Thru axles with rim brakes
 

I keep reading how thru axles on road bikes with disc brakes stiffen up the frame. So why not put thru axles on rim brake versions?

The stiffening of the frame is a by-product effect of the thru axle design. There are 2 primary reasons for using thru axles with disk brakes:

1. Because of the high mechanical advantage of disk brakes couple with the offset moment of rotation (for lack of a better term), there is a tendency for the hub axle to be pulled out of the fork dropouts of an open-ended fork. The thru axle design eliminates this hazard.

2. With open dropouts and standard axles the tendency is that there is often rotor drag on the pads in the caliper when a wheel is removed and replaced often due to less then perfectly round axles. The thru axle design tends to eliminate this issue.

Increased frame stiffness is a big plus though and there is no reason why the design cannot be used on bikes designed for rim brakes...perhaps manufacturers will move to that as part of standardizing production of frames and forks.

-j

Also braking force is exerted on only one fork leg. Coupling the fork legs together helps distribute this load to both legs. With rim brakes, this isn't necessary.

Years ago, when this change was driven into mountain bikes the same reasons were exposed as myth. The actual added stiffness is less than 10%. The real reason is lawsuits, the through axle eliminates accidents caused improper qr use, it's a bigger problem with disc brakes.

The second fork leg is just added weight and should be left to dusty history books. :)

Thru-axles, which pretty much defeat the purpose of quick releases on wheels, exist to indemnify manufacturers against disk-brake wheel ejections.

On my bikes, since I pop the front wheel several times per day, fiddling with thru-axles would be a pain. Hell, the first thing I do with a new road bike is grind the !#@$!! lawyer tabs off of the forks. I know I cannot safely get away with this on disk-brake setups.

Yet another downside of disk brakes on road bikes. Aside from them being heavy, fussy and unnecessary.

I understand the rotation problem with disks and the front fork. Wouldn't this problem be solved if the caliper was on the front of the fork? Then the "rotation" induced by the caliper on the disk would tend to force the axle further into the fork.

Just askin' for my own curiosity. I don't have a disk bike.

Then all the braking force would be on the teeny weeny little screws holding the caliper on. Although the front rim brake is on the front but the bolt for that runs all the way through the fork. You'd need to have bolts running all the way through the fork with front mounted disks too. Plus, with mountain bikes, you'd run the risk of ripping the caliper off if a stick or bush wedged itself between the wheel and front mounted caliper (always make sure QR skewers point backwards on MTBs for this reason).