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Optimal tubeless 28mm tire pressure?

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Optimal tubeless 28mm tire pressure?

Old 07-27-21, 02:51 PM
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Try reducing pressure 5 psi at a time til it gets too soft, then come back up to find your optimum level. Trial and error is your friend!
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Old 07-27-21, 04:21 PM
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Originally Posted by bbbean View Post
Try reducing pressure 5 psi at a time til it gets too soft, then come back up to find your optimum level. Trial and error is your friend!
That works, as long as "too soft' isn't determined by breaking your rim on the first pothole you run through.
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Old 07-27-21, 04:46 PM
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Originally Posted by Sy Reene View Post
FWIW, I believe that that at least for the Silca calculator, the returned values are based on a lot of actual 'real world' testing. Sure, every possible iteration of weight, road surface, tire width, etc wasn't tested, so likely a good chunk of values interpolated. But, to imply that all of it is just theoretical guessing is a bit extreme IMO.
What does real world testing demonstrate with regard to optimal tire pressure? What result would tell you that 65 psi is optimal and 75 psi isn't?
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Old 07-27-21, 04:55 PM
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Originally Posted by Seattle Forrest View Post
What does real world testing demonstrate with regard to optimal tire pressure? What result would tell you that 65 psi is optimal and 75 psi isn't?
I just set mine to the recommended pressure and ride. If the ride feels too harsh I drop the pressure 5 psi on the next ride. Likewise if I think they are squirming about too much I increase by 5 psi next ride. I only make changes of 5 psi at a time and I dont mess around attempting to fine tune them by 1 or 2 psi. Im not that sensitive.
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Old 07-27-21, 05:03 PM
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Originally Posted by Seattle Forrest View Post
What does real world testing demonstrate with regard to optimal tire pressure? What result would tell you that 65 psi is optimal and 75 psi isn't?
Silca's priority is speed. For a given power output, surface, rider and tire, the ideal tire pressure is theoretically the one that moves the rider fastest for a given power output. Or at least, that's what I believe their goal is.
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Old 07-27-21, 05:04 PM
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Originally Posted by Sy Reene View Post
That works, as long as "too soft' isn't determined by breaking your rim on the first pothole you run through.
If you can't miss a pothole that big you should consider a new sport.
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Old 07-27-21, 05:08 PM
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Ah, ok, thanks. I didn't realize optimal meant fastest specifically. Vs most comfortable, least likely to pinch, or whatever balance of qualities any particular rider is looking for. Hence my confusion. 🙂
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Old 07-27-21, 05:14 PM
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Originally Posted by Seattle Forrest View Post
Ah, ok, thanks. I didn't realize optimal meant fastest specifically. Vs most comfortable, least likely to pinch, or whatever balance of qualities any particular rider is looking for. Hence my confusion. 🙂
Your welcome. Now you know.
EDIT: I should also point out that Silca has the caveat that whatever pressure they recommend is not likely to break your setup (ie. flat or break your rim). Finishing the ride is one of their givens.

Last edited by Sy Reene; 07-27-21 at 05:19 PM.
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Old 07-27-21, 05:15 PM
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Originally Posted by cxwrench View Post
If you can't miss a pothole that big you should consider a new sport.
I didn't specify the size of the pothole (or it could be crack, a rock, or whatever). Go try some 36psi in your tires for your next ride. Let us know how it goes.
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Old 07-27-21, 05:34 PM
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Originally Posted by PeteHski View Post
I just set mine to the recommended pressure and ride. If the ride feels too harsh I drop the pressure 5 psi on the next ride. Likewise if I think they are squirming about too much I increase by 5 psi next ride. I only make changes of 5 psi at a time and I don’t mess around attempting to fine tune them by 1 or 2 psi. I’m not that sensitive.
I do the same thing more or less.. "inflate to just below harshness". It might not be perfect but it is relatively simple and repeatable.

I currently am putting 10psi between front and back (70 vs 60psi for me on 30mm tires, 175lbs bike+rider), even with this I have more tire drop in the back. I don't understand why the recommendations (e.g. from the SRAM calculator) are for less difference but there is probably something I don't know.
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Old 07-27-21, 06:16 PM
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Originally Posted by scottfsmith View Post
I do the same thing more or less.. "inflate to just below harshness". It might not be perfect but it is relatively simple and repeatable.

