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I have never raced before but there is a cycloycross race a mile from my house this weekend.

Course has a fast bumpy downhill.

On my mountain bike with fixed carbon fork and tubeless at 40 PSI the downhill is easy but I timed myself and I am slower by a good margin. 6:10 versus 6:30 on a three lap test course. Even riding the CX second when I was more tired.

A buddy of mine raced me and we switched bikes and we both agreed the CX was faster but just getting banged around too much. At the bottom of the downhill on the CX can hardly see. In a heat of 75 riders on a small course I want to see.

On related question someone posted a link to last year's race. The hill I was afraid of is on the course but the decent is three long switch backs. And even then it is late in the course so the field will be spread out.

The CX is clincher and not tubeless - that is not going to change. I cannot put my mtn wheels on my CX as the mtn is disc and the CX is not. I put the biggest tire I could on the CX (38 mm) and the lowest posted pressure for the tire of 55 PSI. Tire still feels hard. Tire does not even come close to bottoming out. At 45 PSI the tire starts to soften up.

So finally a direct question. Is it safe/sane to run a 38 mm at 45 psi when the posted range is 55-90? I weigh 165.

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    If you can't see, you are likely holding on too the handle bars tight and your carrying too much tension in your upper body. You may need to loosen both up, and find your flow. Also the 75 riders will thin out fast as the pace in CX is typically fast, it won't be bunched like a road race.
    – Rider_X
    Commented Oct 29, 2014 at 7:07
  • @Rider_X Agree. With practice I am getting more comfortable on the CX.
    – paparazzo
    Commented Oct 29, 2014 at 13:45

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Yes you can reduce the pressure and it will remain safe (cyclocross isn't a high speed high turning force type of racing).

Your biggest issue will be pinch flats and rim protection.

Weather and ground conditions should drive your pressure choice as well. The muddier the track the more traction you'll need.

I'd look at 30-35 psi to start and try the track on that.

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  • 40 psi on a 2.1 on my mtn bike feel great. If I go below 45 then I will go with the mtn bike. If the track is muddy then I will ride the mtn bike. So far the track is dry and that is the forecast.
    – paparazzo
    Commented Oct 29, 2014 at 0:04
  • And lose the fun of drops on a descent!!
    – DWGKNZ
    Commented Oct 29, 2014 at 1:05
  • Found a video of the course. The decent I was afraid of is not part of the course. I am going to play around at 45 and 50 PSI.
    – paparazzo
    Commented Oct 29, 2014 at 19:19
  • 45 PSI felt good. I am going back tomorrow at 40 PSI. If it gets muddy the city will not even allow us to race.
    – paparazzo
    Commented Oct 31, 2014 at 0:32
  • 40 psi on the 38 mm worked fine.
    – paparazzo
    Commented Nov 1, 2014 at 23:56
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It's perfectly safe and sane to run 45 psi, that 55 psi minimum probably has to factor in the safety and likelihood of someone weighing more than you. Really you just run the risk of a sharp edge giving you a pinch flat, and it depends completely on the course, some courses might have more rocks and pavement than others. Personally I weigh more than you and I've run 45 psi on thinner tires for certain races. It all depends.

You could even borrow a set of wheels and tires, and keep them in the pit in the event you do get a flat, that's pretty common practice for a cross race.

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  • Found a video of the course. The decent I was afraid of is not part of the course. I am going to play around at 45 and 50 PSI.
    – paparazzo
    Commented Oct 29, 2014 at 19:20
  • There as cat 4 rider at the park tonight that is also one of the organizers. He took me through the course and told me to run 40 PSI.
    – paparazzo
    Commented Oct 29, 2014 at 23:36

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