105% rule - is there an optimum ratio?

[hambini]

The trend is back to non-terroidal shapes as seen in the raynolds wheels above. Part of the design behind terroidal was more about wind stability and side forces, not just aero gains. The idea was that the leading edge and trailing edge had similar appearances to the wind. So side forces would not torque the wheels as much... or so was the theory.

But in the end, you want a shape that replicates an airfoil. The trick is that airfoil is reversed on the trailing edge of the wheel. But then airflow has already been disturbed... so maybe it does not matter as much.

Terroidal are not as good as a leading edge, but better as a trailing edge.

And the linked hambini video (god i hate him) relates to the alpha angle i was referring to prior. I.e. the wider the tire, the deeper the rim must be to recapture disturbed air. But he basically reinforces the rule of 105 by advising against tires being wider than a rim. Different rim shapes will tolerate tire sizes differently, so the rule of 105 will vary based on the rim shape/depth. Of note, this is a flaw in his wind tunnel data imho as he used gp4000s2's in 23c at 120psi or something moronic which would put them at 26-27mm wide easy on many of those wheels. So assuming his data is valid, the test is as much about how a rim handles a tire at that width than anything. Don't quote me, but Since then i think he has swapped to testing with a gp5000 that is narrower, and not as aero... so it kind of invalidates any data comparison. But i may be wrong on his protocol changing.

I don't know what you have against me but for info the test tyre has not changed yet.

To clarify this statement to anyone else who is reading so the rule of 105 will vary based on the rim shape/depth.

It's essentially no longer a rule. My guidance is as follows

Pick a tyre that is no wider and ideally the same width as the rim
Ensure the minimum rim depth is as follows:
23mm tyre -> 45mm minimum rim depth
25mm tyre -> 55mm minimum rim depth
28mm tyre -> 80mm minimum rim depth

Picking a rim that is wider than the tyre is no where near as good as getting one that is level.

Thanks

Hambini

[neeb]

To clarify this statement to anyone else who is reading so the rule of 105 will vary based on the rim shape/depth.

It's essentially no longer a rule. My guidance is as follows

Pick a tyre that is no wider and ideally the same width as the rim
Ensure the minimum rim depth is as follows:
23mm tyre -> 45mm minimum rim depth
25mm tyre -> 55mm minimum rim depth
28mm tyre -> 80mm minimum rim depth

Picking a rim that is wider than the tyre is no where near as good as getting one that is level.

Thanks

Hambini

Interesting - but this is completely contrary to the Silca article and the data in that. Why is that wrong?

[AJS914]

Why the mimimum depth for different sized tires? Don't you just match the tire to the rim width? For example, you can find a 45mm x 30mm wide rim these days. That would seem to work perfectly with a 28mm tire.

[NordicSal]

I believe it's something like the wider the tire the more it disrupts the airflow (logic). Then it needs a deeper rim to stabilize the airflow after it's been disrupted by the wider tire.

[RocketRacing]

To clarify this statement to anyone else who is reading so the rule of 105 will vary based on the rim shape/depth.

It's essentially no longer a rule. My guidance is as follows

Pick a tyre that is no wider and ideally the same width as the rim
Ensure the minimum rim depth is as follows:
23mm tyre -> 45mm minimum rim depth
25mm tyre -> 55mm minimum rim depth
28mm tyre -> 80mm minimum rim depth

Picking a rim that is wider than the tyre is no where near as good as getting one that is level.

Thanks

Hambini

Interesting - but this is completely contrary to the Silca article and the data in that. Why is that wrong?

Because hambini is wrong... or maybe oversimplifying? Conceptually he should be right so it is a safe armchair call. A smoother airfoil is faster than one that is not. Would love to see the hambini data, but i am sure he is not willing to drop to that level. But interestingly, he is contradicting his own assumptions in his own video (the why the rather wide light bike rims did well. He presumed the very wide rim edges near the tire acted as a trip strip. It likely depends on the rim design).

I recall independant testing of different tires on a given rim, and the individual found that a 21c tire outperformed 23 or 25c on the same rim. I think it was 25-28mm wide. The point is, the narrower version of the tire was fastest. The silca data shows a similar trend, but to a point, smaller tires do not give much added advantage.

My view is that it is less about the "rules", and we are really talking more about rough guidelines. What is "best" is likely dependant on tire (shape, tread pattern), and the rim shape. At the end of the day, we are looking to get something as close to an airfoil shape as possible... and deeper is better. So what is ideal for terroidal, may not be ideal for v shaped... but it is all on the marginal gains side of things.

Tires are key too. Many wheels now are designed around a specific tire (size). It seems that the lowest crr tires... ones with thin sidewalls often tend towards a rounder shape, which makes for a more jagged interface between rim and tire, even if widths are matched (we know maic strips, or silicone filling this area makes wheels faster). And round is less aero than egg shaped.

A gp4000s2 is still more aero than the 5000, and is consitered to be the most aero tire to date. I think it is partly due to shape (egg), and the tread (trip strip). i will armchair logic this one and assume that a tire with a gp4000 sees no real gains from a rim that also acts as a trip strip, because the tire is the trip strip. Vs a corsa speed with no real tread, that could likely benefit from a rim trip strip, i tentional, or not.

