I mean, I think I know the answer to this...but hear me out.
Situation:
Mate's teenager has bought a Trek Marlin 8. It comes with a QR RockShox Judy Silver. First I know of this is stopping by his on Sunday, at which point we had a conversation over his Ring doorbell as they'd just gone on holiday. Long story short - he reckons if they brake hard the front tyre is rubbing on the fork crown, he'd cranked the QR down as hard as possible, but it's still happening. (He's a mech eng so has a good idea what he's doing). Now, I've had a few disc/QR bikes, and whilst I'm aware that the hub can move in the dropouts under braking...I've never felt it, certainly never buzzed a tyre, so I was slightly mystified. Anyway, he gave me the combination to the shed so that I can try it myself, I gave the bike quick run round the garden, brake hard on grass - which given recent weather obviously isn't offering the best traction - and BZZZZT, front tyre rubs on crown! Wheel is left slightly skewed in the dropout. Seems pretty clear to me that the hub is shifting in the dropout, although there is still plenty (ie a few mm) of clearance around the tyre/crown, so I can only assume that the hub is pulled back a bit in the dropout when the brake is released. There is plenty of clearance above the tyre and witness marks on the left of the crown.
He'd already taken it back (or talked, not sure) to the shop, and I can only assume got the Saturday boy, as their response was that his son is 'braking too hard' etc etc, the sort of crap you'd expect from a bad branch of Halfords perhaps. Honestly. It's embarrassing.
So firstly, I'll assume the consensus is that this just should not happen, talk to the manager etc, and if the shop can't sort it then it's a return not fit for purpose.
Secondly though, I'm quite surprised that this can happen in 2024, esp. given the dodgy history of disks and QRs and Trek (ie here which is admittedly different but I do remember re-designed fork dropouts appearing across the board to stop the pulling issue). Has anyone experienced anything like this? Only possible remedy I can think of is introducing more friction between QR and dropout to stop it moving (sandpaper?), or *maybe* a 'better' QR. If it was my bike I'm not sure I'd want it at all whatever the remedy TBH, the buzzing was just weird. Could there be something else going on? I'm pretty sure not, but who knows.
As it happens if the outcome is that he still has the bike, I think I'll be long term lending a 10 yr old bolt thru Rev that has just become surplus to requirements as it's just a much better fork. Assuming I can sort headset and front wheel economically of course.
All a bit odd really, but as ever, any wisdom gratefully received.
Probably a cheap qr lever. Most of the time a decent Shimano one will apply better clamping force and the issue will go away. Doesn’t need to be fancy - deore will do the job.
Agree it shouldn't!
If the hub is moving on the dropout, does the hub have a short stubby axle that goes into the dropout (like an oldie roadie bike did ?). If so is the slot of the drop out top wide - so allowing hub to move too much once the friction is overcome?
Do you mean “arch” when you say “crown”?
It’s not uncommon on cheap bikes and
as above a better QR is normally all that’s needed.
Genuine Shimano ones have a much better clamping force
Do you mean “arch” when you say “crown”?
ah, yes I do.
It’s not uncommon on cheap bikes
yeah, but it’s a £900 (actually £1000+ rep) bike. Clearly though using a qr fork (when rock shod offer a TA version!) is a Product Manager thing presumably to enable some upsell/differentiation which makes it all the more annoying.
*rock shox , damn you autocorrect.
I thought I’d gone mad, thanks for the clarification
Yeah, the Shimano QR thing is something I've already suggested to my mate if the shop can't or won't sort it....but really it still amazes me that Trek can spec a crappy QR given the history, or at least not test the full system somehow.
I thought I’d gone mad, thanks for the clarification
Yeah...shame I had brain fade and put it in the thread subject too, clearly sounds like nonsense when read back, dammit. Written on a phone not a real screen in my defence.
Make sure the bearings are correctly preloaded. If they are loose, the hub will move no matter how tight the QR is done up.
Otherwise, just make sure the axle is seating properly in the dropouts (turn the bike upside down and remove the QR to check) and that the QR is fitting into the dropouts correctly (i.e. it's not fouling on the "lawyer tabs"). As above, Shimano QRs are the gold standard.
WWID?
Take it back again and insist they sort it out.
Make sure the bearings are correctly preloaded. If they are loose, the hub will move no matter how tight the QR is done up.
This was also my thought. Should be fairly obvious that a QR is moving in the dropouts, but is the axle/skewer moving within the hub? Would seem unlikely if it's a cartridge bearing, but maybe they've put the incorrect endcaps on it or something? If it's cup/cone, maybe it's not been preloaded correctly as above.
Take it back again and insist they sort it out.
Obvious, but excellent advice.
I don't think it's the qr. If the qr was slipping I'd expect the wheel to remain misaligned after braking, but it sounds like the wheel returns to centre? Can you put the brake on full and push the wheel forwards to observe what is flexing?
Could it be a 29er wheel in a 27.5" fork?
I'm doubtful that if the hub was moving then you wouldn't feel it just pushing the wheel to one side. Try holding the wheel between your legs and twisting the bars. The forks will flex quite a bit. But if the wheel looks like it's flexing then it could be a poorly built wheel with very low spoke tension.
Have they put bigger tyres on it ?
I'm struggling to see how the tyre can hit the arch under braking (with the axle still "within" the dropouts) unless the tyre is too big for the specced fork (or just maybe the wheel itself is too big but that's pretty unlikely)
Any displacement within the dropouts would be away from the arch rather than towards it, wouldn't it?
It does sound like the wheel / tyre is too big for the fork. Like has been said above if the wheel is correctly installed in.the dropouts the only way it should go is away from the arch. If the slight flex caused when braking is causing the tyre to rub it must mean the tyre is almost touching the arch to begin with.
