I know for fit purposes that this isn’t the correct dimension. But for how your body presents itself to the environment, it matters.
With staggered tire sizes front/rear, does this actually contribute to your “stack height” as viewed from the road surface upwards? This was why they dictated wheel sizes back in the day. Folks were putting really small front wheels on bikes with larger rear ones. It tilted you forward with no consequence to hip angle.
So, for 23/25 front to rear…that would be 2mm of reduced stack. If you own a really really modern rear wheel or disc built around 28’s or even 30’s…would that mean that 23/28 would be 5mm of reduced stack? A 23/30 7mm?
If so, 5mm is 0.20". That’s about the height of the shorter stack spacers on my cockpit.
So, if a skinny rider won’t get much extra from a 25mm or 28mm front tire on CRR but is pretty quick for the aero to kick in…would running a “funny car” staggered tire setup provide a tangible aero benefit from your “stack height” as viewed from the road surface?
Ok, that’s all the idiocy I have for today. I’ve had too much morning coffee.
Yes. Bike will also rotate a bit so e.g. reach and saddle setback will change a bit.
Fun story. Handful of years ago before bike check at a TT I told my team mate he needed to put his race rear wheel on before getting it measured in the jig or else he would likely run afoul of the UCI 80cm extension tip limit due to his training rear wheel having a larger tyre on. He didn’t believe me and went up to get it checked. Sure enough, it was outside 80cm limit. Came back, changed to his race rear wheel, got it checked again and now it was fine. He didn’t question my TT input and always went to the check with race wheels installed after that day
Your saddle and BB are forward of the rear axle, so they aren’t going to be raised as much. So I don’t think you’re going to get a full 5mm drop with a 5mm tire difference…
Your saddle and BB are forward of the rear axle, so they aren’t going to be raised as much. So I don’t think you’re going to get a full 5mm drop with a 5mm tire difference…
This ↑. Also, tires tend to be “wider” than tall, so a 5mm difference in nominal tire size probably means more like a 4-4.5mm difference in mounted tire height. A 5mm difference in nominal tire size probably results in a net stack difference of 2.5-3 mm. And a slightly steeper or slacker head tube angle, depending on which end of the bike you mount the bigger tire.
Tire pressure and tread shape also matter. Tire stack is rarely precisely the advertised size. I’ll take Cyclocross tires as an example, running a Challenge file tread and a Michelin mud Both advertised as 700x33 tires, but one runs true to size, and the other actually measures closer to 36mm even at low PSI (it was actually a frame clearance issue with the Muds, my older aluminum frame didn’t have enough clearance for them, and we actually had to hammer out the frame, to create enough clearance to run a sufficiently treaded tire to handle some of the nasty muddy races we get in November… That said I tend not to stagger tire sizes, only tire pressure.
Yeah, I wasn’t seriously doing anything related to this. Just a caffeine fueled random thought.
Good TT bike speeds are going to slow as weather cools and clothing to stay warm is donned. So, keeping the mind busy. Maybe not though. Race helmet may keep me warm.
I suspect next to see a 10cm rule update due to Ganna and that forearm pad trend. Smart, but just going to make crap tougher after more rules.
The difference in height from axle to axle is ~75% of the difference in tire diameter (due to the rim/bead interface), so a 23->25 is a difference of only about 1.5mm.
The drop is axle-to-axle, which is longer than saddle-to-armrests…so now we’re at 1.2mm.
The bike is slightly tilted, so this is different from just dropping the bars 1.2mm (unless you adjust the angle of the saddle and bars). You position is unchanged, but the gravitational pull is ever so slightly different since you are slightly rotated but the gravitational field is not.
Just some sample math as you get ready to vault that mouse turd (from my bike):
Wheelbase: 96cm
Drop due to 23F/25R: 1.5mm
Angle off horizontal created: 0.0895246919 deg
I’m quite sure my bars and saddle are currently level with 1/10 of a degree. I’m also quite sure I could feel the difference.
