I've got a steel hack road bike (track ends) which lacks pannier rack mounts.
Could I get away with just drilling and tapping a hole (M4?) in each side? Or will the frame explode?
you'll be fine, as long as you are putting them in the plate material of the drop outs rather than in a frame tube.
Just get some p-clips.
p-clips won't take any weight.
M5 is the standard, should be fine if you leave enough meat around it.
Steel P-Clips are strong.
you could also attached to the wheel axle (you can get/make adapters)
but if dropouts are chunky drill and tape M5 will be ok
p clips work fine if decent quality ones carefully used. However if there is enough metal a proper threaded mount is better
One of the err...dudes on Pinkbike's epic Scott Voltage thread http://www.pinkbike.com/forum/listcomments/?threadid=81521 has drilled the head tube and BB shell to fit internal cables.
The alternative to P clips I used to use for a lower mount are these: http://www.cyclesurgery.com/blackburn%20custom%20eyelet%20kit/racks/bikes-components-bikewear/fcp-product/15897 - they fit into the triangular hole in old-fashioned dropouts.
I've used p-clips with a load on a rack and they did not hold, strength is not the issue, it's sliding down the stay that is the problem.
it's sliding down the stay that is the problem.
So put them at the bottom of the stay?
I've used p-clips with a load on a rack and they did not hold, strength is not the issue, it's sliding down the stay that is the problem.
perhaps they were too big?
If its a rear rack you're wanting to fit you could get one of the topeak racks which clamp to the seat post only. They also have the excellent mtx-quick-track with corresponding bags that slide onto the racks