BXT-MTB-117 150mm AM bike

[veeTee1Pah]

naeTech has posted some great information about this frame. I built up a bike using this frame, but I haven't been able to ride it much due to snow season, but here are some quirks I've noticed so far.

• The suspension gets stuck when the shock is bottomed out with a 210x55mm shock. (Edit: This may happen only on some size frames. See reply #61 for more info) The lower linkage rotates to the point where it gets cammed in the bottom out position, and the suspension won't extend. naeTech says this happened to him at 52mm, so the practical option is to use a 210x50mm shock for 136mm of travel according to his model. This bike is not a 150mm rear travel bike.
• According to naeTech's model, the leverage rate of the suspension has a 39.2% progressivity through 136mm of travel. This seems a bit extreme, and I couldn't find another example of a trail bike with similar travel and that much progression.
• I experienced excessive friction on the upper linkage / lower bolt at low torque. I tried some different washers that helped somewhat. In an mtbr thread, naeTech has found that a spacer that bridges the inner races of the bearing here on the hightower is missing from this frame. A custom spacer might help here. The linkages are not very refined. Many of the bolts experienced friction at torques much lower than SC hightower specs.

Some of the quirks might be addressed with some tinkering, but understand that you are getting 136mm travel with almost 40% progressivity with this frame. It seems like these numbers would be hard to change unless you are able to take some extreme measures like fabricating a new linkage. (Edit: Maybe not accurate for all sizes. See reply #61 for more info)[/b]

If anyone can think of a way to get closer to full 150mm travel and a less extreme leverage progressivity, I'd love to hear it!

[daifanshi]

This frame is basically a clone of the Santa Cruz Hightower V2.  Depending on which model year they copied, the shock specs are actually different depending on the frame size.  I believe S and M specified a 210x52.5mm shock while L,XL,XXL are able to accept a 210x55mm shock.  Travel was either 140mm or 145mm depending on the model year.  So the suspension model may not apply to all sizes.

But I would think that 150mm sounds a bit out of reach for this frame even if they advertise it as such.  Cascade makes a alternate lower link that claims 160mm of travel for the hightower, but who knows if that will fit in the BXT.   For now I would just try it with a 55mm stroke to see if the suspension locks.  And then add enough 2.5mm spacers until it doesn't.   

[naeTech]

I use Size S frame for make measurements for linkage model. Different sizes have different lower link or differen front triangle axle positions?
If you have measurements for cascade alternate lover link (between axles) - then i make calculation for such setup
Upd: Usually inside the positive air shock chamber there is an oring 2..3 mm thick that holds the volume spacers. It also limits the stroke that can be squeezed with your hands.
I haven’t fully assembled the bike yet, but it seems to me that the hand-locked suspension can be unlocked simply by sitting on top of the bike...
Those. probably this is a false alarm - there is latching, but you can only encounter it by squeezing the deflated shock with your hands)

[naeTech]

I make spacers! Now levers works great not depend on tightening force.
Drill holes at lower link 17mm and upper link 19mm.
Spacer size for lower link 12.1x16x37.2 (+0.2 to bearing space), 12.1x16x44.2. For upper link 15.1x18.5x51.2. Why bxt not did that in stock??? (its cheap operation in industrial environment)

[daifanshi]

Maybe I'm not getting what this is for, but can't you just put two separate spacers on the outside of the bearings using the existing through bolt without redrilling the linkages?

[naeTech]

Without drilling spacers not fit
(bearing spacer (what i custom) - red color, outside separate spacers - yellow)

[veeTee1Pah]

naeTech, very impressive modification. Did you bore out the linkage to make the hole wider, and can you do this easily with a normal drill press?

I tried to free the stuck suspension at bottom out by sitting and bouncing on the bike, but it did not break free. Even if this method worked to free the stuck suspension, it would still be bad to have the linkage jam in the bottom out position on a big jump. Let me know if your experience is different.

