When I checked, the tyres' spec does indeed say 85-115. Because I was surprised I asked
Wiggle and was told "60 is fine". They couldn't say why Schwalbe had such a minimum or its rationale (searched Schwalbe sites too). But given your 28mm tyres could be run by a 100kg giant (bloke) on a 12kg bike (say) and he's on 100-115 ie max, and they don't suffer from snakebites: at 54kg plus your bike (which is 10kg btw not 5) your tyres are carrying half the load (give of take) and for the same tyre deflection you'd need to run pressures circa half. The read off for you on the chart is: 44 front and 61 rear. Also see xl sheet attached.
Formula for 28mm is 1.76 x wt(kg) - 7. This gives the total pressure needed divided between front and rear as you determine (see recommendations in article). For 25mm the figure is 2.1 (instead of 1.76) and for 37mm it is 1.13 (instead of 1.76).
Besides the
BQ Vol 5 No 4 (2006) Optimizing Your Tire Pressure for Your Weight article I attached to my last post, there's this:
http://problemsolversbike.com/blog/comments/optimal_bike_tire_pressure
https://docs.google.com/spreadsheet...MYkrWs4-jH6FGwpT-L0JXq2XygMIgm0&revision=1358
BQ Vol 5 No 4 (©2006 Vintage Bicycle Press)
Optimizing Your Tire Pressure for Your Weight
Inflating your tires to achieve 15% tire drop will optimize your bicycle’s performance, comfort and handling. Our tests of tire
resistance have shown that tire resistance is high at very low pressures. As pressures increase, tires roll faster, but the performance
levels off at a certain pressure. Beyond this point, higher inflation brings only negligible performance improvements.
Optimum pressures
Riding your tires at this “cut-off” pressure optimizes both comfort and performance. At lower pressures, you roll slower. At higher
pressures your bike is no faster, but much less comfortable. Our tests of the same tires at various pressures determined the
optimum pressure for each tire for our rider/bike combination. How does this translate for other riders and other tire widths?
I compared our results with Frank Berto’s charts for tire drop, and found that the “cut-off” pressure corresponds roughly to a
tire drop of 15%. Perhaps not coincidentally, 15% is the tire drop recommended by several tire manufacturers.
Tire drop is the amount the bicycle is lowered as the tires deform under the load of bike and rider.
Tire drop
Measuring tire drop is not easy. Fortunately, Frank Berto already has done it for us. The chart below shows the pressures required
for different rider/bike weights to achieve a tire drop of 15% with tires of various widths. Berto measured this for 700C tires, but the
values apply to other tire sizes as well. Note that the weights are wheel loads, not the weight of the entire bicycle.
Determining the wheel loads of your bicycle
[Use front 40-45%; rear 55-60%]
Over- and underinflation
The table below shows that narrow tires require very high pressures, otherwise the tire drop exceeds 15%. On the other hand,
wide tires do not require high pressures for optimum comfort and speed. Inflating tires to the maximum pressure recommended by
the manufacturer tends to underinflate narrow tires and to overinflate wide tires.
Conclusion
Tire pressures that correspond to 15% tire drop will optimize your bike’s performance and comfort on average road surfaces. On
very rough roads or unpaved roads, it may be useful to reduce the pressure. On very smooth roads, increasing the pressure slightly
may improve the performance of your bike.
Example: Rider and bike weight: 100 kg. Weight distribution:
45%/55%. Wheel loads: 45 kg/55 kg. Tire pressures for 20 mm tires: 125 psi/155 psi. Tire pressures for 37 mm tires: 45 psi/53
psi. For heavy riders/bikes, narrow tires require very high inflation pressures, and wide tires are a better choice.
