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How much can we change our bike wheel diameters before needing another bike fit?

If we change the size of our bike tires, it raises or lowers the front or rear of the bike. That means it can affect the reach, handlebar height, setback, saddle angle, and handlebar angle. That means even after tilting the seat up or down to make it flat, we need the setback readjusted, and we likely need a new stem to readjust the reach and handlebar height. Adjusting the setback affects the reach.

It seems like even a mm of adjustment can be significant in terms of optimizing our bike fit. If we extend our knees, our hamstrings are engaged when extending our hips like on the downstroke but if our knees are bent, they don't get activated so we get less power. Most of us notice the stretch in our hamstrings when when trying to reach our toes with our knees extended. There's a reason why a bike fit can take at least 2 hours. Some bike fitters recommend another bike fit when we get new shoes as they can have different sole heights in addition to needing the cleats adjusted.

Sometimes we need or want to change the diameters of the wheels.

Examples include:

  • Tire clearances are different for both the front and rear wheels while the rider desires fitting the widest tires possible. Adding a fender also decrease tire clearances.
  • Even replacement tires with the same width markings can have slightly different actual widths when installed onto the rim.
  • Rim replacement since changing the rim inner widths can affect the tire widths.
  • Spare wheels since some cyclists have multiple wheels for different conditions, race days and training, different gear ratios, etc.
  • Upgrading the hub to a hub dynamo, internal hub gearing, coaster brake, power meter, etc. The new hub may have different hole counts so a new rim, which may have a slightly different inner width may be needed.

How much can we increase the difference in the two wheel diameters before we need another bike fit?

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    "It seems like even a mm of adjustment can be significant in terms of optimizing our bike fit." Not really. Stems, cranks and so on aren't generally sized to the millimetre. – David Richerby Dec 12 '18 at 10:43
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    "If we change the size of our bike tires, it raises or lowers the front or rear of the bike. That means it can affect the reach, handlebar height, setback, saddle angle, and handlebar angle. " I don't agree with this. All the geometry you list is independent of tyre diameter. It is based on hard points in the frame. Unless you are building some kind of custom project, you will always be running wheels of virtually identical diameter. Extreme changes in wheel diameter might affect standover height or BB clearance, but they don't affect rider fit in motion, just usablity. – Baracus Dec 12 '18 at 10:55
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    This will sound rude and I apologise for that. But a lot of your questions really do seem to be based on hugely over-thinking minute details that really don't make any difference to most people. Have you actually had a bike fit? Have you actually changed (or contemplated changing) your wheels? Or, rather, changed just one of your wheels, since changing both of them will make no difference at all. Just get on the bike and ride it; if it doesn't feel right, then consider fit adjustments. And, as I understand it, you're riding a hybrid anyway, so minute details of fit probably don't affect you. – David Richerby Dec 12 '18 at 11:42
  • The only thing that MIGHT affect "fit" (in a minimal way) is changing the outer diameter of only one tire, so that the bike tilts forward or backwards more than before the change. But this would be minimal. Tire diameter does affect standover height, but that measure is rarely so critical that a change of, say, 15mm would make any difference. – Daniel R Hicks Dec 12 '18 at 12:47
  • @DavidRicherby I had 2 bike fits on different bikes but they weren't properly done as one was the wrong size and the other was done with improper pedalling and too low of power. I'm considering a hub dynamo. Do you think bike fit is less critical for hybrids b/c the posture tends to be more upright? Or is it b/c it's associated w/ recreational riders? What if an amateur or elite rider uses a hybrid bike? The difference is that they ride longer and often harder so I feel that fit may be as important. I understand that if we ride occasionally and for a short time, we can get away w/ a poor fit. – Han-Lin Dec 13 '18 at 4:14
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The thing about bike fit is that it's almost exclusively about relating your body position to the mechanical parts of the bike. The relationship between handlebar, saddle and pedals is unaffected by changing the wheels.

If you make a change to one wheel diameter it will be equivalent to the tiniest of slopes. Let's take a worst case: Say you go from 35mm tyres to 25mm on the same rim (a 10mm difference in nominal radius, which is what actually matters) on a bike with an unusually short 1m wheelbase. That's a 1% change. Does your fit change on a 1% slope? More typical changes on more typical bikes would work out closer to 0.2%. I doubt you can even measure whether the saddle is level to that precision, for example, never mind whether it should be. You'd have to change the bead seat diameter for it to make much difference, and while people do put 650B wheels on 700C frames they normally do both at once.

Shoes are very different, in that they come between your body and the bike, and in a particularly important way.

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You would need a really big difference in effective front and rear tire diameter to create enough tilt in the frame to effect rider position.

Replacing one rim with a slightly wider or narrower one, or replacing a tire with one of nominally the same size will make no appreciable difference. You would get the same level of effect by increasing the difference in tire pressure between the two wheels.

  • In fact as the back wheel carries more weight, you shoudl already account for compression with pressure. – Chris H Dec 12 '18 at 11:54
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If I understand your question correctly, you are worried about your bike fit might change when you tilt your bike to the front or to the back.

I'd ask you another question - do you change your bike fit when riding up the hill or down the hill?

Imagine you are climbing a 7.5% hill (say it's Mount Ventoux). This incline means that for every meter distance you climb 7.5 cm - on the average.
The axle distance is usually around 100 cm for the road bikes. Which makes your bar 7.5 cm higher than usual. Will you change your bike fit for the occasion of climbing and then again when you're about to descend?

If your standard tyre size is 700C-25 and you happen to put 23 in the front and 28 in the back, the front of the bike will be 5 mm lower than the rear of the bike. You can compare this to a 0.5% descend. Since you didn't change your bike fit for Mount Ventoux why would you do it for a mere 0.5% descend? Even if you make 1 cm difference in tyres, it's still 1% decline. Do you notice it while riding?

TL;DR; - the tyres that would cause the necessity for bike fit change are not available for purchase.

  • The tyres are avilable for purchase, but wouldn't fit the same rim or frame. I'm sure you could build a wheel that would take 23mm tyres and put it on a fatbike with a 4.8" (120mm) tyre on the other wheel – Chris H Dec 12 '18 at 12:39
  • If I built a bike entirely for climbing or descending, I would use different setup than one do it all bike. Also, check the difference between usual DH and XC setups in MTBs. – ojs Dec 12 '18 at 14:59

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