Having to do With Court's New Review of the Biktrix Juggernaut Ultra FS

[reed scott]

It's a very well done review. Seldom have I seen an equally thorough treatment. In his review he talks about a bit of flex in the rear triangle due to the power of the motor at high torque:

This 'flexing of the rear triangle' is a new thing to me. Can't say I have heard anything about such a thing from any body. I don't doubt it exists as if you think about it the motor and chain are initiating force from one side only ... not down the middle of the axle ... and 160 Nm is about double anything a Eurobike motor delivers. However Court's mention of hearing noise from the rear brake disc as indicative of such frame movement mystifies me. Why would this happen and how? He mentions this twice. The second time at about 31 minute mark of his video.

How would this flex be eliminated and how do manufacturers deal with the problem? Is that extra piece of material vertically mounted in Frey bike frames to do with this? I've always wondered why Frey is the only manufacturer that puts this extra material in the rear triangle of a bike.

 

[CodyDog]

"Juggernaut Ultra FS eBike the best-built full-suspension electric bike available."

Far from it! Marketing is one thing but making that kind of claim make me wonder about these companies.

 

[smorgasbord]

This 'flexing of the rear triangle' is a new thing to me. Can't say I have heard anything about such a thing from any body. I don't doubt it exists as if you think about it the motor and chain are initiating force from one side only ... not down the middle of the axle ... and 160 Nm is about double anything a Eurobike motor delivers. However Court's mention of hearing noise from the rear brake disc as indicative of such frame movement mystifies me. Why would this happen and how? He mentions this twice. The second time at about 31 minute mark of his video.

If you watch closely at the first "flex" mention (24:50) you'll see that Court is wrong about the cause. The swing arm itself isn't deflecting, it's the center pivot arm (not sure what that's called) which is pivoting on its bearings. This is the arm that translates the swing arm up/down motion on one end into the in/out motion for the rear shock at the other end.
Here's an arrow pointing to the center pivot arm:

Here's a shot from the video:

When watching the video on your computer, go full screen and place the mouse cursor at the rear arrow (leftmost in the above photo), which is where the swing arm joins to this center pivot arm thing. As Court uses throttle and the shift interlock re-enables you'll see that point move laterally (up/down on the screen relative to the cursor). The swing arm itself doesn't appear to deform as much as there is play somewhere in this multi-pivot mechanism. Note that the distance from the center pivot to the rear swing-arm pivot is pretty big so any small play in the center pivot gets magnified out by leverage.
Maybe this is something that can be adjusted or maybe it's something in the quality of the bearings used, or maybe it's just this particular example.

I also note that in both this section and the later section, where Court is climbing a short steep hill, he's in the top or almost top gear. In the second section of the video, he's again using only throttle and is going too slow and up too steep for that top gear. The top gear (small sprocket at the rear) isn't meant for hill climbing, it's meant for high speed on level or downhill ground.

I'm also not understanding the rear brake "zing" thing, either. The wheel, the brake disc, and the brake caliper are all attached to the swing arm. So, even if it deflects (or pivots as shown above), that shouldn't affect alignment.

 

[reed scott]

Interesting analysis. I hope we can get Court to elaborate. You make a lot of sense in your argument. It seems clear to me that with that many pivot points there almost has to be some play ... allowing side to side 'slop' or movement. Have to go back and watch the relevant portion again, maybe at slow speed. Now I'm curious if we can see any side to side movement in the single large bottom pivot.
I would think this situation exists in most bikes with this sort of suspension configuration.

 

[Admin]

Hey guys! Yeah @smorgasbord I agree. To me the play or "flex" as I am used to calling it, seems to be originating at the joints. When I use the phrase "rear swing arm" I'm not being as exact and detailed as I could be... and I might have just been sloppy because I'm so used to saying "frame flex" and most frames don't have pivots, unless they are folding designs

In short, the entire rear swing arm has some play when the motor pulls hard, which seems to originate at the pivot joints. I did not see as much flex in the seat stays or chain stays... but I suspect that there is still some. Your screenshot captures the exact moments I was thinking about and reflecting on after the ride test. You can literally see the position of the rear wheel changing, moving down in the shot as the motor pulls. Whatever part has play or is flexing, the result is that the cassette changes position a bit, especially if you're riding in a higher gear (the smaller rings, which are further away from alignment with the chainring). This is where I noticed the most phantom shifting under heavy motor power.

