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Re: Welcome to Chinertown - Introduce Yourself! Hi everyone,

I've only recently stumbled upon this topic and now I'm totally obsessed with it... thank you all for this awesome community!

After being a blind reader for a while, things are now starting to get a bit more serious. I'm planning to build an FM936 for myself and an FM1001 for my gf.

Currently thinking about the right wheelset for the FM936. I thought Lightbicycle AM928 rims and sapim CX-Ray spokes for proper "down country" capability. And then maybe light China hubs like the ZTTO M1 or similar. Fovno/Venfort seem to offer nice looking straight pull star ratchet hubs too, but I have no idea if they're any good. Hoping to find out more from the bold test pilots among you!

Cheers,

Julian
April 16, 2021, 03:50:52 AM 		1
Re: Carbonda FM936 Sizing I agree with both of the previous answers. Riding style and body proportions are more important than declared frame size and height.

Generally, the FM 936 has really long reach numbers, but also really low stack heights. So if you compare the geometry to other bikes, it makes more sense to think of the actual distance and angle between BB and grips when the handle bar is set at the desired height (like pinkbike's "RAD"). You can do this using https://www.bike-stats.de/en/

For example: The size medium FM 936 with a 120 fork has approx 466 reach and 599 stack. This is exactly the same as a size large Specialized Epic Evo with 460 reach and 611 stack, only that you could run the bar slightly lower on the FM 936 than on the Epic.

So a size medium FM 936 is pretty much an average size large down country bike. If that's what your're after, go with medium. The size large however isn't huge if compared to an average size large modern trail bike, but those usually have more travel. Doesn't mean that you can't still put a short stem on a large FM 936 and ride it like a trail bike. It just depends on what you want to do with it. Although in any way - even with the long geo of a size large - the suspension design will never feel the same as more downhill focused trail bikes like the Transition Spur for example. Considering that, I'll personally go with a size medium, even though I'm 182cm (6ft) with 87cm inseam. Because I want a down country (race) bike, not a short travel trail bike.
April 21, 2021, 04:03:33 AM 		3
Re: Ican S3 XC Frame
Quote from: mtbluvr68 on April 23, 2021, 04:16:27 PM
I appreciate the explanation but I think the ratio is off because 165X38 shock is 100 mm on most bikes so taking the trunnion in consideration 140 trunnion should be about 105 ,  the 142.5 should be about 112 and the 145 should be about 120.  Maybe the stroke changes that slightly but unless you're compressing the shock to get in the frame then the numbers are still skewed.  Not to mention that if it doesn't have capability to full compress the shock and bottom out then what's the point?

I'm not quite sure if I fully understand what you mean... But there seem to be some errors in your thinking. Please correct me if I'm wrong:

You say that "165x38 shock is 100mm on most bikes", do you mean that a frame with a 165x38 shock usually provides 100mm of travel? Well it may be, that many bikes with these shock dimensions have 100mm of travel. But the amount of travel that the shock stroke translates to, depends on the leverage ratio of the frame. And that cannot be "off", it is what it is. Every bike is different. That just depends on how the linkage is designed. Like some bikes with a 210x55 shock have 140mm of travel, others have 160mm.

The Ican here for example has an average leverage ratio of 2,5. That means that for every 1mm of shock compression, the rear wheel moves upwards by 2,5mm - on average. It's probably a progressive frame, so the leverage ratio will reduce throughout the travel.

Now for this, the only important number is the stroke length of the shock, which is the second number, like "38". The first number - 165 - is the eye to eye, so the overall length of the shock. That should usually not be messed with, since any changes will drastically affect the geometry of the bike. Also, it doesn't matter if its trunnion or standard mount. The eye to eye is what counts. In this bike, you should only run 165mm, trunnion or standard.

What the guys are talking about, is keeping the eye to eye length the same, but changing the stroke length of the shock. So when the standard shock would be fully compressed, a longer stroke shock still has some more to go, resulting in more travel. The limit of this is reached when the seat stays hit the seat tube. What Ben did, is go to this point of maximum compression and measure the remaining eye to eye distance, which is 115mm. A 165mm shock would need to be compressed by 50 mm to get to this point. But you need some safety room to avoid breaking the frame at the first bottom out, also because the bottom out bumpers of the shock still give way a little when it's compressed hard. That's why Ben suggested a 45mm stroke shock may work.

The rest is math. Let's say the leverage ratio is 2,5 at the end of the stroke (it's probably a little lower), then an additional 2,5 mm of shock stroke will give you 6,25mm more travel. Take that twice and you have your 112mm with a 45mm stroke instead of the standard 100mm with a 40mm stroke.

Sorry, a lot to read, I know... I hope I didn't misunderstand you, just thought this might help. Took me a while to get behind this linkage stuff too.
April 23, 2021, 06:10:02 PM 		8
Re: Ican S3 XC Frame For the linkage nerds among you (who haven't done it themselves yet), I've done some analyzing of the S3 diagram using Linkage X3.

