Will I feel a difference between 42psi and 22psi (going from 700c to 650b)

[scotsbikester]

I have a "touring" bike. I don't tour, at least not yet. It's built around a touring frame - all steel, drop bars, long wheelbase, lowish BB, all the braze-ons, 27x3 gears etc. etc.

It's got disc brakes. So I could change wheels.

At the moment it's got 700c wheels with 35mm tyres (tubed). I run 49psi front, 53psi rear. Silca's site says I should/could have 42psi. If I change to 650b and run 55mm tyres (to get the same overall diameter) Silca says I could run 22psi.

Will that be a lot more comfortable over rough tarmac/gravel/off-road? Is it enough of a difference to notice?

I ask partly because I would like to get a dynamo front wheel built up for longer and/or night cycling. So wondering about making the change.

Thanks.

 

[Brandane]

At the moment it's got 700c wheels with 35mm tyres (tubed). I run 49psi front, 53psi rear. Silca's site says I should/could have 42psi. If I change to 650b and run 55mm tyres (to get the same overall diameter) Silca says I could run 22psi.

All those pressures seem very low. I have a Ridgeback Panorama steel framed tourer, 700c wheels with 35mm tubed tyres. If loaded with luggage, front 80 psi, rear 90 psi.. Unloaded, front 65 psi, rear 70 psi.. My own weight is about 13 stone 7 lbs.

 

[Punkawallah]

I would imagine your biggest problem would be the space in the frame to fit they tyre. While 55mm might give you the same overall diameter, will it fit between the chain stays, and/or the forks?

 

[PaulSB]

I have a "touring" bike. I don't tour, at least not yet. It's built around a touring frame - all steel, drop bars, long wheelbase, lowish BB, all the braze-ons, 27x3 gears etc. etc.

It's got disc brakes. So I could change wheels.

At the moment it's got 700c wheels with 35mm tyres (tubed). I run 49psi front, 53psi rear. Silca's site says I should/could have 42psi. If I change to 650b and run 55mm tyres (to get the same overall diameter) Silca says I could run 22psi.

Will that be a lot more comfortable over rough tarmac/gravel/off-road? Is it enough of a difference to notice?

I ask partly because I would like to get a dynamo front wheel built up for longer and/or night cycling. So wondering about making the change.

Thanks.

You seem to be on low pressures already. I ride tubeless on 700c/35mm. On road I'm at 55 front, 60 rear, on gravel I ride 50/55. Yes, I could probably lose 5psi all round. It's a hangover from my tubed days.

Whenever I've used Silca the pressures seemed far too low for me. I wouldn't rely on a website calculation which cannot take account of the two most important factors. First at what pressure do you feel confident and secondly what's comfortable for you?

Those are the key factors. I ride higher pressures than I need to but it gives me confidence. That is crucial.

 

[chriswoody]

As the other posts have said, these suggested pressures do seem incredibly low.

I run 700 x 47mm wide tires on my Gravel bike and they are set up tubeless, I run these at 35/38 PSI and find them great at these pressures. I even go bike packing with just a couple more PSI in them. Much less than 35 and you do start to feel the bike squirm a little, but 35/38 PSI seems the optimum for on the mixed surfaces I ride on.

The Mountain bike in contrast also runs tubeless and there I run 26/28 PSI on 65mm wide tires, I never could go that low with the gravel bike, but much more than that on the mountain bike and it starts to feel like Bambi on ice.

I really wouldn't entertain going to these low pressures though with tubes in my tires. Whilst I agree with Paul above, I would still be tempted to go onto SRAM's tire pressure guide and see what they recommend. Using my figures they actually recommend a slightly lower pressure than I run on my Gravel bike, so I would use this as a ballpark figure and then like Paul suggests, play around with it to find what makes you comfortable and confident.

https://axs.sram.com/guides/tire/pressure

 

[scotsbikester]

All those pressures seem very low. I have a Ridgeback Panorama steel framed tourer, 700c wheels with 35mm tubed tyres. If loaded with luggage, front 80 psi, rear 90 psi.. Unloaded, front 65 psi, rear 70 psi.. My own weight is about 13 stone 7 lbs.

Interesting. My bike is also a Ridgeback Panorama, the "deluxe" frame (853). I'm not sure the type of steel matters, I only bought the frame because Biketart was selling them cheap (for an 853 frame). And 35mm Continental tyres, whatever the puncture resistant touring/commuting ones are called.