I hope this helps to acknowledge your points. Thanks for taking the time to post a picture and share so we could clarify the situation a bit

 

[smorgasbord]

@Court , thanks for the quick reply and confirmation.
Now the question is whether that pivot bearing play is endemic to just the particular example you tested and/or whether those pivot bearings can be adjusted to remove/reduce the play. I looked again at my own full suspension Bafang Ultra powered bike (on which I've not encountered that problem), and it uses a different design for that middle pivoting piece, with it being shorter and with the attachment to the rear swing arm much much closer to the central pivot (so less leverage) and with the rear shock horizontal instead of vertical. I also wonder is the lateral spacing of the pivot points has anything to do with this as well. Might take a Mechanical Engineer to really say.

EDIT: Also might be worth reading this older thread. If I'm reading it right, those linkage bolts are adjustable and one person reported them loose while another reported that it was actually missing pieces! Court, do you still have the bike to check?

 

[reed scott]

It could be this is a more solid less likely to allow side to side 'slop' in the linkage:

 

[reed scott]

This is the way the venerable Turbo Levo SL does it. Doesn't look nearly as solid as the Wattwagon's configuration.

 

[smorgasbord]

I suspect the issue is more related to the quality and adjustment of the pivot bearings as well as the horizontal distance between the center pivot point and the rear swing arm attachment point. Both alternate bike photos posted here have a much shorter distance than the Ultra FS.
But again, I wouldn't be surprised if Court's bike's pivots could simply be adjusted to be tighter to improve things.

 

[reed scott]

I mentioned it in my OP but I still wonder why Frey puts that vertical 'strut' or whatever it is called in their rear triangle. Additionally this pivot linkage looks highly susceptible to the problem at hand.

 

[MartsEbike]

To be honest... I think we need more reports before we can really be certain.

It would also be worth investigating the cassette before putting all the blame on the linkage.

 

[roshan]

Court brought this up with me and on our phone conversation I confirmed the following with him:
- if the bike is used the way it should be: using lower gears to get going and higher gears when at higher speeds, then there is almost NO flex/rear movement
- if the rider is on the wrong gear (usually higher that he should be) and gives 100% throttle, there will be some movement in the rear triangle due to the sheer 160NM of torque at the sprocket

I personally found the pull on the rear only when I give full throttle from standstill on the highest 2-3 gears. I think this is more of a user education thing than a bike design improvement. Our previous generations had quite a bit more flex than our current gen and so did about 6 other frames that we tried when we put this amount of power in the rear.

We've even tried some really expensive suspension designs like that from Specialized and that also has similar issues if used improperly.

 

[Deafcat]

Full suspension electric fatbikes are definitely an engineering challenge, all of the best suspension technology evolved around much narrower tires and wheels, pedal power, and so forth. Electric full-suspension fatbikes are definitely a new trend, Biktrix has been continuously improving theirs and it's cool to see how quickly they've dialed it in.

I've test ridden three generations of them and the newest ones definitely have greatly reduced sideplay in the rear under load. Ultimately, I think the Biktrix FS absolutely achieves it's goal of being the smoothest riding Ultra bike out there, I also found that sideplay didn't occur in lower gears (geometrically this makes sense, and confirms what others have found).


edit: to add my thoughts from a mech engineering background, ultimately I think it's impossible to eliminate all rear end sideplay in a heavyweight electric full-suspension fatbike with a wide 1x rear cassette (197mm rear hub). Dimensionally, it's a huge challenge and adding more structural members only compounds the weight gain of rear suspension. More complex bearing and bushings don't solve it either, it's simply an equation of number of pivot points multiplied by the distances of a frame/swingarm wrapping around a big fat tire, with the required chainstay lengths. From a bike builder perspective, I think Biktrix has pretty much nailed the solution. Cheers!

