Oracle frame flex

Thought I would create a new topic instead of highjacking the hatchet review with these posts.

I recently bought an oracle 29, and noticed the tire (duro crux) rubbed the frame with moderate climbing force. Didn’t think too much about it as the tire is big and pushes the limit on frame width. Then saw another person mention the same issue. I went out and pushed on the wheel to flex it and was surprised at how much it flexed and how easily. It’s not the wheel flexing, I can definitely see the frame flexing where it connects to the bearing holders.

I used to ride the steel nimbus 29 and it didn’t flex nearly as much if at all.

Just wondering if other Oracle riders have the same problem?

I run a 2.8” tire on my 29’er Oracle frame so no rubbing but I decided to flex the frame by hand like you mentioned and was surprised how much it flexed! Looks a little rubbery down low

I would be interested to see how other 29’er frames compare

My KH29 has rim brakes and it doesn’t take a big effort to hear the pads squeak against the rim. Even a rough free mount is enough to hear it!

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Currently I run a 3" wide tire in my KH27.5 and I also experience tire rub when I’m really pushing it while holding on to my M4O Handle Saddle. But that saddle/handle setup is also very stiff so that may have an impact. Downsizing to 2.6" wide tire “solves” this.

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Flexing the frame can really be a problem. I’ve been riding unicycles since 1981 and have ridden quite a few forks. It can actually become a problem:

  • In the case of rim brakes, the brake pad can be briefly present under load, especially when starting off or driving uphill when you stand up

  • With thick tires that have little space to the fork.

I’m not sure if it’s just the forks to blame though. I can well imagine that the spoking is also a factor.

I know this problem from aluminum and steel forks.

But there are huge differences: If the forks are not installed, press the end together with your hand. Certain products flex extremely, with others you need more power. I am writing here as a dealer, I will be careful not to make specific statements. But everyone can test that for themselves


In my experience Oracle frames are some of the stiffer options, but I’d love to build a jig to do some scientific testing.

I’d expect the Impact hydroformed frames to perform very well in such testing.

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When I did this test - squeeze the empty fork legs together - my 26" Oracle was noticeably stiffer than the Mad4One frame I replaced it with. I think this is because the Oracle has round legs, and the M41 has flattened legs (stiffness is all about the diameter of the leg, and the Oracle legs are wider). But flattened legs give more clearance, so it’s a trade-off.

I think the bearings and the bearing holders also affect stiffness; when you are pulling up on the saddle, all that force is concentrated on a bearing in a holder that is basically hanging from a couple of bolts.

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interesting way of thinking. I didn’t even think of that. I guess it’s a mix of several things. I’m currently running steel forks with round tubes. The bearing mount is the simple version with open cups. I have no problems. Sometimes it will probably be difficult to assign the problem specifically

Squeezing the forks together without a wheel will give you the stiffness at the crown. Where I have noticed my frame flexing is where the legs connect to the bearing holders. Granted it comes with a tire that leaves very little tolerance for any movement, kind of seems like they are asking for trouble. I could probably shave the most outward Knobby’s by 1/4 Inch and that would solve the rubbing. It really only does it on one side, even though the wheel is centered. I may be stronger on my right side.

Squeezing the frame legs together we are only testing the stiffness in one direction (inwards), and are assuming a solid hub and a solid connection to it, but if the legs down by the hub were more flexible than up by the crown, that would be flexing as part of this test. It’s all connected so (assuming the ideal hub connection), the results are still valid.

If it does seem like the frame legs are moving at the hub, then it may well be an issue with the connection to your bearings (are your housings tight?), your bearings moving (are you using crank spacers, with cranks done up tightly?), or the bearings themselves (are they old - is there any play in them?)

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All frames out there have flex and it is normal. Obviously, you dont want a frame to feel like a spaghetti, but you also wouldn’t want a frame that doesn’t move at all. Anything too stiff will tend to break faster since the material isn’t able to take forces in all directions. Very stiff materials have their uses in certain areas (like tooling), materials with flex will be used where there will be vibrations and variation of the direction of forces.

I’d be curious to see the different flexes in frames, like @mowcius said it would be interesting to build a setup to test them. I already have a jig I could use for that, I guess I need frames other than Flansberriums now hahaha.


I think as well as testing flex side to side (mounting a frame sideways above a dial indicator and hanging something off the end), it would be super interesting to see if there are any different results when loading off-centre. When we push down super hard on a pedal we’re twisting things in all sorts of weird ways.


If I can get my hands on a couple frames of the same size I’ll try that out. Obviously it’s more than side to side, pushing on the pedals while pulling on the handle when climbing generates alot of torsion too!

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Thanks for starting this, I was going to as well.

The duro crux 3.25 tire on my new Oracle 27.5 (Jan 2022) rubs against the frame, but it doesn’t take super hard pedaling, merely mounting, or slightly more pressure due to feeling close to losing balance causes it. (I’m a beginner moving up from a club 24, before I’m really ready to).
I am heavy, close to 240lbs/108kg, so that doesn’t help I am sure.

UDC support suggested I check the bearing caps hadn’t loosened)… I tightened them some, which helped slightly but they seemed tight (almost wonder if it’s maybe a hair too tight now- the wheel will free roll when propped upside down but slows to a stop faster than my other uni or bike wheels as if there is some slight resistance)
The spokes seem tight too, when plucked they make a “ding” sound as you would expect something under tension would instead of a “thud”

I can’t quite tell where the movement is other than seems near the hub, and i would guess it takes near 10lbs(4.5kg) pressure with my thumb on either side to push the rim far enough that the tire touches the frame.
There’s not much clearance between the tire and frame (about 2mm / ~1/8"), pics below…

I am thinking i need to reduce tire size to maybe 3.0 instead of 3.25 to avoid the rub.
I have very little time and distance of easy beginner riding on it.
It is annoying and disappointing because one of the reasons I upgraded was to have a stronger uni with stronger ISIS setup. (I thought these were “built to go big and shred hard”?)

Maybe it should come with a 3.0 tire stock? (The M4O’s do).
I hope that would provide enough extra clearance over stock to avoid the rubbing.



Wow, that looks like the 27.5 has less clearance than the 29er. I do like the crux tire so far, but am looking for a replacement that is under 3 inches. I’m not sure which tire to get though.

I would be careful with your bearing cap, if it’s so tight that it impedes movement in the wheel it’s too tight and not so great for the bearings.

I’m sure there is some flex in the wheel, but on mine things feel tensioned properly, when I push on the tire laterally I can see the hub moving, so I know it’s not the wheel flexing.

I bought a bontrager xr2 29x3, put it on today and went for a short ride. For only being 0.25 inches narrower it looks much smaller than the crux. It’s a very light tire and I noticed that right away. There were a couple of climbing sections I’ve been working on, hadn’t completed either one up until now :grin:, cleared both hills today and no more rubbing. Pretty happy with it so far, probably not a tire for mud as it has pretty small Knobbys.

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You might want to put a tape measure on it and check the width. I’ve found that tires are frequently smaller than advertised.

Yeah, measures at about 2.8 inches. No biggie.

The Maxxis 26x4.8 on my Hatchet is actually about 4.4" and the 24x3.0 Duro Wildlife on my 24er is actually 2.8". I never actually measured the Duro Crux I had on my 27.5, but it did look to be pretty close to 3.25", so I don’t find it surprising at all that your Bontrager appears much smaller. A half inch difference is pretty massive.

How do you measure these widths? Are the tires on the rims and inflated? These measures would likely depend on the rim internal width as this alters the final shape of the tire…

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