27.5 Plus air pressure setup

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27.5 Plus air pressure setup
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Posted: Aug 12, 2019 at 13:59 Quote
Fairly new to biking. I’m currently running 27.5 plus tires on my hardtail. I’m typically running 26-28psi in both tires. But I’m curious how you set yours up...

Harder front, softer rear? Harder rear, softer front? What pressure do you like? I am riding single track, some jumps, birms, and lots of roots. I wouldn’t say my style is too aggressive, but I’m also not pedaling gingerly through the woods.

Posted: Aug 13, 2019 at 12:38 Quote
Running 27.5x2.8 on a 36mm ID rim. I'm about 210 in full riding gear. On the front tire I run about 16 psi and on the rear I run 20psi. If the rear gets lower than 20 psi then my back end will fold and get squirrely in high speed berms.

Edit: Forgot to mention I run mine tubeless.

Posted: Aug 14, 2019 at 2:29 Quote
NoMaam wrote:
Running 27.5x2.8 on a 36mm ID rim. I'm about 210 in full riding gear. On the front tire I run about 16 psi and on the rear I run 20psi. If the rear gets lower than 20 psi then my back end will fold and get squirrely in high speed berms.

Edit: Forgot to mention I run mine tubeless.

Thanks for that info. Wow that’s soft! I weight 210. I notice at about 24psi my rear tire is really soft and it looks like I’m flattening it out just sitting on it. I’m afraid that I might roll it if I ride too hard. Maybe this weekend I will try running them a little softer and see how it feels.

With the low psi that you run, do you notice any added resistance or trouble keeping speed?

Posted: Aug 14, 2019 at 6:29 Quote
No problem holding speed and rolling resistance should technically decrease as tire pressure decreases on rough surfaces. I haven't had any rim strikes and the grip is amazing. In terms of the rear tire folding at speed it isn't too bad. It feels like quick loss of traction as you back end shifts over an inch or so and you just grab a little brake and back off. As I said I am running tubeless so if you're running tubes you will be 2 to 4 psi higher.

One thing to make sure of is that your pressure gauge is accurate.

Posted: Aug 14, 2019 at 8:45 Quote
NoMaam is correct. To add my support for his points:

Within the range of tires we're discussing, wider tires roll faster on rough surfaces. The rougher, the greater the advantage. [1, 2]

For thin casings (downhill casings were not tested), a similar effect is observed with rolling resistance falling as tire pressure is reduced.

These effects are attributed to the micro-suspension effect of tire compliance being more beneficial to rolling resistance than the detrimental effects of increased casing flex.

I'm lighter than you two and I run 2.6" tires on 36 mm (internal) rims at about 16 - 20 psi front and 18 - 22 psi rear. Lower if I'm on rough, natural trails (i.e. no berms); higher if I expect large, sharp rocks and/or high lateral loads.

Rim width makes a huge difference. I once tested 3.25" tires on 29 mm rims and couldn't keep them from laterally collapsing at any pressure that produced a tolerable ride quality. 2.4" tires on 35 mm rims wouldn't collapse at pressures that were too low to avoid pinch-flats.

Regarding gauges: Many people mistake the precision of digital gauges for accuracy. Any gauge can display as many decimal places as you like, but that doesn't make it accurate! One of the best steps toward a good measurement is to use a gauge that maxes out around the target pressure. This is a cheap, widely available option that tops out at 30 psi.

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