TOST Drum Brake Adjustment
Every time that the TOST wheel or brake is serviced, this procedure must be followed in order to ensure that the brake is effective.
The challenge is that the brake is not inherently very effective – and in most gliders, under most circumstances, only gentle braking is required. An adverse outcome of a poorly tensioned brake cable is that the brake shoes become glazed. This requires that the brake pads be freshened with emery paper of about 250 grit to remove the glazing.
1. Adjusting the brake cable tension.
The brake lever is operated by the cable, which ends at the control stick. The lever’s fulcrum is toothed and fits over a fluted bolt that rotates to press the shoes against the drum. It is impossible to get adequate tension on the cable without adjusting the mechanism at the control stick.
The adjustment mechanism
The image at the right shows the end of the cable at the control stick. This is a bicycle cable adjustment mechanism, familiar to most cyclists (who have worked on their own bikes).
This brake lever adjustment photo is from a Schempp-Hirth Ventus CM. The location of the brake tension adjustment may vary depending on the sailplane and sailplane manufacturer.
In order to properly adjust the brake tension any time that the lever arm must be removed one must first fully loosen the adjustment mechanism, circled here.
First, release the lock nut at the top of the mechanism.
Then, back the fitting out of the control stick until it wiggles slightly, indicating there are few threads still in the stick (to save the annoyance of re-threading it in this tight space).
Now you are free to lie down under the glider and adjust the brake lever arm.
After that is done, you can return to the cockpit, screw the fitting back into the control stick until the brake lever on the control stick meets resistance when squeezed (does not feel loose!)
This can, of course, be readjusted any time.
Do secure the lock nut before walking away…
2. Assembling the brake lever at the wheel.
It is impossible to remove the wheel (typically to change the tire) without removing the brake lever.
This means that the brake tension must be adjusted every time the wheel is removed. The only other task that requires removing the wheel is dressing or replacing the brake shoes.
On right, the brake lever arm is dangling after removal. Don’t bother to remove the brake cable, because replacing that is more work than you care to imagine when the core task is merely changing the tire.
Above left is the fluted bolt over which the lever arm is placed after the wheel is back in place. It is not possible to adjust this bolt.
3. Pretensioning the lever arm.
Even though the cable housing has been adjusted to its greatest looseness, life will be easiest if the brake lever arm is tightened against the cable before sliding it over the flutes and replacing the nut that holds the arm on. You can hang an elephant from it, or use a long screwdriver as a lever. This would be easier with an assistant if you can both fit under the glider.
4. After tightening the nut, return to the cockpit and tighten the cable.
Reprinted by permission, Copyright © 2022, DrDan Johnson