 

[Admin]

Hi @smorgasbord, I had a professional shop look over the bike with me before doing the review. I think everything was setup as it should be, without over-tightening which could stifle suspension movement. I may still have access to the bike, but am on a work trip with my girlfriend right now so don't have it in front of me. I appreciate what @roshan has said here. I did check with him during and after the review filming. It's always a challenge introducing critiques of a product when you're getting the marketing or "this is the best that is possible" language, which I interpret more as "this is the best we could do given the price point and available parts". I remember years ago when Pedego would say "gearless hub motors are the best" and then eventually they started using geared hub motors and mid-drives. It can take some time to adapt, and the financial incentives and business model has to be taken into account. I really enjoyed seeing the different suspension designs shared in this thread, and I agree that there is probably room for improvement. Biktrix has taken the design forward from their last gen product, and if it is shifted and ridden thoughtfully, it does perform well. But, me and some of my friends who test rode the bike did all experience some phantom shifting when we were just messing around and pushing the bike to its limits. I hope these insights are informative and empower people to make informed decisions and use it optimally

 

[roshan]

Hi @smorgasbord, I had a professional shop look over the bike with me before doing the review. I think everything was setup as it should be, without over-tightening which could stifle suspension movement. I may still have access to the bike, but am on a work trip with my girlfriend right now so don't have it in front of me. I appreciate what @roshan has said here. I did check with him during and after the review filming. It's always a challenge introducing critiques of a product when you're getting the marketing or "this is the best that is possible" language, which I interpret more as "this is the best we could do given the price point and available parts". I remember years ago when Pedego would say "gearless hub motors are the best" and then eventually they started using geared hub motors and mid-drives. It can take some time to adapt, and the financial incentives and business model has to be taken into account. I really enjoyed seeing the different suspension designs shared in this thread, and I agree that there is probably room for improvement. Biktrix has taken the design forward from their last gen product, and if it is shifted and ridden thoughtfully, it does perform well. But, me and some of my friends who test rode the bike did all experience some phantom shifting when we were just messing around and pushing the bike to its limits. I hope these insights are informative and empower people to make informed decisions and use it optimally

You did an excellent job at this review, Court. Your attention to detail is impeccable

 

[smorgasbord]

...it's simply an equation of number of pivot points multiplied by the distances of a frame/swingarm wrapping around a big fat tire, with the required chainstay lengths.

I can't help but focus on the Ultra having a long horizontal distance (parallel to the ground) from the center pivot to the swing arm attachment, compared with some of the full-suspension bikes we've mentioned here (Frey, Luna, Watt Wagon, Specialized). Any play in the center pivot gets magnified by the length of the center arm to the swing arm attachment - the shorter that is the less side to side motion is produced given the same pivot bearing play.

I do understand that such play can't be fully removed, but the issue is the magnitude of the resulting deflection.

I also do appreciate Roshan's stance that when ridden properly this isn't an issue. Holding down full throttle and then shifting to see how the cut-off works, which results in the throttle being off then abruptly full on in high gears is not proper riding. Climbing short steep hills in a very high gear at slow speed with throttle only is also not proper riding. So I would expect that ridden properly this wouldn't come up except in rare circumstances, and even then it's certainly not catastrophic.

But, just general knowledge-wise, it would be interesting to hear from bike frame designers what the considerations around this are.

 

[Taylor57]

So I'm guessing u guys are not using these bikes for casual riding?

 

[scout592]

Does this thread only apply to the fs model or could the same concerns exist with the Juggernaut Ultra 1000 model? Thanks

 

[reed scott]

Does this thread only apply to the fs model or could the same concerns exist with the Juggernaut Ultra 1000 model? Thanks

I doubt a hard tail bike would have any such problem.

 

[greeno]

You are absolutely wrong as far as a hardtail is concerned. I have personally broken (3) aluminum frames at the same point and some friends have had failures too. Drive side lower chainstay close to the front curve portion. All you have to do is hit the throttle and hold the brake to see the torque stress. And knowing aluminum it will flex, and flex until it cracks and then your done. My last hardtail ( Surly Wednesday ) was a steel frame with a 170mm rear horizontal dropout. I built a torque bracket for me and my friends to clamp to the motor housing and lay flat to the non-drive rear chainstay and we use a hose clamp ( painted black ) and have not had any frame breaks in over a year . My buddy has a Juggernaut FS Ultra and the rear dropout is 197mm! So I'm guessing a wider footprint means less stability and more flex? Not bashing at all just my opinion. More pivot points means less stress per pivot yeah? I got a frame from WW and the rear dropout is 148mm and I see ZERO flex in the rear end and shifting is silent and has so far never made any noise at all. Flex is about transferring energy through the complete spectrum and the wider it is the more deflection. Thats why the 150mm Rockshox Bluto noodles a bit more than it should. Running a 110 or even a 135mm front hub keeps everything tighter, in my opinion.