Technical FAQ: Minimum tire pressure and more

Lennard Zinn /
Photo: Caley Fretz | VeloNews.com

On minimum tire pressure

Dear Lennard,
I read with interest your reply to Manny with regard to 25mm tyre pressures. I recently purchased 25mm Vittoria Open Corsa SC tyres, and these are clearly marked with “Clincher MIN to MAX pressure: 115 to 145 PSI.” This seems quite unreasonably high to me, so I’m wondering what is a safe minimum pressure to use with 25mm clinchers (would be interesting to hear from your contacts at other tyre manufacturers), and why does Vittoria see the need for such a high minimum pressure?
— Simon

Dear Simon (I sent this answer directly to Simon),
Here is the response to your question from Vittoria:

According to ETRTO, bicycle tires are allowed to deflect 30% of its height at maximum load only. We respect the ETRTO, but we do not limit the body weight of our customers. 115PSI minimum air pressure is the consequence for our high-end 25mm tire with its very flexible casing; the minimum air pressure is related to the worst case: heavy load, rear wheel, aged tire.

We will address this matter more precisely on our MY16 new models.
— Christian Lademann – Product Manager
Vittoria S.p.A.

I’m not going to tell you to do anything differently with your Vittoria tire than Vittoria’s product manager just told you. However, since you’re asking generally what minimum pressure you can with 25mm clinchers in general, I can answer it generally. The safe minimum tire pressure is certainly a function of rider weight, and you’ve not given me yours. I, at 174 pounds, have ridden safely for extended periods on many different 25mm clinchers, both standard clinchers and open tubular clinchers on smooth roads, at as low as 75 psi in the rear and 65 psi in the front. I haven’t measured to see if I get over 30% tire drop at those pressures, but tire squirm is not an issue for me at those pressures.
― Lennard

Dear Lennard,
Thanks for getting a half-sensible answer from Vittoria, although I do have to ask what they classify as a heavy load – do they consider worst-case to be a 300kg version of Sagan pulling wheelies? For the record I’m 80kg (176 pounds), so I don’t consider myself to be at the heavy end of the spectrum.

I decided to run my own test, as I’m sure wheel rim width is also a consideration. I’m using HED Jets, which have the 23mm wide C2 rims, and used butyl rubber inner tubes. The setup was a simple stationary test. I simply put my bike on its rear-wheel wheel stand, inflated the front tyre to the desired pressure, then stood in front of the bike with my hands on the handlebars and pushed down until I lifted myself off the ground. To capture the measurement I simply tied a steel ruler loosely to one of the spokes (not fixed to the spoke, rather just enough to keep the ruler upright during measurement), rotated the wheel so the ruler was perpendicular to the wheel contact patch with the floor, and setup my bike camera to capture the tyre deflection. I did the test on both Continental Gatorskins and Vittoria Open Corsa SC, these are the results rounded to the nearest millimeter:

Conti Gatorskins
No load tyre height (rim to floor): 26mm
60PSI : 6mm deflection, or 23%
70 PSI : 5mm, 20%
80 PSI : 5mm, 20%
90 PSI : 4mm, 15%
100 PSI : 4mm, 15%
110 PSI : 3mm, 12%
120 PSI : 3mm, 12%

Vittoria Open Corsa SC
No load tyre height (rim to floor): 26mm
60PSI : 7mm deflection, or 27%
70 PSI : 6mm, 23%
80 PSI : 5mm, 20%
90 PSI : 4mm, 15%
100 PSI : 3mm, 12%
110 PSI : 3mm, 12%
120 PSI : 3mm, 12%

While this is a static test and therefore doesn’t take into account Paris-Roubaix style cobbles, potholes, or road curbs, it is essentially bearing my full weight on one tyre. It seems you’d need to be running really low pressures to obtain 30% tyre deflection, and it also shows that regardless of the tyre construction being a “supple” cotton casing or a “bullet proof” rubber/Kevlar construction the deflection is similar until you get to really low pressures.
— Simon

Dear Simon,
Cool experiment! Thanks for doing that and giving us the results!
― Lennard

