Working out the Best tyre pressures

[nobby]

How do you work out the best tyre pressure for the weight carried by the bike?
In this case it is a Moulton TSR with 35 - 406 Schwalbe Marathon plus tyres

[perpetual dan]

Long ago I found an article about this. I can’t find it now, but this has a similar thrust https://www.biketinker.com/2010/bike-resources/optimal-tire-pressure-for-bicycles/ - although it makes no reference to the surface ridden in the formula or whether tubeless changes things.

In reality I put about 3/4 of the max in the rear, 1/2 of max in front and drop a bit for chutney season or higher than usual levels of silly off road. I think that’s lower than their numbers, on the basis that the maximum recommended pressure in my tyres is lower than their number!

[LittleWheelsandBig]

Supposedly, optimum tyre pressure is based on 15% drop in tyre height when loaded. The 15% comes from Michelin’s advice to Frank Berto back in the 1980s. This tyre drop pressure is modified by tyre stiffness and stability and by transient loading (e.g. landing jumps) vs. average loading.

[Brucey]

15% drop explained plus a handy chart

https://www.renehersecycles.com/wp-content/uploads/2015/03/BQTireDrop.pdf

cheers

[robgul]

Hmm, that article seems to waffle a lot  . . .

Being quite heavy (and tall!) I tend to look at what it says on the tyre-wall as the max - inflate to that plus a little bit more.  Works for me.  My bikes are either road or touring (no MTB, gravel etc)

The other issue of course is the unreliability of pressure guages for bike tyres ... out of curiosity, on a slow day at robgul towers, I checked the same wheel (Presta valve) using 5 different devices . .  2 hand held pressure gauges (one analogue, one digital) and inflation with three track pumps with dials (a Park Tool, Joe Blow and Bontrager) . . . yep, all different although with a total range top to bottom of about 15%.  Between test air loss is possible but fast removal minimised that.

The thumb gauge is often the best!

[hubner]

Optimal, best, for what? Rolling resistance, comfort, wear?

This is a bit like crank length. Some riders want a formula and hard numbers. So to provide an answer, someone makes up a "rule".

For cranks:
inside leg length divided by inside leg length, add 169. If you're 1.5m or shorter, take off 5. If taller than 2m, add 5.

For tyres:
pick some arbitary but technical sounding figure, eg some percentage of the max pressure marked on the tyre, or the amount the tyre bulge out when loaded multiplied by some secret constant.

Then, increase pressure if it feels too soft, or if you're heavy, or if you want lower rolling resistance.

Or reduce pressure if it feels too hard, or you want more comfort, or if it's a heavy touring tyre, or you want more grip etc.

 

[nobby]

Ah, I had an interesting (to me) result.
Me, bike and battery is 102.6 kg. So, on a 45%/55% split 46kg weight on front wheel and 56kg rear wheel.
For a 35mm tyre that is 50psi and 60psi. Trouble is the tyre wall - Schwalbe Marathon Plus 35-406 - says minimum 65psi maximum 100psi.

To Hubner - comfort always comfort!

[robgul]

. . .  . the other factor with tyres and inflation is the material the tube is made from - butyl/latex etc - some will "leak" air more quickly than others .... so how long does the "optimum" or "desired" pressure last for?   

Then there's the quality of the tube -  is a pound shop tube any different from the offerings from Messrs Continental, Schwalbe, Michelin et al?

[nobby]

Then there's the quality of the tube -  is a pound shop tube any different from the offerings from Messrs Continental, Schwalbe, Michelin et al?

They'd better be. I always buy Schwalbe! 

[rogerzilla]

Since a badly underinflated tyre (say 50%) doesn't noticeably drag, I venture that the optimum pressure is a bit of a unicorn.  Typically 60psi causes about 4-5W more drag from a mid-range road tyre than 120psi, and 60psi is ridiculously soft.

I have my tyres quite hard because it reduces the risk of snakebites and makes little difference to comfort.  A good supple tyre at 130psi is a lot more comfortable than a stiff tyre at 100psi (hello, Durano Plus).

[quixoticgeek]

Since a badly underinflated tyre (say 50%) doesn't noticeably drag, I venture that the optimum pressure is a bit of a unicorn.  Typically 60psi causes about 4-5W more drag from a mid-range road tyre than 120psi, and 60psi is ridiculously soft.