I'd understand if it was the crown being touched due to a too large wheel or tyre being installed.
Is there a photo of it all installed under no loading.
Any displacement within the dropouts would be away from the arch rather than towards it, wouldn’t it?
If there is play in the bearings, the wheel will twist sideways under braking and the tyre will rub against the side of the arch.
I am a mechanic, my first thought is poor quality, shoddy quick release lever. Major brands use really, really cheap sub-standard QRs on their cheap bikes.
First thing to try - replace the QR with a Shimano made one, with an internal cam structure.
External cam qr is probably what it now has, very likely with a plastic construction as well, that will wear with time, and probably give some when tightened properly.
https://brainybiker.com/internal-vs-external-cam-quick-release-skewers-my-comparison-analysis/
Also possible as said above - too big tyre, or play in the bearings. These should be checked too. Too big tyre is really the only thing to explain this, if the tyre hits the center of the arch. All other scenarios would have the wheel moving somewhat sideways under braking force.
That cheap wheel will also probably / very likely have cup-and-cone bearings from some high quality brand like Quando or Joytech. The bearings are usually way too tight and will eventually self-destruct, but it is also possible that they are way too loose and have tons of play there (they are never set just right from the factory, they will either be too tight or too loose, and possibly a lock nut might not be tightened against the cone securely).
wow, that'd be a LOT of play ! Should be easy to "detect", I guess
(just had a google and they do use shorter forks on the smaller bikes. Makes "the wrong fork" just a bit more likely I suppose)
I'm reasonably confident (prepares to duck and hide) that the current Judy chasis is good for up to a 2.8'' tyre (both 29 and 650b)
It would be odd if it were a 650b fork supplied on a 29er bike though, but IMHO is more likely the culprit than the QR as if the QR was loose to this level it would impact braking as the disc would be moving in the calliper. I'd be surprised if the calliper would allow the wheel to move "that much" as Shirley the disc would catch/limit such movement.
What does the label on the back of he legs state/serial number say about the fork?
Edit ....
Seems that the non boost chasis has a published maximum tyre clearance of 62mm so circa 2.4":
https://www.sram.com/en/rockshox/models/fs-jdys-tk-a3
Trek list the tyre used as being a 2.4'' Ardent across the wheel sizes so could it simply be too big a tyre?
Size: XXS
Maxxis Ardent, tubeless ready, EXO casing, folding bead, 60tpi, 26x2.40"
Size: XS , S
Maxxis Ardent, tubeless ready, EXO casing, folding bead, 60tpi, 27.5x2.40"
Size: M , ML , L , XL , XXL
Maxxis Ardent, tubeless ready, EXO casing, folding bead, 60tpi, 29x2.40"
If the tyre is hitting the crown on the fork, is the fork needing more air in it to prevent such massive compression during braking? I'm assuming the crown is the top of the fork that sits by the bottom of the headset...if so, it won't really matter if the QR wasn't holding the wheel properly as that would be side-to-side play/rubbing (also a possibility with disc brakes and a QR not done up properly) - however, having a QR done up properly is very important, just not for this paragraph of verbage that I'm typing as the QR won't make the top of the wheel move up and down, (just side-to-side), but the crown is above the tyre, so is the fork diving fully when brakes are applied?
If the tyre is hitting the crown on the fork, is the fork needing more air in it to prevent such massive compression during braking?
The OP has already confirmed that he actually meant the arch.
looking at the images of the stock bike, there is not a lot of clearance around the fork\tyre interface so you wouldn’t need a lot of movement to get a bit of rub. Look at 8.12 in this vid to see how close it is
If the tyre rubbing is 'as designed' under hard braking as the shop said then you would assume it's something a sharp mountain bike reviewer would have picked up on.
My son has the same bike, and never had an issue with front wheel/fork. I have changed the wheels to a lighter set (thanks to CRC), so don't have the same qr lever now.
It’s likely the QR that’s the issue here. Pop a dab of grease on the cam, as standard they feel like they’re done up tight due to the friction but they’re not.
A dab of grease means you can actually do it up tight enough.
Source: work at an IBD selling Trek and have encountered this exact issue before.
pals son had exactly rbis issue on an older Marlin - Shimano skewer sorted it.
Some amusing suggestions above, why do people guess/speculate with insufficient experience/knowledge?
Some amusing suggestions above, why do people guess/speculate with insufficient experience/knowledge?
Are you aware that you typed that on the 'internet' and didn't just say it to yourself?
Upgrade the forks and wheel to thru-axle. Hey presto!
(well the question is WWSTWD!)
Thanks all, pretty much confirming what I thought, and somewhat re-assuring to hear that a couple of people have seen this on the very same bike. Did seem weird that I'd haven't felt anything so bad before. Will suggest lubing the cam or using a Shimano QR.
And apologies for introducing confusion by writing 'crown' when of course I meant 'arch'.
Just to clarify - the disk side QR slips down in the dropout and stays there, you can see that the hub is skew after braking, and the tyre is off centre in the arch. It does pop back a bit once the brake is released, but as I say, clearly skew still.
btw, the tyre is the standard supplied tyre, a 2.4" I think, and sits with plenty of clearance around it to the arch normally. RS say that max tyre width is 81mm, so well within their spec.
And yes, I should have taken and posted pictures... and I will be forcing my thru axle RS Rev onto them, as even the thought that this could happen just makes me a bit jittery.
Nothing new to add other than to say non shimano QRs (cheap alternatives anyway) are universally crap. I had to replace the QR on my Bizango years ago because I pulled the rear wheel out of the dropout several times under my enormous power.
They're cheap and ugly but effective.
Maybe they need to stick some air in the forks?