Back to hambinis comment of "picking a rim wider than the tire is inferior to a matching width" could make sense at low yaw. The silca data, and the bontragger data linked in their article (the rim with the tire smaller than the rim with had the most delayed seperation at -12.5 degrees yaw) is not wrong, for those tires and rim shapes tested, but at low yaw, the extra rim width is just more surface area. So depending on the average wind speeds and yaws one would see on a ride, the fastest option might differ.

In reality is it all very complex, and the best option is far from clear.

Too wide a tire and you hurt aero (surface are and cda), but you can improve crr to offset any cda/sa losses. The faster you go, the more aero (low yaw) is key. The higher yaw you see, the more high yaw aero performance is key. The faster you ride, the less yaw you see.

A while back i stopped stressing, got gp5000's in the widest width that made sense for my rims (25c), and i ride.

[RocketRacing]

Double post

[pmprego]

A while back i stopped stressing, got gp5000's in the widest width that made sense for my rims (25c), and i ride.

This!!

As for hambini, he's the only one with a phd in aerodynamics. All the others just have fake phd and are dumb as donkeys.

[hambini]

If I park trip aerodynamics to one side

When air is moved a certain distance from is original starting point it takes a rough linear distance in the flow direction to move back smoothly

This is why I have quoted widths versus rim depths.

The light bicycle wheels are an anomaly which I have discussed on my zipp video.

Rocket racing did aerodynamics from his computer desk by making papery planes and quoting zipp and other wheel manufacturer data.

Listen to him at your peril.

[hambini]

To add some more.

One source you may have heard of is NASA

Their standard aerofoils are NACA. A popular and well studied one is the 0024.

That has a width to length ratio of 5. That ratio is fixed with scaling. It is technically called cord length but I have simplified it.

Same principle applies to bike wheels. The wider you go, the deeper it needs to be. The alpha critical angle cannot be exceeded.

Hope that helps. Hambini

[emotive]

If I park trip aerodynamics to one side

When air is moved a certain distance from is original starting point it takes a rough linear distance in the flow direction to move back smoothly

This is why I have quoted widths versus rim depths.

This makes sense to me, but I see a trend to wider wheels to support the trend to 28mm tyres. What are your thoughts on the Hunt Limitless 48's?

According to Hunt and their aero trials against big-name brands their wheels outperform due to their 34mm width.

[AJS914]

That has a width to length ratio of 5. That ratio is fixed with scaling. It is technically called cord length but I have simplified it.

Same principle applies to bike wheels. The wider you go, the deeper it needs to be. The alpha critical angle cannot be exceeded.

Hope that helps. Hambini

Thanks for explaining.

I guess people will need to compromise. Lots of people probably want to run 28mm tires (maybe even measuring 30mm on a wide rim) but I'm also guessing that they don't want to run 80mm depth rims at the same time.

[jeanjacques]

Ensure the minimum rim depth is as follows:
23mm tyre -> 45mm minimum rim depth
25mm tyre -> 55mm minimum rim depth
28mm tyre -> 80mm minimum rim depth

@Hambini, It's seem logic to keep a cross section patern almost identical but can you explain why keep it strictly identical isn't enough to acheive optimal aero ? The number with an identical patern as a 23/45 will be 25/49 and 28/55.
Is it the same for an airfoil with the non-linear chord lenght ?

[jlok]

I think he has explained Alpha Critical Angle
viewtopic.php?t=157371&start=60

[hambini]

Ensure the minimum rim depth is as follows:
23mm tyre -> 45mm minimum rim depth
25mm tyre -> 55mm minimum rim depth
28mm tyre -> 80mm minimum rim depth

@Hambini, It's seem logic to keep a cross section patern almost identical but can you explain why keep it strictly identical isn't enough to acheive optimal aero ? The number with an identical patern as a 23/45 will be 25/49 and 28/55.
Is it the same for an airfoil with the non-linear chord lenght ?

The following is a simplified answer so it's not 100% correct but you'll get the idea.

NACA 0024 is deciphered as follows

00 - indicates it's a symmetrical aerofoil with no camber
2 - multiply by 10% gives you the maximum width, in this case 20% of the chord length
4 - multiply by 10% gives you the position of the maximum width, in this case, 40% of the chord length.

On a bike tyre and rim, the position of the maximum width drops as a percentage as the rim gets deeper so it's not a linear arrangement. This must be considered in conjunction with the major (and Huge) difference that the wheel is rotating.

The aerodynamics are not linear and there is a bit of empirical testing in there.

Be aware that toroidal rims were the rage 5 or 6 years ago, now we have gone back to V shaped rims. The claims are that new aerodynamics have come out. My answer to that is; it's total BS and they want to sell more wheels.

There are some combinations that do not fit my rules of thumb perfectly but they are rare.

Hambini

[hambini]

A while back i stopped stressing, got gp5000's in the widest width that made sense for my rims (25c), and i ride.

This!!

As for hambini, he's the only one with a phd in aerodynamics. All the others just have fake phd and are dumb as donkeys.

I could not have put this better myself.