I currently am putting 10psi between front and back (70 vs 60psi for me on 30mm tires, 175lbs bike+rider), even with this I have more tire drop in the back. I don't understand why the recommendations (e.g. from the SRAM calculator) are for less difference but there is probably something I don't know.
Probably less difference between F&R for optimum speed rather than comfort. Some guides do offer alternative settings for speed vs comfort and those often involve dropping the front pressure more than the rear. Personally I keep my front pressure within 5 psi of the rear because I prefer a more solid feel at the front when climbing out of the saddle.

I just put my own stats into the Sram and Silca calculators and they recommend 67/71 & 69/71 respectively. I'm actually currently running 70/74 based on my specific tyre manufacturer's guide (Pirelli in this case). Note that Pirelli suggest reducing the front pressure by 5 psi for more comfort.

Last edited by PeteHski; 07-27-21 at 06:30 PM.
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Old 07-28-21, 08:49 AM
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My impression is that for optimal speed and comfort you want the same % drop in the tires, the old "15% drop" standard. But if your weight is really 40% front 60% back it really should be 10psi or more pressure difference for this optimal point.

There is another calculator at PSI Calculator which applies this 15% rule and it shows a 17psi front-back difference for me. Since I'm not sure how accurate that calculator is I also "faked" the SRAM calculator into making a 60-40 weight difference by putting in 20% less and then 20% more for my weight respectively, and averaging the font-back results in each case. I got 57 front 67 rear using that, a 10psi difference. So, the SRAM calculator seems to be applying something like a 55%/45% weighting difference to front and back. My best guess as to their logic is for climbing the weight can be more like 50-50 (?) and they are averaging things out. Or maybe there is something else going on.

PS the calculator at https://docs.google.com/spreadsheets...LTY/edit#gid=0 might make this more clear, you enter weight per tire here, so start by halving the weight of you-plus-bike. If I put in 80% on front and 120% on back (a 40-60 split) I get 45psi front - 70psi back.

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Old 07-28-21, 09:09 AM
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Originally Posted by scottfsmith View Post
My impression is that for optimal speed and comfort you want the same % drop in the tires, the old "15% drop" standard. But if your weight is really 40% front 60% back it really should be 10psi or more pressure difference for this optimal point.

