Jim’s Tech Talk
By Jim Langley
This week, I’ve got some wheel-compatibility suggestions for Mark Pryor, the co-founder of Alameda Velo (“we are goal-focused everyday cyclists”). They’re in the San Francisco Bay area. If you’re interested, learn more here www.alamedavelo.com.
It turns out that Mark and I met during the Low-Key Hill Climbs, fun events that took place for a few years in the fall on famous and not so famous steeps all over the Bay Area.
Mark reached out for help with his new all-road rig. He wrote…
One Bike, Two Wheelsets
“I bought a gravel bike (steel frame, Shimano GRX 800 mechanical with hydraulic disc brakes) and I recently purchased a second wheelset (Hunt 4 Season 24-spoke) to ride tubeless road in addition to the original DT Swiss Spline wheelset with tubeless Gravel Kings.
I have ridden the gravel wheels for about 2,000 miles and the road set-up for about 1,500 since buying the bike. I love the “do it all” nature of the flexible set-up, but I do have some issues with the road wheelset.
Both wheels have 160mm rotors and thru-axles, but when I switch to the road wheels I get the following:
• There is an occasional but thankfully brief rotor rub with both brakes that may last for a few blocks at the start of the ride then stops, but then shows up at random times during the ride, but always just for a few blocks. The gravel wheelset is completely silent when I use it.
• I do get a bit of rotor rub on the road wheelset when out of the saddle on a steeper pitch, but again, it lasts briefly then the rotors spin quietly, even out of the saddle. Again, silence with the DT Swiss set-up.
• The shifting with the road set-up is slightly hesitant when shifting through the gears, but the gravel wheelset shifts crisply without hesitation. I am too lazy to adjust the cable tension at each wheelset swap, but it seems to me that there is a fraction of a millimeter difference in the DT Swiss rear hub (using a 34 tooth 11 speed cassette) vs. the Hunt (30 tooth 11 speed cassette). Is the differing cassette size the issue?
I have run through the basic disc wheelset maintenance checklist like loosening the caliper and retorquing the bolts while engaging the lever and ensuring the rotors are properly installed and torqued, but the noise persists, albeit briefly.
This is my first disc brake bike, so is it something that I’ll have to live with as it’s just the way it is with discs, or is something fundamentally incorrect with the road set up vs. the gravel?
One last question: is there a trick to installing the rear wheel with disc brakes? It always seems to hang up just below the drop out so I can’t get the thru-axle in there unless I turn the bike upside down.
I am sure a lot of us disc-brake noobs could use some advice living with them vs. the good ol’ days of calipers!”
Great questions, Mark. I actually just answered similar ones for another reader. So, this time, I’ll share the advice to help any other dual wheelers dealing with these frustrations.
For riders who might not know, I should explain first that using two sets of wheels like this is not specific to disc-brake or gravel bikes. It’s a common practice for racers who train and warm up at events on heavy-duty hoops and then switch to their lightweight, high performance tubular (sew-up) or tubeless wheels or aero, etc., just before the race.
At some events the other wheels can then ride in the race support vehicle if there is one or rest beside the course for lap courses like criteriums. That way should you flat, you’ll have a spare wheel that fits your bike at the ready.
Probably a Hub Issue
On the rotor and shifting issues, usually, as a mechanic or bike builder, if you want two sets of different wheels to go in and out without any problems (true compatibility), you do it by using the exact same hubs, rotors and cassettes. That way there should be no spacing differences.
Since you’re having issues, Mark, I’m guessing it’s because one hubset is different from the other since the hubs are the key thing that set the spacing for the cassette and rotors. Hub manufacturers in theory would make everything the same as all the other hub makers but that only holds true in theory, unfortunately.
Spacers/shims to the Rescue
For fixing the rotors you can find rotor shims (they go beneath the rotor) on Amazon and from bicycle shops and bike parts companies. For example, Tektro/TRP, SRAM and Syntace make them. They come in a few thicknesses, such as 0.2, 0.5 and 1.0mm. Those should do the trick to move the rotor that tends to rub into the right position.
And, there are also cassette spacers that may (keep reading) do the trick to slightly nudge the cassette into the same position as the one on the other wheel. These, too, can be found at bicycle shops or online and come in a few thicknesses.
I said “may” because it depends on the hub’s cassette body whether one of these spacers will work. There has to be enough cassette body width for the spacer to fit AND the last (smallest cog) and cassette lockring to be fully engaged and tightened respectively. (Wheel makers using their own proprietary hubs should be able to tell you if a spacer will work.)
If not, don’t use a spacer. Instead, just fine tune the shifting with the cable adjustment barrel every time you swap the wheels.
Dealing with Stubborn Wheels
Now, on getting that thru-axle rear wheel in place, I think that the derailleur and chain are interfering with getting the axle lined up in the dropouts. The derailleur spring is really strong now on a lot of the modern gravel derailleurs like yours. That’s to help prevent chain slap and chain drops, but it can mess with easy wheel removal and installation.
Note that on your derailleur, the Shimano GRX, there’s a clutch. It has a gray on/off switch near the top pulley. If you move the clutch switch forward all the way, it turns off the clutch and this lets the pulley cage and body of the derailleur move freely.
With the clutch off you can take the derailleur out of the wheel-install equation by pulling the derailleur – and the chain along with it – back, and holding it like that the entire time you put the wheel in. That way you should be able to get the wheel in and push the thru-axle through without too much of a struggle. Be sure to turn the clutch back on before riding.
10,095 Daily Rides in a Row
Jim Langley is RBR’s Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his “cycling aficionado” website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim’s streak of consecutive cycling days has reached more than 10,000. Click to read Jim’s full bio.