I have done some quick "back of the fag packet" calculations to try to quantify the loads and stresses on the disc bolts.... ASSUMPTIONS - A sports motorcycle + 2 chubby occupants . Weight = 200kg [bike] + 100kg passenger/rider. Mtot = 400 kg - All braking force through front brake only. No aerodynamic or friction drag - All loads transferred by shear through disc bolts - Steady state deceleration "a" at -1g (-9.81m/s2) factored by 2 to account for safety (pessimistic) Geometry Data Wheel Rolling Radius R = 290mm = 0.29m [measured off my 749] 2 discs Disc Bolt PCD D = 90mm. Bolt PC Rad "r" = 45mm = 0.045m [measured off my 749] Disc Bolts: 5 x 8mm diameter Braking Loads: Braking Load "F" at tyre contact point F = Mtota = 400 x 2 x 9.81 = -7848 N Torque on wheel "T" due to Braking Load F: T = FR = Mtota R = 400 x 2 x 9.81x 0.29 = 2275 Nm Assume this Torque T is reacted equally at each disc through 5 disc bolts at radius "r". Load per disc bolt "P": P = T / (2x5r) = 2275 / (2 x 5 x 0.045) = 5057 N Shear stress "t" on each bolt: t = P/(Bolt CSA) = 5057 / (PI x 4x4) = 5057 / 50.27 = 100 N/mm2 Allowable Shear Stresses (difficult to find reliable data, so bear with me...) 10.9 Alloy Steel: Allow Shear Stress = 400 N/mm2 A2-70 Stainless: Allow Shear Stress = 300 N/mm2 Safety Margins (Reserve Factors R.F.) Based on a VERY pessimistic braking assumption for a heavily loaded bike (-2g deceleration, 2 passengers) Alloy Steel 10.9 bolts: R.F. = Allowable Stress / Applied Stress = 400 / 100 = 4.0 (Bolts can accomodate 4 times the load!) Stainless A2-70 bolts: R.F. = Allowable Stress / Applied Stress = 300 / 100 = 3.0 (Bolts can accomodate 3 times the load!) SUMMARY Based on my simple sums, there appears to be sufficient margin in either type of bolt. I have probably made an over simplification of the analysis, and of course I have not factored in the effects of fatigue or dynamic or thermal loads or taken into account environmental degradation effects....etc.
I love a good technical discussion that requires me to get my old Casio calculator out and drag up "O" Level Physics equations of motion... Triste...moi??? Oui, c'est vrai.. :tongue:
Might want to check the calculations using the information here: Article: Shear and Tension Capacity of stainless steel bolts http://www.blacksfasteners.co.nz/assets/BoltShearCapacity_14-15.pdf And here's another little gem I just dug out of my reference books....I trust it doesn't need explaining, other than alloy steel is usually what 10.9 bolts are.
I think even I am a bit tired of this subject now. The horse has bolted............ Risible attempt at humour...I think I'll stick to my day job. :tongue:
And, more importantly, can richgilb use stainless steel bolts ? With reference to your picture, the calliper mounting bolts, (2-off per calliper) should be M10x35 & the thread is a 1.5 pitch. Sorry, can't help with the other bolt as it would mean removing one from my bike to check. If you are struggling for an answer though & nobody with a loose calliper knows I could try removing one of mine to confirm exactly what they are. Just had a look in my workshop manual & it refers to the calliper "screws" as being M8 with a 1.25 pitch, but does not give a length. However, it should be easy enough to measure the length of one of these bolts with a steel rule. Just tried measuring the M8 bolts for length & 40mm looks to be the correct length. The heads of these bolts though are only about half the height of a normal M8 cap head bolt, which will almost certainly make them harder to source if you are looking to replace your existing bolts. Might be better to get your existing bolts re-plated, (roguard plated & yellow passivated) depending upon how concerned you are with originality.
You can get genuine Brembo spares at good prices from Guzzibits, those specific bolts should be available I would have thought