[veeTee1Pah]

Maybe I'm not getting what this is for, but can't you just put two separate spacers on the outside of the bearings using the existing through bolt without redrilling the linkages?

daifanshi, naeTech is talking about a spacer between the outboard bearings that appears as a tube labeled "spacer" in the attached picture of the hightower v2 linkage. They bridge the inner races of the bearings accrding to naeTech's diagram. The hightower has them and the bxt doesn't. I experienced some binding and friction at low torques from these pivots. The spacers may help with this, but it looks like you have to modify the linkage and fabricate the spacers.

naeTech, did you add 2 spacers to the lower linkage as in the attached diagram?

While the BXT 117 may look like a hightower v2 copy, the kinematics, weird stuck suspension problem, and lack of linkage refinement like spacers suggest the copy isn't very good . The progression of the leverage curve is significantly greater on the BXT (some might say extreme)  if naeTech's model is accurate. I don't think you could call the kinematics the same.

All sizes of the hightower 2 accept a 55mm stroke shock, but the same stroke on the BXT 117 will cause suspension to jam at bottom out. As I understand, the 2020 hightower 2 had a 52.5mm stroke shock for "140mm" travel and the next year they shipped the same frame with 55mm of stroke for "145mm" travel in all sizes. I tested and jammed it with 55mm stroke and low air pressure on a size medium. naeTech says it happened at 52mm of stroke on size small.

If you revert to a 50mm stroke shock, the actual travel on this frame seems to be about 136mm, and the progressivity may make it difficult to use full travel. The takeaway is that the suspension travel falls short on this frame not because of marketing exaggeration but a defect that prevents you from using a 55mm stroke shock as designed. (Edit: This may happen only on some size frames. See reply #61 for more info)[/b]

If you have this frame, I suggest you try for yourself because it could be dangerous on the trail (although the high progressivity might make it difficult to reach the bottom out point). I believe you will need at least a bit of air pressure in the shock to get the suspension to jam. It's the extension pressure that cams the linkage into bottom out position.

[daifanshi]

I left about 25psi in the my son's Fox DPS 210x55mm shock and I confirmed I can't get it to "lock" or "cam" when the suspension bottoms out with the full shock stroke.  It is a size Large frame.  For us this appears to be a non-problem.   And I do not see any issues with friction in the pivots.  He purchased this frame more than a year ago. Maybe something changed over the last year. 

[naeTech]

Can you unscrew the big axle and take a picture of the internal space between the bearings? Check if there is a spacer there?
In such manner :

[naeTech]

naeTech, did you add 2 spacers to the lower linkage as in the attached diagram?

Yes - in lower same situation.
Aluminum drills easily by sharp drill. The main thing is to securely fix the parts.
It seems to me that for such a progressive frame it is not particularly important whether it gives 135 or 150 strokes. This is more critical for linear frames. Branded bicycles also have bump stops and full travel can only be achieved in extreme conditions.

[daifanshi]

Can you unscrew the big axle and take a picture of the internal space between the bearings? Check if there is a spacer there?
In such manner :

This might take a while.  Don't hold your breath. Not my bike.

I just remembered I posted a picture of the top link a year ago in reply #22 of this thread. See if it shows something.

[naeTech]

This might take a while.  Don't hold your breath. Not my bike.

I just remembered I posted a picture of the top link a year ago in reply #22 of this thread. See if it shows something.

I not see spacer tube here.

[veeTee1Pah]

daifanshi, seems like size Large may not get stuck at bottom out. That's good news for Large frame owners. I'd still suggest that owners test their frame just in case to prevent a potential dangerous situation on trail.

It seems to me that for such a progressive frame it is not particularly important whether it gives 135 or 150 strokes. This is more critical for linear frames. Branded bicycles also have bump stops and full travel can only be achieved in extreme conditions.

I just hope I'm able to use full travel on a bike with such a progressive leverage curve. As a light rider, I'm afraid the 135mm of travel will feel like 120mm. I will see in the springtime when I can test the bike.

[FullCarbonAlchemist]

Eh, using full travel in anything short of super extreme situations is kind of a <2015 thing. Suspension has improved now to the point where impact force should be well managed even if you don’t use more than 70-80% travel on most rides even if they’re pretty intense. Only large drops and jumps should get you beyond that.