I have my tyres quite hard because it reduces the risk of snakebites and makes little difference to comfort.  A good supple tyre at 130psi is a lot more comfortable than a stiff tyre at 100psi (hello, Durano Plus).

I beg to differ. I can notice that my conti GP5k tyres are 4 bar rather than their usual 5. And on the Brompton with it's Schwalbe Marathon's, I can notice when i gets to about 4, rather than 6. The later becomes really draggy. Noticeably so.

J

[Brucey]

Ah, I had an interesting (to me) result.
Me, bike and battery is 102.6 kg. So, on a 45%/55% split 46kg weight on front wheel and 56kg rear wheel.
For a 35mm tyre that is 50psi and 60psi. Trouble is the tyre wall - Schwalbe Marathon Plus 35-406 - says minimum 65psi maximum 100psi.

To Hubner - comfort always comfort!

comfort requirement + Marathon Plus = 'low' pressures (vs what is written on the sidewall) and riding the bike will be very hard work.

If you are not attuned to such things you can easily ride M+ tyres around underinflated (they are less likely to wallow, pinch flat etc) and this is part of the reason they say to inflate to 65psi min; if you deliberately set a lower pressure than this, by the time you get round to topping the tyre up again it may have been run soft enough to damage it.

A few years ago a Dutch organisation did some tests on 700x~35 tyres and they (unsurprisingly) found that a nice supple tyre (Vittoria Voyager Hyper) was the most comfortable of those tested, at any given pressure.  They lowered the tyre pressure in Marathon plus until they got the same vibration meter reading (over cobbles) as was obtained with 4 or 5 bar in the hypers.  The M+ tyres were 'equivalently comfortable'  (according to the vibration meter readings) at about 3 bar.  At this pressure the rolling resistance was almost exactly double vs the hypers, and probably the tyres were in danger of being damaged too.

Bottom line  (with a caveat that a Moulton might be different because of the suspension) ; if you want comfort, don't choose marathon plus!   IME the occasional puncture with better (faster rolling, more comfortable) tyres is much less of a problem than most people imagine and furthermore you might not feel the need for a motor, batteries, and all that stuff.

cheers

[Kim]

Since a badly underinflated tyre (say 50%) doesn't noticeably drag, I venture that the optimum pressure is a bit of a unicorn.  Typically 60psi causes about 4-5W more drag from a mid-range road tyre than 120psi, and 60psi is ridiculously soft.

I have my tyres quite hard because it reduces the risk of snakebites and makes little difference to comfort.  A good supple tyre at 130psi is a lot more comfortable than a stiff tyre at 100psi (hello, Durano Plus).

I beg to differ. I can notice that my conti GP5k tyres are 4 bar rather than their usual 5. And on the Brompton with it's Schwalbe Marathon's, I can notice when i gets to about 4, rather than 6. The later becomes really draggy. Noticeably so.

Absolutely agreed about the Brompton.  (Though pretty much anything that isn't a Brompton brake will slow down a Brompton.)

Drag is also the primary means through which you detect rear wheel punctures on a Streetmachine, though that's mainly because the suspension does too good a job for you to notice a change in comfort, and by the time you're wondering what's wrong with your legs you're in danger of snakebites.

I find that in general, soft front wheels tend to make the steering go squiffy before the drag is really obvious.  Probably depends on how prone the particular bike is to squiffy steering.


(While we're on the subject, lockdown weight-loss has made all my usual tyre pressures wrong...)

[nobby]

Thanks all for the input.

I'll stay with the Marathon family for their puncture resistance.

100psi and a little more as robgul says and I'll let the suspension handle the comfort excepting on the Trice Q where I use Big Apples on the front wheels for that purpose.

The 'scientific' bit:-  if I pinch the sidewalls between fingers and thumb and they just move it is hard or soft enough.

[rogerzilla]

In my experience (borne out by the independent rolling resistance tests), Marathon Pluses roll well, much better than you would expect from a tyre of such thickness.  The weight, however, is very noticeable and you can feel it as you accelerate from standing.

[nobby]

In my experience (borne out by the independent rolling resistance tests), Marathon Pluses roll well, much better than you would expect from a tyre of such thickness.  The weight, however, is very noticeable and you can feel it as you accelerate from standing.