Mounting a Campagnolo battery

Dear Lennard,
I am lucky enough to have a Record EPS group being installed on a Cervelo S3, size 48. The shop doing the build said that the internal Campy battery I have will not mount internally in the down tube because the bolts provided are not long enough. They also said the seat post and seat tube are too small to house the battery. They are mounting the battery with the Campy external mounting kit, but I was wondering if there is a way to get longer mounting bolts. I cannot seem to find anything about this online or in the Campy catalogue. The bolts, as it was explained to me, are pretty unique. If there is another solution to getting the battery to rest safely inside, I’d be very grateful.
— Ryan

Dear Ryan,
I don’t think it requires all that much imagination to come up with a solution to use the internal EPS battery in the down tube of your bike, if the supplied bolts are indeed too short. For those wondering, the internal Campy EPS (V2) battery screws in behind the bottle bosses inside (ideally) the seat tube, or inside the down tube in the case of an aero seat tube too narrow for the battery to fit. If memory serves, the kit comes with three bolts, all of different length, and you only use two; I wonder if one of them is long enough …

Yes, the EPS V2 internal-battery-mounting bolts are unique, having M4 thread that will pass through the M5 threaded bottle boss bole and into the battery, then a hex head, and then M5 thread above that for a nut to hold the bottle cage on. But with some M4 (4mm X 0.7) all-thread, you could certainly secure that battery in your down tube.

You could use pieces of the all-thread long enough to thread into the battery holes, then drop a washer followed by a nut onto each, and snug the nut up against the face of the bottle boss. Then you could drop the bottle cage onto the extensions of the all-thread sticking up from the nuts, and put another washer and nut on each to tighten the cage on. Then you’d cut off the excess length of the threaded rods so they wouldn’t tear up your bottle.

You still need the special Campagnolo installation tools to mount the EPS V2 battery in the down tube. Those consist of a pair of cables with a threaded end to screw into the end of the battery and suspend it inside the down tube while you secure the battery by tightening it in place with bolts through the bottle bosses. Thread the tools in loosely enough that you can unscrew them by twisting the cables counterclockwise after the battery is secured in place.
― Lennard

Staying hydrated

Dear Lennard,
Your reader, Jeff, appears to be looking for a solution so he does not have to carry water on his back. There is a wonderful product available that solves this problem: showerspass.com/veleau
— Matt

Regarding drilling holes in carbon frames

Dear Lennard,
In this week’s FAQ the question of owners drilling holes in carbon frames was discussed (in this case it was holes in the down tube), and it was considered to be a bad idea. However, in the past I have seen you advocate drilling bottom bracket drain holes in bikes that didn’t have them. Is this still true? Or shouldn’t you drill drain holes in carbon BB’s either? The center of the BB seems to be a relatively low stress area that also has a large wall thickness so I wouldn’t think it would be a problem. What is your take?
— David

Dear David,
While I was thinking about metal frames when I wrote that, I’m confident that drilling a small drain hole in the underside of a bottom bracket shell would not lead to failure there. That’s a thick area that sees minimal stress.

On the other hand, a carbon frame can’t rust, so the necessity for drainage is not the same as in steel, aluminum, or magnesium frames.
― Lennard

Dear Lennard,
Though I agree drilling big holes in frames is a huge risk, and understand comments from manufacturers, there are situations where this could be OK.

Last spring I built up a Pinarello FCX with Shimano hydraulic disc Di2. To route wiring internally I had to drill two small holes for the front and rear dérailleur e-tube wires. I was careful to choose locations that would be loaded in compression rather than tension – understanding that areas with carbon in tension would likely lead to failure. I realize I voided the warranty, but a year later I’ve had zero issues.

I also work part time at a shop and have done the same thing on two other bikes with no issues.

Not a job for the faint of heart or with an unsteady hand, but not something that creates big concerns IMHO.
— Paul

Dear Paul,
I got a number of letters from people trying to figure out how they could safely drill holes in their carbon frames. I just can’t get behind any of them, unless you have the okay of the manufacturer. While your method is safer than not taking tube stress into account, it requires an understanding of frame loading. And, you have no idea of the internal construction of the frame; the manufacturer may have already minimized material in that area due to its relatively low stress.
― Lennard