Hi Ali,
I can offer this bit from the Park Tools website...
The recommended tension for spokes in bicycle wheels can be as low as 80 Kilograms force (Kgf) and as high as 230 Kilograms force. As a rule of thumb, it is best to set tension as high as the weakest link in the system will allow, which for a bicycle wheel is usually the rim. Therefore, to obtain a spoke tension recommendation for a specific wheel, it is best to contact the rim manufacturer.
...from:
http://www.parktool.com/blog/repair-help/wheel-tension-measurementBased on your average measurements, you're a bit low in the overall range as referenced by Park (midway in their overall regardless of rim weight is 155Kgf; they would recommend higher tensions for heavier rims, as shown below). As to the specific range recommended by Rigida for the Andra 30...that's where things get a bit more murky, thanks to Rigida's recent history:
http://www.thorncycles.co.uk/forums/index.php?topic=3571.0 I don't at present know where to find the factory-recommended figures for the Andra 30. If I can track down the specs for you, I'll post them in a followup.
Looking back at the full article on the Park site, they correctly state that "correct" tension will vary according to a number of factors, including spoke gauge and rim "weight" ("weight" being a proxy for not only mass, but construction, i.e. lightweight rims will be built of thinner extrusions than heavy-duty rims). Their general rule is, go with lesser tensions for light rims and higher tensions for heavy rims. According to Park's own figures, a "heavy" rim is one weighing more than 450g. The presence and absence of eyelets makes some difference (though the Andra eschews eyelets in favor of a heavy longitudinal central rib and centering sockets for the spoke nipples), as does anodizing (in my experience, Mavic MA-2 rims were always able to take much higher tension than their MA-40 brethren. The anodizing on the MA-40s was often breached by the factory setting/clinching of the eyelets, creating stress risers at the eyelet edges that would frequently propagate cracks if built with higher tensions).
Park further state...
Tire pressure will have the effect of lowering the tension of the wheel. Generally, do not try to account for this drop by adding more tension then recommended by the manufacturer.
...and this is true; compression caused by high tire pressure has the effect of squashing the rim smaller, thereby lowering spoke tension -- and the rim will pull toward the drive side, which has higher tension in a dished wheel. Ali...did you measure your spoke tension with a tire mounted and inflated, or as a bare set of wheels? This could account for some variation. I once built a set of track wheels for a friend who was using extremely high pressures in his tires. I could not keep the wheels
perfectly tensioned/centered unless I tensioned and dished them with the tires on and pressurized or under-dished them slightly with the tires off. The compressive effect on the extremely light rims was just too great, as was the variation in spoke tension for the bare wheel compared to one with the tires mounted and aired-up (225psi/15.5bar). The downside was effective spoke tension soared when the tires were deflated after use, as was the custom. Fortunately, there was never a rim failure, but I can see it could happen. Honestly, I wouldn't expect them to last long with a great number of de/compression cycles, but these were his special "race" wheels and saw little actual use.
Ali, what I have always tried to do is to concentrate on reducing variations in spoke tension so the wheel so the tension is as consistent as possible across (one side of) the wheel. I don't want my highest-tension spokes to exceed the recommended variation, yet try to get the lesser spokes to vary from the highest by the minimal amount possible (an average spoke tension based on a narrow overall range). The result is a wheel with high average tension. The reality of the situation is rims are often less than round, and there is a lot of variation in spoke-hole concentricity -- the hub flange holes aren't always the same distance from the axle.
You asked...
Is it normal for back wheel spoke tensions to be higher than front?
On a derailleur rear wheel, the dish can account for variations in wheel tension compared to the symmetrical (undished) front. For a dished rear wheel, go with the drive-side tension for a maximum. As Park correctly state, "it is important to only compare the tension of a spoke relative to spokes on the same side of the wheel". If all other parameters are the same i.e. equal flange height and spoke length and the wheels are symmetrical in dish -- or nearly so as in the case of a Rohloff-hubbed wheel -- then front and rear wheel tensions will be similar. Usually, they differ for the reasons listed above.
May I suggest a couple additional links you might find helpful?:
http://wheelfanatyk.blogspot.com/2009/08/wheel-building-tip-no-5-be-wheel.htmlhttp://wheelfanatyk.blogspot.com/2011/12/wheel-building-tip-no-14-set-spoke-path.html <-- Elbow setting makes a difference!
Having gone at some length, I think your
reported spoke tensions may be a bit low for a heavy rim like the Andra 30. If you check again, make sure you're taking the average per side of wheel, and that you're taking the tension with bare wheels (preferred) or deflated tires (acceptable). A personal concern: Given the rare but occasional incidence of Rohloff flange failure, I would err a bit on the slack side to avoid stressing those flanges unduly.
Sorry I can't be more specific at present; perhaps this will still be of some help?
Best,
Dan.