There is another calculator at PSI Calculator which applies this 15% rule and it shows a 17psi front-back difference for me. Since I'm not sure how accurate that calculator is I also "faked" the SRAM calculator into making a 60-40 weight difference by putting in 20% less and then 20% more for my weight respectively, and averaging the font-back results in each case. I got 57 front 67 rear using that, a 10psi difference. So, the SRAM calculator seems to be applying something like a 55%/45% weighting difference to front and back. My best guess as to their logic is for climbing the weight can be more like 50-50 (?) and they are averaging things out. Or maybe there is something else going on.
I just completely ignore the "15% drop" standard because whenever I've tried running with pressures set to that rule the front feels horribly soft and squirmy when I'm powering up over a crest in the big ring. Maybe because my local terrain is very undulating, so I'm often in and out of the saddle. I guess I just prefer a more solid feeling platform on the front when I'm pressing on it and moving the bike from side-to-side. The trade-off is a little loss in ride comfort, but the D-fuse bars on my Defy and thick bar tape take out most of the road buzz anyway. I have found that 70 psi on the front is a good compromise for me on 30 mm tyres. I've certainly tested them at 65 psi and didn't like the feeling. It might also be a factor that my rims are relatively narrow (17 mm internal). On wider rims I can imagine running with lower front pressure without the sidewalls feeling too soft.
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Old 07-28-21, 09:26 AM
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Originally Posted by PeteHski View Post
I have found that 70 psi on the front is a good compromise for me on 30 mm tyres. I've certainly tested them at 65 psi and didn't like the feeling. It might also be a factor that my rims are relatively narrow (17 mm internal). On wider rims I can imagine running with lower front pressure without the sidewalls feeling too soft.
Yup. On my old Assaults (17mm int width), I didn't like going below ~70psi on 28mm/~65psi on 30mm tires. With 28s on the Zipps (23mm int width), I've never had that uneasy squirm in the corners, and that's even when letting the tires run down in to the low 50s (I assume; I inflate to ~60psi and will sometimes often go close to a week without topping them off again).
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Old 07-28-21, 09:34 AM
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Thanks, this is starting to make some sense now. My regular rims are 21mm ID and I don't recall feeling any squirming at 60psi in front. I could probably go even a bit lower, I might try out 55psi in front given these 15% drop numbers.
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Old 07-28-21, 09:53 AM
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Originally Posted by scottfsmith View Post
Thanks, this is starting to make some sense now. My regular rims are 21mm ID and I don't recall feeling any squirming at 60psi in front. I could probably go even a bit lower, I might try out 55psi in front given these 15% drop numbers.
Yep, some of the calculators factor in the internal rim width. Going from 17 to 21 mm rims on the sram calculator drops the pressures by about 5 psi. It applies that to both front and rear tyres, but it's the front I notice squirming too much at lower pressures.
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Old 07-28-21, 10:05 AM
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Originally Posted by PeteHski View Post
Yep, some of the calculators factor in the internal rim width. Going from 17 to 21 mm rims on the sram calculator drops the pressures by about 5 psi. It applies that to both front and rear tyres, but it's the front I notice squirming too much at lower pressures.
The only problem with those calculators (eg. the Zipp one), is they have to use some predetermined guess as to what your actual tire width (and therefore volume) will be after inflation. I think we all mostly know that brand vs brand or model vs model, the nominal size labelled tires (eg. 25mm) are often different sizes once inflated even on the same rim (GP4000 vs GP5000 is good example).
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Old 07-28-21, 10:13 AM
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After reading this I now know why I find pumping my track tubulars up to 120+ to be so satisfying.
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Old 07-28-21, 11:49 AM
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Originally Posted by WhyFi View Post
Yup. On my old Assaults (17mm int width), I didn't like going below ~70psi on 28mm/~65psi on 30mm tires. With 28s on the Zipps (23mm int width), I've never had that uneasy squirm in the corners, and that's even when letting the tires run down in to the low 50s (I assume; I inflate to ~60psi and will sometimes often go close to a week without topping them off again).
My Enves are 25 mm internal, I remember them feeling squishy and not precise when I got them and went through finding the right pressure. Don't remember what it took.
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Old 07-28-21, 11:54 AM
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Originally Posted by Psimet2001 View Post
After reading this I now know why I find pumping my track tubulars up to 120+ to be so satisfying.
The bone shaker!!
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Old 07-28-21, 12:15 PM
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Unless my gauges are wrong, I ride 125 psi rear and 100 psi front on the MUP here. I don't get my bones shaken. However lately I've been on the roads which have a few segments of rougher road. So I've dropped the pressure in the rear to 110 psi. 25mm GP5000's, tubed version.

I'm both faster and less tired at those pressures than when I've tried 90 psi. But your roads aren't my roads. So I'm not going to say what you should run. But if you just let a calculator tell you what to put in them, you are doing yourself a disservice, IMO.
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Old 07-28-21, 12:21 PM
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Originally Posted by Iride01 View Post
Unless my gauges are wrong, I ride 125 psi rear and 100 psi front on the MUP here.
🤯

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Old 07-28-21, 03:12 PM
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Originally Posted by Sy Reene View Post
The only problem with those calculators (eg. the Zipp one), is they have to use some predetermined guess as to what your actual tire width (and therefore volume) will be after inflation.
At least for the Silca, no. Silca uses the measured inflated tire width.
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Old 07-28-21, 03:56 PM
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Originally Posted by asgelle View Post
At least for the Silca, no. Silca uses the measured inflated tire width.
True that, and probably why I'd prefer the Silca calculator, though worth nothing that Zipp (and Silca doesn't) presumably might be building in safety (e.g. ETRTO) refinements or otherwise, since its calculator also asks for rim profile info (hookless TL, hooked TL, tubed, tubular). I Haven't seen yet a calculator that's got every variable. We need Silca and Zipp to have a baby.
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Old 07-28-21, 05:03 PM
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Originally Posted by Sy Reene View Post
True that, and probably why I'd prefer the Silca calculator, though worth nothing that Zipp (and Silca doesn't) presumably might be building in safety (e.g. ETRTO) refinements or otherwise, since its calculator also asks for rim profile info (hookless TL, hooked TL, tubed, tubular). I Haven't seen yet a calculator that's got every variable. We need Silca and Zipp to have a baby.
For me at least both those calculators recommend very similar pressures. Silca 69/71 Sram 67/71
I don't fuss over a couple of psi here or there. I don't find it that sensitive.
The only big variation I see between the various calculators are those that recommend significantly lower front pressure based on the 15% drop rule e.g. I've got one that recommends 52/79 with a 60% rear weight distribution.
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