I haven't toured yet, though when I commute a16"macbook and 12.9" iPad add a surprising amount of weight. I'm a little lighter than you, at just over 12 stones. But your comments on pressures are very useful, thank you.

I haven't experienced any pinch flats yet, though I am always aware of the possibility, so am careful about dropping off kerbs and similar.

 

[scotsbikester]

I would imagine your biggest problem would be the space in the frame to fit they tyre. While 55mm might give you the same overall diameter, will it fit between the chain stays, and/or the forks?

I measured them a while ago, and I think they will. It is a touring frame. The figure of 55mm was a bit arbitrary, just done to come up with the same overall diameter as my current 700c/35mm combination.

As a matter of fact, it's the space for mudguards that is more of a restriction.

 

[Dadam]

I've used Silca's calculator but agree it seems to come out on the low side. It doesn't have a UK pothole roads option on the drop down!

 

[Yellow Saddle]

I have a "touring" bike. I don't tour, at least not yet. It's built around a touring frame - all steel, drop bars, long wheelbase, lowish BB, all the braze-ons, 27x3 gears etc. etc.

It's got disc brakes. So I could change wheels.

At the moment it's got 700c wheels with 35mm tyres (tubed). I run 49psi front, 53psi rear. Silca's site says I should/could have 42psi. If I change to 650b and run 55mm tyres (to get the same overall diameter) Silca says I could run 22psi.

Will that be a lot more comfortable over rough tarmac/gravel/off-road? Is it enough of a difference to notice?

I ask partly because I would like to get a dynamo front wheel built up for longer and/or night cycling. So wondering about making the change.

Thanks.

There's a bunch of superflous information in your question. If I drill down, I think you're simply unsure what tyre pressure to run for a given terrain.

Specific numbers are absolutely useless other than the manufacturer's maximum recommended pressure, which should not be exceeded. If you exceed that, you could have blow-off. However, if you are a 20kg child, you could happily go below the lower limit. Thus, just one rule - don't exceed the upper limit.

Now all you need to do is figure out, via a few short experiments, what feels comfortable for the terrain you encouter. The upper end would be whatever you consider discomfortable and the lower limit as just hard enough to not cause pinch punctures. Anthing within that range is purely to taste. No-one could put a number to that. There's generally no need to measure tyre pressure, through time you will develop a feel that negates the need for special measuring equipment.

 

[scotsbikester]

Thanks for the replies folks. The consensus does seem that the pressures Silca came up with are very low. I was a bit surprised myself. At the moment I'm running about ~50psi. I'm 12 stones, the bike is about 16kg*

Is there a better/more realistic pressure calculator?

I suppose the real question I was asking was will I notice a difference of about 20psi between 700c/35mm and 650b/55mm? Though I appreciate that the difference between 80psi and 60psi is a lot less significant than between 42 and 22.

*Yes, mixed measuring systems, I know. I used to work in construction, not long after the move to metric. We were all very used to moving seamlessly between imperial and metric.

 

[PaulSB]

Is there a better/more realistic pressure calculator?

Yep, you're backside. If you're comfortable and confident the pressure is OK.

 

[Yellow Saddle]

Thanks for the replies folks. The consensus does seem that the pressures Silca came up with are very low. I was a bit surprised myself. At the moment I'm running about ~50psi. I'm 12 stones, the bike is about 16kg*

Is there a better/more realistic pressure calculator?

I suppose the real question I was asking was will I notice a difference of about 20psi between 700c/35mm and 650b/55mm? Though I appreciate that the difference between 80psi and 60psi is a lot less significant than between 42 and 22.

*Yes, mixed measuring systems, I know. I used to work in construction, not long after the move to metric. We were all very used to moving seamlessly between imperial and metric.

OK, I get it. You don't want to experiment for your own unique situation. In that case, the Shrader-Presta Alliance (SPA) algorythm calculates that the difference in feel, for a 12 stone person on a 16kg bike with a 58%/42% rear to front weight distribution, will be 12% noticeable. The margin of error is + or - 5 atmospheres.

 

[twentysix by twentyfive]

Just pump the tyres up.

 

[figbat]

You should notice a difference but it might not be what you want. I don’t like riding the ultra-low pressures that tubeless can enable as I hate the squirmy feeling that comes with it, particularly on corners.

 

[scotsbikester]

You should notice a difference but it might not be what you want. I don’t like riding the ultra-low pressures that tubeless can enable as I hate the squirmy feeling that comes with it, particularly on corners.

That's very useful feedback, thank you.