do a lot of crits with high speed cornering. . .so 25's are a no brainer. When I am doing a time trial, I put a 23mm tire on the front. If your riding is on flatter terrain, without a lot of turning, and the roads are in good condition then a 23mm tire will be a great option.
If you are doing turning, climbing, descending, or riding on rougher roads, then a 25mm tire is definitely the way to go.

Either way, both sizes work great with the wheels.
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Old 02-05-16, 08:41 AM
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Great reply, CoachBoyd, and thanks for noting the bead seat width increase on the newer Altamonts, which I neglected to do.
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Old 02-05-16, 12:35 PM
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Thanks so much for your reply. and they are not the newer ones, I wish haha , they are a early 2015 altamont build from my you guys at Boyd. and yes 25mm is what I use and I feel no problems with them great wheels set with great specs thanks again.
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