Just got my new carbon wheels fitted this morning ,however when fitting the rear wheel I spun it (foolishly) before fully tightening the sprocket bolts and snagged the speed sensor,it has scored the plastic and when I went for a ride the speedo was flickering about all over the place,i'm assuming because of the scoring to the plastic sensor,i didn't realise they were so sensitive to damage ,but I guess the word ''sensor'' says it all...i'm guessing theres no way of repairing this and I will have to fork out for a new one because of my hamfistedness...just going to try and source a new one.lesson learnt.
Yes why?...I had heard that titanium sprocket bolts can mess with the speedo but I thought stainless would be ok.
Just been out and swapped the bolts back to OE and hey presto, speedo working as normal again...thanks mate you just saved me the best part 0f £90 for a new sensor....why wouldn't it work properly with stainless bolts, they're both steel after all?
The speeds sensor is basically a "Hall Effect" sensor. As a bolt head passes over it, it induces an electromagnetic field in the sensor, the frequency of which (the interval between each passing bolt head) is calculated as a speed through a pre-determined calibration. Stainless steel has a lower ferrous content than ordinary steel and therefore does not induce a large enough change in the electromagnetic field to allow the sensor a reliable reading.
Common stainless steels like 304 and 316 are often (but not always) austenitic in structure due to the nickel content, this effectively makes them non magnetic or at most slightly magnetic. You can check with a magnet how weak the effect is. Most mild steels (including the steels used for common nuts and bolts are ferritic which is magnetic, the sensor is reading a magnetic effect as the bolt passes it
I replaced mine with titanium and it didn't work at all, so it seemed a reasonable guess that you'd replaced with stainless for your lovely new wheels. My thinking was that stainless is a little more magnetic than titanium but probably not enough to enable your sensor to work properly. The good news is that if you have allen-head type stainless bolts, you can still use them All you need to do is get some steel ball bearings from Halfords - they come in a race for a bicycle bottom bracket - and embed them in clear silicone in the allen bolt heads. Works a treat!
