I have a few questions that I’ve been researching for a while with no resolution:
It seems that ideal tire pressure is based on everyone’s feel. There are a number of calculators, and I use a mix of the figures from this one: http://www.dorkypantsr.us/bike-tire-pressure-calculator.html 5 but I’ll see people say go as low as possible until the tire feels a bit…off. I think I know what that means, but I’m not sure, and testing a tire to go low as possible seems dangerous - you have to push the tire to know when the pressure is too low right, and the more you push it (high speeds, hard turns)…
It’s all very subjective and I don’t know if I’m doing the right things. I am fairly light (65KG) so I don’t ever need to run (25 and 28mm) tires at 100PSI so perhaps I’m lucky here.
Related to the above, people agree that tubeless lets you run lower pressure. But how much? All calculators I’ve found are for clinchers, I believe. Am I just testing these out until I “think” it’s good?
Should tubular tires be at the same pressure as clinchers? Technically, these are tires with tubes in them.
On Weight Weenies, I read someone saying that tire pressure should be based on the actual width when fitted on the rim, not spec width because the same tire on a wider rim has higher air volume. Thus, if I have a 25mm tire that stretches out to 30mm on a wide rim, I can use the tire pressure suggested for a 30mm tire. Is this true? I can’t wrap my head around this - a big bag should hold more air than a smaller bag no matter how how wide you make the smaller bag when you put air into it, right? If it’s true, that means I can get the comfort of a larger tire with the aero and weight benefits from a smaller tire if I put the tire on a wide rim and the tire is stretched out.
Wider tires have less rolling resistance at the same tire pressure, Ex. 28 @ 60 PSI has better RR than 23 @ 60 PSI. Although many suggest having PSI as low as possible without slippery/sluggishness in the tire, isn’t there a case to have the HIGHEST PSI you’re comfortable with to take advantage of this lower RR? For example, maybe my butt and hands can handle 80 PSI, so I should do that even if I feel sluggishness at 60 PSI.
Ideal depends on what you’re looking for. If speed is the goal, you want lower rolling resistance (which comes from higher pressure, up to a point). If comfort or grip are the goal, generally lower resistance works best (to a point).
A good resource to check out for different tire pressures and types is https://www.bicyclerollingresistance.com. This can help you choose the right tire, but also look at the impact of running different pressures on your rolling resistance. To find the ideal point, trial and error at different pressures is likely the best approach.
I don’t know about calculations, but I’m 5kg heavier than you, and I ride and race in the UK on 25mm tubeless tyres pumped to 75psi in the back and 70 in the front. It’s grippy, comfortable, and fast.
If I’m going somewhere with very smooth roads I might go up to 85/80 or 90/85. And the 23mm tyres on my TT bike are generally pumped up to 95/90.
It really depends on a lot of factors, obviously the tyres and rims, but also how smooth your roads are, how are the corners, is it wet or dusty, how do you like the bike to handle, how much roughness can you smooth out as a rider… Its hard to have an objective optimum with so many factors.
If you really want to find whats best for you for an event, test ride sections of the course with different pressures, keep the power constant and record the times. If its just for general riding, just whatever feels good for you.
As an answer to your point 4: Yes exactly. Given relatively smooth tarmac you should absolutely run a rather high PSI as long as you are comfortable with it. This reduces hysteretic resistances. But there is something different at play also - and this is the induced movement and vibration from the surface over tire, rim and frame to you and effectively on your system bike and rider as a whole. These are dampening resistances or impedance losses. As the tire pressure rises and surface roughness increases these are increasing steadily. There is a sweet spot where the curve of the hysteretic rolling resistance and impedance resistance are intersecting and therefore their sum is minimal.
So - the rougher the surface (rough tarmac, gravel) the less PSI you should your tire inflate to else you are getting slower despite trying to cope with the uncomfortable ride. You are getting double punished if you so will. You are paying with discomfort but not only not gain speed but forfeit it!
Listen to these podcasts basically…
According to josh - Yes it’s measured width of the tyre on the rim that matters.
I can’t remember the exact number he gave in another podcast - but I believe for a measured 25mm with a 70kg rider it was 70-80psi as a range pending other factors.
The other thing he talked about was that cyclists need to better distinguish between feeling fast and actually going fast.
The other thing he talked about was that cyclists need to better distinguish between feeling fast and actually going fast.
Yeah! That’s my problem with the subjective test until you like one (essentially) testing method. And it’s not that easy to test at constant power on the same roads for a long enough time to be able to tell whether a certain PSI is having an impact.
The performance with a latex tube will be roughly the same at the same pressure as tubeless but there is the higher risk of pinch flats. If you’re running tubes, go for 5-10 more psi than these enve recommendations as a start.
Just for reference I am about the same weight as you at 65kg. My 28mm marked tires on a 21mm internal rim width wheels balloon up to 30.3mm at 55PSI and that’s about my ideal pressure.
Yes what they say at Weight Weenies is true. That’s because a tire is only like 2/3 of the bag size. The tire does not close around perfectly to contain all the air so your reasoning that a bigger tire is a bigger bag does not hold true. The rim is also major factor to the total volume. I think this picture helps visualize things.
This is measured cross sectional data from HED. As you can see, wider rims changes the shape, width, height and overall volume of the tire. From here, you can see how a 23mm tire on a wider rim will have an actual size bigger than a 25mm tire on the 19mm rim.
