Softer tyres - cause what problems?

[allen-uk]

Ice and snow plus a christmas cold meant my Wisper sat in the hallway for over 3 weeks, and the tyres deflated from their normal 70-plus psi down to about 35.

Just for the experience, I took it out and rode it on the (to me) fairly soft tyres, and it was certainly a softer ride, probably more comfortable than before.

My experience is only with conventional bikes, where high tyre pressures were my norm as they made for more efficient pedalling, so when I first got my Wisper 905 with Marathon Plus tyres I kept them at the top of their pressure range for similar reasons - i.e. it would be more efficient for battery life.

What are the long-term implications of using tyres like these at a lower than recommended pressure? I bought them primarily to avoid punctures, so will I be defeating that object? And what about power? I don't want to sacrifice hill-climbing ability just for a slightly softer ride.


Allen, Wisper 905se, 2010.

 

[indalo]

I'm glad you asked that question allen-uk as I've often wondered about the practical ramifications of much reduced tyre pressure long-term. I'm sure someone will provide an answer as I'm sure not everyone opts to maintain tyre pressure at the top end of the scale. Like you, I keep mine around the max for similar reasons.

Indalo

 

[Fordulike]

I do believe that they will be more prone to punctures at a lower pressure.
I always keep mine at 70psi, and yes it is a pain in the bum to inflate them on a regular basis to maintain this, but well worth it for the puncture free trips.
I find that there's not a lot of difference in ride comfort between extremes of pressures, so 70psi it is

 

[Streethawk]

Lower tyre pressures do give a much greater chance of pinch punctures (also often called "snakebites". The sympton is that a square edged bump pinches the tyre and tube within against the wall over the wheel rim, and the wall of the wheel itself punctures the tube.

These tend not to happen too often above 30psi on big MTB tyres, but on thinner tyres the pressure to avoid this need to be higher.

Obviously no amount of puncture proofing on a tyre can prevent these, simply keeping your tyres within the recommended range on the sidewall should though.

 

[Mussels]

I found MP tyres gave a pretty awful ride at lower pressures but are good at full pressure, other tyres I have tried have been fine at lower pressures especially the Big Apples which gave a very nice ride without any noticable loss of speed.

 

[aseb]

I thought tyre pressures should be set according to your weight not one size fits all. Heavier = higher pressure.
I use Michelin City and the tyres have moulded on the walls 'inflate to 2.5 Bars (36psi) minimum - 6 bars (87psi) maximum'. The cardboard label has a little michelin man smiling at 4.5 bar and looking sad above and below this next to + and - signs.
Harder gives a more uncomfortable ride jarring on our slightly less than smooth sections of road, whereas softer can cause punctures through nipping of the inner tube.

(btw the Michelin City tyres have the protek lining to prevent punctures like the Schwalbe Marathons that seem so beloved by many in the forum. I got the Michelins because they were a better price, I'll try the Marathons next but I have no complaints about the Michelins- no punctures!)

 

[BLACKPANTHER]

I keep my Marathon Plus Tours near their maximum. When I tried them near the minimum they didn't seem to roll any where near as well, so I suspect the battery range would be a fair bit lower/human input would have to be higher. They'd also wear faster at minimum pressure, and heat up more.

Also, if you set off on minimum, after a few days/weeks (let's face it, how often do people check their pressures?) you'll be below minimum, which is where real damage could occur.

 

[Synthman]

On regular bicycles in the past, I've always preferred slightly under-inflated tyres, as the roads and paths are so bumpy round here, it gives a better ride. On my e-bike I keep the pressure as close as possible to maximum, to reduce drag. The ride has improved a bit recently so I reckon I need to top up now.

 

[tangent]

It requires a certain amount of energy to deform the tyre as it rolls along the ground. In a perfect world this energy will be returned as the tyre shape is reformed. Unfortuneately though you do not get back all the energy that you put in to deform the tyre. This energy loss shows up as "rolling resistance".

Lower pressure means the tyre flexes more as it meets the ground. This means more energy loss, increased rolling resistance, although I have no figures to indicate how much.

 

[10mph]

It requires a certain amount of energy to deform the tyre as it rolls along the ground. In a perfect world this energy will be returned as the tyre shape is reformed. Unfortuneately though you do not get back all the energy that you put in to deform the tyre. This energy loss shows up as "rolling resistance".

Lower pressure means the tyre flexes more as it meets the ground. This means more energy loss, increased rolling resistance, although I have no figures to indicate how much.

My cycling science bible is the book by David Gordon Wilson, Bicycling Science Edition 3, NIT 2004. I strongly recommend the book for a good introduction especially to the energetics of cycling. It is all based on human power rather than electric power, but the data collection is of great value, and he gives enough of the theory and formulae that the engineer can extract useful data to apply to ebikes.

There is a useful table on page 228 which shows the effect of low inflation values on a 51mm crossection tyre with a load of 120 lbf (53N) at a speed of 20 mph (8.9 m/s)

10 psi gives a coefficient of rolling resistance of 0.016 and 75 watts of power is needed
18 psi gives coeff= 0.011 and watts 52
30 psi gives coeff 0.008 and watts 37

There is a curve of coeff against pressure from another experiment on page 227 which shows that the drop in rolling resistance starts levelling out at higher pressures and does not improve much more as the pressure is raised from 50 psi to 150 psi, ie from 0.008 to 0.006.

These are of course particular tyre sizes and composition so could be different for other tyres.

 

[NRG]

There's also some detail in the excellent Schwalbe technical PDF with a graph, free download from their web site.....

 

[Mussels]

It requires a certain amount of energy to deform the tyre as it rolls along the ground. In a perfect world this energy will be returned as the tyre shape is reformed. Unfortuneately though you do not get back all the energy that you put in to deform the tyre. This energy loss shows up as "rolling resistance".

Lower pressure means the tyre flexes more as it meets the ground. This means more energy loss, increased rolling resistance, although I have no figures to indicate how much.

This is where a fatter tyre works well as they deform less at lower pressures, the trade off is that they are heavier so accelerating takes a bit more effort. I found the trade off well worth it.

 

[Old Timer]

Funny that, for that little bit of extra comfort(old bum) and a tad more grip on the crud I`ve been running my marathons at 50psi instead of 60psi and don`t notice much difference in running resistance. However I nearly always notice when my tyres need a pump up and are at 40psi because the rolling resistance seems to hold me back a touch so the 50psi you mention as a possible break even point might be working for me.
As far as comfort goes then the rule of quality in place of quantity does it for me.

Dave