I've never been fast enough to accelerate but I go into 4th on the Alfine 8 and set the motor power to 2 nowadays for a standing start. It is not searing but it is good for my knees 

[giropaul]

It was much easier when the correct pressure of a silk tubular was the sound of the “ ping” when you flicked the tyre.

[quixoticgeek]

The 'scientific' bit:-  if I pinch the sidewalls between fingers and thumb and they just move it is hard or soft enough.

Depends how strong you are...

J

[Brucey]

In my experience (borne out by the independent rolling resistance tests), Marathon Pluses roll well, much better than you would expect from a tyre of such thickness.

isn't this damning with faint praise? 

IIRC a typical M+ tyre  drags between 25 and 30W where a nice touring tyre at the same 'normal' pressures drags as little as 17-18W.  And joy of joys the M+ rides quite like it is made of wood; mahogany perhaps...? If you deflate the M+ to make it comfortable it drags even worse, as per the example I quoted above, double the drag isn't unusual in this case.

It is just as well that these tyres don't puncture often because getting them on and off the rim can be a bit of a nightmare.  For most purposes the standard marathon HS420 is more than adequate puncture resistance-wise and it both rolls easier and rides nicer.

M+ is a great tyre for what it is, but what it is is a tyre that is either incredibly slow and reasonably comfortable or it is a tyre that is still slow and  incredibly uncomfortable.

cheers

[mrcharly-YHT]

IME the occasional puncture with better (faster rolling, more comfortable) tyres is much less of a problem than most people imagine and furthermore you might not feel the need for a motor, batteries, and all that stuff.

Back in the days when I cycled a lot I worked this out. Used one of the speed calculators to compare riding on something like M+ with a fastish tyre (like hypers).

Even though there were weeks when I was getting 2 punctures a week, on average my commute would be faster if I rode on the fastish tyre. Plus, my body would be less hammered by vibration.

[nobby]

M+ is a great tyre for what it is, but what it is is a tyre that is either incredibly slow and reasonably comfortable or it is a tyre that is still slow and  incredibly uncomfortable.
cheers

But on a Moulton with a motor neither matters.   

[rogerzilla]

Just to get this into perspective, there is 4W between a Marathon and a M+ at 75psi.  I run more like 90-100, so even less difference.

https://www.bicyclerollingresistance.com/tour-reviews/schwalbe-marathon-plus-2015

I don't like Marathons (or Duranos) because I've had a couple of duff ones:  Schwalbe QC seems lacking for the cheaper tyres in the range.  If you want to go cheap, Vittoria are less likely to be wonky or to have a bead that won't stay seated.

[Kim]

I had a M+ with a bulge in it once.

Other than that, I've had countless problems with Pro Ones not seating properly, but I suspect that's user error expecting them to work with a rim that I've actually got.

[andrew_s]

Just to get this into perspective, there is 4W between a Marathon and a M+ at 75psi.  I run more like 90-100, so even less difference.

https://www.bicyclerollingresistance.com/tour-reviews/schwalbe-marathon-plus-2015

If you run a tyre at pressures higher than is comfortable, it's likely that it's also slower, regardless of test figures.

The trouble with the bicyclerollingresistance is that they only measure the energy losses in the tyre itself; they ignore the losses in the human body that's being carried by the tyre, and these can be considerably larger. Losses of well over 200 W have been found when deliberately choosing very rough roads.
These energy losses go into shaking your body about, and getting converted to heat. You experience them as discomfort.

By way of an example, Mr T. Boonen (80+ kg) used to race Paris-Roubaix with 60 psi in his tyres, despite the risk of snakebites.
He did that not because he's a wimp that can't cope with a bit of discomfort, but because not having to produce the power to cover the vibrational losses in his body in the cobbled sections gave more overall gain than soft tyres lost in the tarmac sections.

[Brucey]

I agree with the above. One way of looking at it is that, if one tyre of a given width and pressure is more comfortable than another, you will be seeing lower suspension losses too.  So regardless of what a roller test might be telling you, the more comfortable tyre is likely to be faster on 'real' (bumpy) roads.

This is why I mentioned the tests I did previously; it isn't as simple as roller tests alone might lead you to believe. Another way of looking at it is that it is probably only safe to compare roller test figures between two tyres if they are about as comfortable as one another.

As to the idea that 'it doesn't matter' if your tyres are draggy and uncomfortable on a Moulton with a motor, I do not agree.  What you will find is that (amongst others) this affects the range of the thing.

cheers