Last modified: 2026-01-18 16:36:16
< 2026-01-17 2026-01-19 >The axle bolt on the tractor sheared because the shear strength of the bolt was not enough to handle the force going through it?
So what force was going through it?
It sheared when trying to drive up the step onto the patio, so I guess assume it is lifting materially all of the weight of the tractor and driver. Which I think is a fair assumption given that the driver is right over the rear axle.
So maybe the tractor is 50kg and the driver is 100kg? So the tractor is lifting 150kg, 1500N.
The rear wheel diameter is 430mm, so 215mm radius, and the shear plane is at 10mm radius, so the force in the shear plane is 21.5x the weight of the tractor, so 1500*21.5 = ~32kN.
What is the shear strength of an M6 bolt?
According to https://eurocodeapplied.com/design/en1993/bolt-design-properties the shear strength of a grade 8.8 M6 bolt is about 7.7kN, so in my application it was overloaded by a factor of about 4. So that would do it.
So then we come to the car. If we assume I am going to drill through the motorbike wheels and bolt to custom hubs, like this:
Then how much shear force is going to go through those bolts?
If the overall wheel diameter is 600mm and the bolts are at 100mm PCD.
Again let's say each wheel sees the entire vehicle weight in load. (In reality it would be distributed over the 4 wheels, and would only be the entire weight if you are lifting the car vertically, or accelerating/braking with coefficient of friction 1.0).
Pessimistically, the car weighs 250kg empty, and has 200kg of passengers, so 450kg. 4500N at the wheel, so 6*4500 = 27kN at the bolts, if you have 4 bolts then 7kN per bolt.
A grade 8.8 M8 bolt has a shear strength of 14kN so I think we're probably good. That's a safety factor of 2 even assuming the pessimistic scenario.
Plan:
I did another coat on the wheels. I also did an initial coat of the black hammerite on the bits that are easier to access now.
It looks good but goes on very thick! It seems to have closed up the holes for the brake cable so that may need addressing once it is dry. I tried to wick paint out of the gaps with the brush but that only seemed to make it worse. I think it is just very thick paint.
Apparently it is "quick-drying", but I don't know how quick is quick.
After lunch I think the plan is to realign the layshaft block and then see how dry the black paint is looking. If dry, start reassembling the tractor, else find something else to do. Maybe knock bearings out of the car wheels?
Front brake disc carrier:
Obviously will want re-making in aluminium once I'm happy with it. I need to check that the caliper won't foul the wheel, but currently the caliper has a piece of plastic jammed in between the pads which I can't remove without potentially introducing air into the system so I'm umming and ahhing about it. Probably I just have to do it, I'll probably have to replace the lines with different lengths anyway.
And the wheel hub with bearings removed:
I think we want 25mm front axle, which wants "6905" deep groove ball bearings, 25mm ID, 47mm OD, 9mm thickness.
To get the bearings in/out of the hub, the hub needs to be heated.
Also I have chipped the register on the side of this hub in trying to get the bearings out, so I think this particular one would rather be a rear wheel, where I don't intend to use that register.
< 2026-01-17 2026-01-19 >