Heinz,
You describe the problem as a strong shudder. It is possible that the problem is not caused by warped disks.
This symptom can also be caused by damage or breakage to the front crossframe.
The first check is to ensure that the two diagonal tiebars that form part of the crossframe are not broken or loose. These tiebars run from the bottom tube to the top and prevent the crossframe from flexing everytime the car is driven over a bump or the brakes are applied. Use a spanner or similar to hit the tiebars - the sound should be that of a sharp 'ping' not a dull sound. It is the same principle as a piano tuning fork.
The second check is to ensure that the crossframe mountings and tiebars that run from the bottom of the crossframe to the bottom chassis flange are neither broken nor loose.
Finally, check that there is no damage associated with a minor accident such as anything that is bent such as the crossframe tubes, stub axle or wheel. Checking the crossframe tubes is quite easy: remove the wheels and look through the length of the tube with your eye - you will see if it is bent. It is not a major task to straighten the tube.
Good luck!
