You are correct that those levers are designed to be opened with your bare hands. Unfortunately it's usually easier to push the lever closed than to pull it open, so it's easy to over-tighten it.
- Start with the lever open, and facing back as shown in the picture
- Wind the nut on the other side back 2 or 3 turns.
- Close the lever. It should be very easy, and the wheel is not properly attached.
- Wind the nut in until it is tight (finger tight, do not use tools!)
- Open the lever.
- Wind the nut in half a turn.
- Close the lever. It should be firm, but not impossible.
- If it's too hard to close, back the nut off a small amount and try again.
- It it's too loose, tighten the nut up a small amount and try again.
Unless you're unusually weak, not being able to open it means it is on too tight. Even if you are somewhat weak, unless you have disk brakes "as tight as you can" should be ok.
Note that if you have disk brakes it is much more important that the quick release be tight. When you use disk brakes the reaction force is pushing the wheel down, out of the fork. It is better to have the QR slightly too tight than slightly too loose with disk brakes.
Also, the QR lever should always point backwards as shown. That makes it less likely to catch on anything and be pulled open (or throw you off the bike).
The link broke, so here's key bits of the article via archive.org and another article that summarises the problem
Disk brakes and quick releases - what you need to know
(Updated Jan 2006 with the first out-of court settlement)
Yes, that's right - a UK cyclist has recently settled out of court with a fork manufacturer, in what is I believe the first successful case of its kind. There are a few more details in this thread here - the rider himself is hoping to avoid the limelight but thought that others might like to know.
The purpose of these web pages is to explain the danger of using a disk brake with a standard quick-release fastening for a front wheel on a bicycle.
[NB for 'quick release' please read 'conventional axle in slotted dropouts'. A simple nutted axle may be slightly better than a QR in theory, as it can provide a higher clamping force. However I'm not convinced that in practice it is safer, as it will be less obvious if it loosens or slips, and in fact the greater bolt diameter will generate a stronger unscrewing force. The safe alternative is a properly clamped axle combined with fork ends with a blocked exit slot, as shown in the pictures below.]
A brief `Executive Summary'
There are two main aspects to the failure.
The disk brake generates a massive force largely downwards in the direction of the open fork ends. The friction of a quick release skewer is often not sufficient to stop the axle slipping down in the dropout slot. This is explained in more detail here,
The QR is initially restrained by the retention lip on the fork (assuming it is present), however over time the slipping of the quick release leads it to unscrew, which is described here. Once it has unscrewed enough, it can be forced over the retention lip and the rider will crash.
Below the analysis, we have a section of anecdotal and experimental support, and I've put the small number of manufacturers' comments on a separate page.