Oracle frame flex

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.

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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.

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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.

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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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Inflated and on the rim. If I’m trying to be precise I’ll put a c clamp over the tire and then measure that. The Duro Wildlife was measured in that manner. Not sure on the rim width, but it was on a UDC 24" rim. The Maxxis 26x4.8 was just measured by placing a tape measure across the tire and eyeballing it. It appeared to be 4 3/8" across on an 80mm rim. Perhaps on a 100mm it would be 4.8".

I measured my duro crux 27.5 x 3.25 using a cheap electronic digital caliper I got from Harbor Freight tools years ago (works great, seems to be within .001 in accurate… I think it was like $14?)
Anyway it came out to 3.194 in wide.
I can’t measure the frame at the place where the tire rubs without removing the wheel, maybe later…

The tire width is supposed to be measured when the tire has a full contact on the ground (all knobs are flat on the same surface). They are still generally smaller than labeled though.

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What a ludicrous way to measure it.

Side knobs are there for turning grip - you’re not ever going to have them all on the ground with some tyres.

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I’ve got a few 29" frames (M4O medium, Oracle 29, Nimbus steel 29, old non-disc brake KH 29)
Are you in Quebec? I live in Michigan, and was on a road trip and in Ottawa last November!

I recently tried the Oracle, Qu-ax carbon post, and M4O Long Handlesaddle, since I wanted the lightest hill climbing setup…It worked, but I found it incredibly flexy. Not sure if it’s just in my head or not… My test was to hold the brake and press down on the front of the handle saddle.

Switching to an aluminum KH adjustable seatpost (my only other 25.4mm seatpost), seemed a bit better; seemed like about 50% less flex.

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I live right across the river from Ottawa haha.

Bring those frames over next time! We can take a few stats on those frames! :grin:

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We have a test jig for testing flex on frames (it tests both deflection and to destruction). Due to the leg profile it is stiffer that a lot of frames on the market. Of course it will still flex but it should not be enough to cause a problem. One of the areas of flex is actually movement in the bearings, this can appear to be frame flex.
The clearance on the frame (height) is lower than it should be for that tyre. anton005 where did get the unicycle from? I will then check on the batch number and determine what it should be.

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UDC used to sell this Oracle 27.5 with a 27.5 x 3.0 Kenda Havok tyre. They must have changed it recently. The Duro 3.25 doesn’t seem to fit. I had a similar experience (long time ago) when I bought a Qu Ax 29 with a tyre that rubbed the frame.