I’m running a 23mm wide front tire on a pretty wide 80mm deep rim and it looks fast but there is a fairly large lip between the tire and rim. What do you put in there? Silicon sealant or something? I’d like to be able to peel it out of there in the future without making a huge mess or damaging the tire (corsa speed).
Joe
Hmmm not sure what your looking for? The 105% rule of thumb applies to tire width vs rim width. The rim should be 105% of the tire but that comes from Zip wind tunnel test 10+ years ago well before 28mm tires. I generally try and follow this rule but I don’t know if any other data has been released to update the thinking.
Maybe you are about to invent a new product that we all cannot live without!
There’s an excellent aerodynamic reason for doing it. You could use some sort of tape? I believe Mavic did that years ago but it was outlawed by UCI or something… 
Yeah I’m fairly certain they had a whole aero system of wheels, tires, and then a tape to fill in the gap. Because no matter how “compatible” a rim and tire are they just are not going to line up perfectly straight. Josh Poertner from Zipp/Silca said on his podcast that it made a fairly large difference at higher racing and TT speeds. I’m not going to try to guess the wattage saved though
Yeah the Mavic thing was a plastic strip that sorta snapped into the gap and yeah the UCI didn’t allow it. But back in the day people filled in the gap with some kind of caulk or something. What did they use?
The 105…I’m not sure where I am with that, the tire is pretty narrow and the rim is pretty wide so it might be more than 105%. I have trouble measuring the width of the rim because it’s back a fair bit from either edge (if that makes any sense).
On a side note…Is 105 the best or just the most reasonable? Could 110 or 120% be faster?
Joe
Noted, thank you 
Those guy were using something back in the day. Any old time TT riders out there??
@Joe get yourself some calipers man. Cheap plastic ones from the hardware store work great. Or go digital for $9 on Amazon.
Josh Poertner briefly mentioned this and said that you should NOT use caulk because that will be aerodynamically worse. (edit: actually he said it will make rolling resistance higher and that wastes more energy than the aero gains, see below).
Think about it: the caulk is not going to be really smooth and it’s going to create a bit of a lip at the edge on the rim. It’s also not going to be uniform. You are better off just leaving that gap alone.
For the 105% rule, it is a minimum amount that your rim should be wider than the tire, using actual widths measured. More is better, but you can’t realistically get more than about 110% with light weight rims because there’s only so much difference between inner width and outer width of the rim (usually 5-8 mm) and your Tire has to be wider than the rim internal width by a big enough factor or it will roll off.
My rims are 32 external, 25 internal. I have 28 mm tires on them that stretch out to 30 to 30.5 mm due to the wider rim. That barely hits 105%. To get to 110%, I would need tires that measure 29 mm on the wide rim, meaning they would have to be tires that are labeled 27 mm (and actually true to label, which is based on a 17 mm internal rim). But then I would be at higher risk of the tire coming off on hard cornering.
There’s ERTO guidelines for what width tires can run on what width rims. Too wide or too narrow can cause issues. Unfortunately those ERTO guidelines are outdated and too conservative. There’s been work in updating them, but committees are slow.
You just gotta know what you are doing…I can lay down a bead of caulk that is ultra-smooth. Gotta moisten your finger or a rag and then wipe away the excess caulk.
Never did it in a tire though…
+1
The other problem with silicone is that, once dry, you won’t be able to get anything to stick to it. Even more silicone.
You’ll be stuck sanding, or scraping the rim back to the base material.
I’ve got carbon kayaks that I’ve used Sikaflex on before so it can be removed if needed. It’s flexible and can be sanded once fully cured.
I have no idea if it’s a good idea to caulk the tire/rim interface, but if I was going to do it I’d use a silicone applicator and some soapy water in a spray bottle to smooth it into shape. Once dry you can further smooth it with fine sand paper.
Silicone caulk is the way to go.
I just listened to an episode of the Margin Gains podcast. They talked specifically about this and how people used to do it. Essentially it makes the tire/wheel more aero, but it increases rolling resistance because it increases the hysteresis of the tyre; Josh’s advice was don’t do it - the aero gains are not enough to outweigh the rolling resistance increases.
That’s the discussion I was trying to remember. I knew he said not to do it, but couldn’t remember why. That makes a lot of sense. It’s a net negative
I can definitely see how that might be. It’s just…such a huge gap, it’s gotta be slowing me down…doesn’t it? I’m not sure what to do…maybe go back to a 25mm tire.
Don’t let it get into your head! I think the most important things are aero, rolling resistance, tyre pressure, weight, and then probably everything else like the shape formed between the tyre and the rim.
You’ve already got a deep section rim, and a super fast tyre. Make sure your tyre pressure is dialled - a smooth ride is fast!
On the tyre size, the rule of thumb is that the maximum rim width should be at least 105% of the inflated tyre profile. My tyres are 27mm (inflated on the same wheel) so I would want my wheel width to be at least 28.35mm. This basically helps the wheel recapture the disturbed air coming off the tyre.
I’m sure there is an aero effect from the gap between your tyre and rim; BUT I’m also sure that it is much less than the negative effect of increasing the rolling resistance or getting the tyre pressure wrong.
I think the best advice is run the biggest tyre you can without breaking the 105% rule; dial in the pressure; and then focus on producing more power 
I would love to see what the other low hanging fruit looks like on your setup before even entertaining these changes. Specifically optimizing your position
absolutely position is the most important thing on the list; then I assume wheels are next; possibly followed by helmet/head position. I’ve never seen an actual list in order of priority/importance…
Helmet over wheels…
is that definitive? I know how much difference my 78mm deep dish wheels make, is the helmet really more important over all?
position, clothing, helmet (i forget if helmet is before/after clothing), then wheels.
But in terms of actual gains, position is MASSIVE, helmet clothing is big, wheels are minor
