Ride Long – Tyre pressure and some other thoughts
3 minute read
Welcome to some more ramblings from an old bloke who rides quite a long way in some discomfort whilst wearing RCC kit. This time I’d like to go into the subject of tyre pressures in more depth, which should be of interest to a good proportion of club members.
In my post concerning hand comfort, I said that I ran my tyres at 50 psi front and 55 psi rear. For 25mm and 28mm tryes respectively, these are significantly lower than most would consider for a road bike, and would normally only be ridden on when attempting to get home with a slow puncture. So how did I get to the position where it looks as though air is now so expensive that I can’t afford to pump my tyres up properly.
One factor is that I’m using Yeoleo carbon rims on my audax bike. The depth is 38 mm on the front and 60 mm on the rear. Importantly from a tyre pressure point of view, the rims have an internal width of 21 mm and hence they tend to make the tyre width, once fitted to these rims, a couple of mm wider than stated on the tyre, ie a 25 mm tyre comes up closer to 27mm. The rims are in many ways like the HED Jet Plus wheels that I use on my time trial bike. There is an excellent article on the HED website https://www.hedwheels.com/tyre-pressure-article. I like the HED wheels as they are essentially a aluminium rim, including the brake track, with a lightweight carbon shroud, giving the aerodynamic profile. HED stayed with an aluminium braking track long after most other manufactures had gone to full carbon rims despite the fact that rim brakes were scarily ineffective in the wet. I know the sensation having driven a 1979 MG Midget when I was used to modern servo assisted brakes. Consequently I tend to believe their claims which I see as being ‘engineering led’ rather more than I would with a company that might be described as being ‘marketting led’.
If you read that article you will see a table making recomendations for tyre pressures that will be fastest on real world road surfaces, taking rolling resistance and the losses from bouncing up and down into account. Taking my weight into account, of 70kg, the recommendation for a 25mm tyre is 65 psi, and for a 28 mm tyre is 60 psi. So how did I get to 50 front and 55 rear. Firstly I adjusted the pressure for the front and back tyres based on the fact that appoximately 45% of my weight is taken by the front wheel, and 55% is taken by the rear tyre. This now changes the recommendations to 60 psi front and 65 psi rear (to the nearest 5 psi interval). These are the pressures I used when I started riding audax events when they briefly resumed in the latter end of 2020. I rode out to Cholsey for the Upper Thames 200 just after it had been cancelled, so I had to ride it on my own as a DIY event. The day was pleasant enough for November and I finally got back home around 8pm without particular saddle discomfort, but the following day I had a very sore back from riding a total of 165 miles the day before. The sore back led to me making a change to the saddle angle which then led me to seek out saddle comfort from a range of changes which I will go through in my post on being comfortable on your saddle. It has more twists and turns that Indiana Jones and the Temple of Doom.
So over the past year in the relentless hunt for saddle comfort I’ve reduced the HED recomendations by 5 psi, which was reasably justified by the fact that audax routes have more lanes and poorly surfaced roads than your average route. I then went down another 5 psi after 300 km and 400 km rides in early summer which were far from comfortable. It’s only in the last few months have I realised that the cause of my problems wasn’t primarily caused by tyre pressure, but was mostly due to the saddle angle, which put too much pressure on the rear of my sitbones that ended up being bruised and uncomfortable.
To put this theory to the test I went out yesterday on what I call my Arborfield loop, which is 38 miles and one I’ve ridden repeatedly to allow me to assess changes in equipment and clothing. It has a fair proportion of lanes in it with repairs and irregularities. Thankfully both my hands and saddle area were comfortable during the ride. I tend to feel that you have sufficient saddle comfort if you are able to forget that you are sitting on your saddle because the brain is no longer taking any notice of the signals from the sitbone area. I was always sensing pressure on the side of my hands if riding on the hoods, but I suspect that may be that the need to steer means the brain always keeps the feeling from your hands as it would with your feet if you were walking or running.
So my advice now for those of you using rims with a 21 mm or more internal width is to see what the HED table suggests you should use, and think about whether your current choices are above or less likely below their recommendations. Personally I will be trying pressures that are a little above their recommendations as I use latex inner tubes which slowly deflate. Depending on the length of the ride I can set them so they are a little too hard at the start of the ride, whilst being a little soft at the end, but not risking being so soft that snake bites become an issue.
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