I've noticed a rubbing sound on my Canondale Synapse the last two times I've ridden. The sound usually occured while putting load on the crank (usually going up hill). This afternoon I checked to see that the rear wheel is tight and noticed that although the wheel appears to be secured (it takes preasure to close the quick release) the wing nut on the opposite side of the quick release turns freely. I assume this isn't right and I'm worried the bike may not be safe to ride. Am I right? Is this something I can fix myself? Thanks in advance for the help.

If by wing nut you mean the knob that is on the opposite end from the q/r lever, no that generally should not turn freely, though there are some rare levers that have a cover on them. Leave the wheel in place and remove the q/r lever completely. Check to make sure the q/r springs have the large end toward the outside, the small end toward the center of the hub. Also, the end of the axle should not protrude past the dropout that it is sitting in, which is the most likely problem. The locknut for that side may have backed off. You would need a cone wrench and perhaps in-person assistance to tighten the locknut and adjust the hub.

By "wing nut on the opposite side of the quick release", I presume you mean the right side of the bike, with the quick release being on the left side of the bike. The wing nut should not spin freely when the quick release is closed. Open the quick release, and tighten the wing nut a turn, then close the quick release. If it is still loose, repeat. Stop when the QR is difficult, but not impossible, to close. Keep in mind that you'll have to open it if you have a flat - so too tight isn't good, either. If nothing you do tightens the nut - take it to the shop.

Make sure your wheel spins freely, too, when the QR is closed, and doesn't rub against your pads. If it does, loosen the QR, readjust the wheel and then close the QR. You may need to widen the gap on your pads by turning the barrel adjuster a bit, too.

IF the QR is too loose, the wheel may flex under a load and rub against the brake pads.

The other scenario, which is what happened to me, is your rear wheel needs attention. The spokes may becoming loose and need tightened. Or, the wheel is possibly not rigid enough for your size and power.

the axle maybe broken and will need replacing .

Yes, by "wing nut" I mean the bolt on the right side opposite the quick release lever. I unscrewed it from the axle and discovered that the plastic over the actual bolt was cracked -preventing it from tightening. I replaced it with a "wing nut" from an old wheel and it now tightens properly. Although I won't know until tomorrow's ride, I suspect the rubbing issue will be fixed as well.

Thanks everyone. Obviously I have a lot to learn about bike mechanics but you guys are helping.

I'm also guessing I cracked it by screwing it too tight on the trainer. Any words of wisdom about how to keep the bike secure on the trainer without over tightening?

Find an old-school steel quick release and swap it in whenever you use the trainer.

Good idea. I'll track one down.
Thanks.

Many trainers come with a replacement all-metal quick release skewer to take the load from the clamps without damaging the one that came with the wheel. If not, any bike shop should have a suitable sacrificial qr.

I found an all-metal skewer in my box of bike stuff so my trainer apparently came with one, too. Now I know what it's for. Thanks.

Look, I'm a mechanical genius so maybe I have a skewed perspective, but I'm convinced the Bicycle is simple and straightforward enough for anyone to be able to figure out like 80% of what goes on with one, just by looking at it and scratching their head.

I'm always baffled by folks asking the about the most elementary stuff, and my first impulse is not to answer the question but to tell them to stop looking at me, and really look at the bike.

This stuff is way easier than most folks imagine, even wheelbuilding. Just expect to be able to wrap your head around it by looking, and you're off and running; it's only simple intuitive physics - you just need logic.

Sorry if it seems like a bit of a rant (it's just a general comment, not directed at you; I'm not saying you shouldn't have made this post), but this medium (the forum) is best employed to find out about that last 20%...

For help with the first 80%, check out https://www.sheldonbrown.com/ and https://www.parktool.com/blog/repair-help ... I think there are a few other good sites, but they're prolly the best place to start.

It's fun. Enjoy

The key with any quick release is that the mechanism is "closed" like the one illustrated. "Closed" refers to the eccentric mechanism which does the clamping being internal; you should not be able to see how it works. The "open" type are weak and inferior in their clamping force.

You should tighten the nut opposite to the quick release lever enough that the lever makes a mark in your palm when you swing it closed; it should take more force than you can exert with your fingers. Face the lever towards the rear of the bike so that it does not get caught and flipped open by passing objects, but put it where you can get your fingers behind it to get it open, not against a fork blade or stay.

The OP already found a problem with the q/r nut end, and he indicated from the outset that he knew how to use the q/r ("it takes pressure to close the quick release").