Firstly, two purely subjective design aspects I don't like about the S3 when compared to the FM936:
- They had to redesign the rocker arm, due to the horst link rear giving more travel. To counter that, they moved the top shock mount further away from the seatstay, lowering the leverage ratio. The new rocker arm looks a bit dull to me.
- The angle of the seatstays does not allign with the angle of the top tube. That's something I love about the FM936 design. The S3 looks a bit more old school there in my opinion.

Now, moving on to the objektive stuff:
- The leverage ratio looks good, fairly progressive for an XC frame (around 11% from SAG). Also, the software confirms what has been said before: 40mm shock = 100mm of travel, 45mm shock = 112mm of travel.
- Anti squat is relatively high, so a bigger chainring is recommended to not increase it any further. A 34t should be okay, while a 32t will already give way too high anti squat at SAG, which may result in a lot of pedal bob. Pedaling technique also plays a role of course. But generally, 100% anti squat at SAG is preferable.

And here some screenshots. Enjoy :)
May 18, 2021, 02:17:05 AM 		8
Re: Ican S3 XC Frame
Quote from: jetpilot on June 10, 2021, 10:02:28 PM
Seems like the S3 has a much better suspension design than the FM936.

Attached is the leverage curve sent by Adam at Carbonda and the S3 (thanks Julian)

As you can see the S3 is progressive to linear, similar to most modern XC bikes designed for an air shock

The FM936 has a really pronounced falling rate in the end of travel, this would mean it has very little bottom out resistance. Almost no bikes are designed like this today. We want a rising rate.

I have been looking at these bikes for my next downcountry bike but seems the S3 is a better choice for anyone who will bottom it out. Too bad its heavier than the FM936 by more than 400grs (comparing the SL version)

You're welcome :)

Your analysis is correct, but I'm not sure if you can trust Adam's material. It seems like he's also using X3 to calculate the leverage ratio, but with that software it's absolutely impossible to calculate the actual leverage ratio of the FM936, because it does not have any means to factor in the flexing seat stays. If you only put in the actual existing links, the software will have a calculation error. I'm no engineer, but I'm pretty sure a flexing part does not have one fixed pivot point, but one that moves throughout the bend (similar to a VPP). That's why I have refrained from posting any leverage ratio stuff on the FM936. I've tried to get close to the actual leverage ratio by using a classic swingarm 4-bar design (so one additional pivot point in the seat stays) and placing the rear link pivot in the middle of the seat stay, where it's the most bendy. But placing it there will cause the seatstay to shorten a lot more than the actual bending does. So neither leverage ratio nor actual travel can be properly calculated.

I've tried out various different positions for that rear pivot point and the leverage ratio was always fairly progressive. That is also supported by what NS Bikes claim for the not-so-different Synonym and by what FM936 riders have reported so far. So I believe that the actual leverage ratio is absolutely fine for a down country bike.

I've attached some examples for different pivot placements in the middle and further back as well as one where it pivots around the rear axle. The "truth" could be somewhere between those estimates.


June 11, 2021, 01:30:36 AM 		7
Re: Ican S3 XC Frame
Quote from: jetpilot on June 11, 2021, 11:06:02 PM
Awesome Julian!! I was hoping you would do that  ;)
Yes seems they are both good bikes then, the leverage ratio from Carbonda did seem a bit odd but I found it plausible given what I have seen been done in China ("designers" that dont even know how to use the thing they are making, happens in many products).
I actually prefer the FM936 as its lighter and that's kind of the whole point of this bike (I already have a 160mm bruiser), plus flex stays is the new thing right  :P
Can you post the antisquat curve at least for one of the flex points? ("further back" seems like a good middle ground) THANKS!

Sure thing :)

Absolutely true about the chinese frames sometimes having weird linkages... but yeah, the FM936 seems pretty fine.

The anti squat is actually not affected by the placement of that rear pivot point, but very much so by the center of gravity, and I honestly have no idea how to correctly estimate that. I've just kept it where the software suggested it to be.

Anyway, it's probably very similar - if not identical - to the NS Synonym, and the magazine reviews haven't reported any terrible ride characteristics.

So, here you go, anti suqat for 32t and 34t (the highest line is with 50t, the lowest with 10t).
June 12, 2021, 05:05:46 AM 		3
Re: Carbonda fm936 Thoughts Has anyone actually measured the travel of this bike with 40mm and 42.5mm shocks?

With the X3 linkage software, I always got more travel than it's supposed to have. I thought it must be the flex stays that I can't properly account for.

But now I compared it to the NS Synonym and it seems like all the linkage points are exactly the same (see image below). Only the flexing seat stay is slightly higher up, but that can't make that much of a difference.

I think the only real difference between the NS rear triangle and the Carbonda version is that NS put the bridge between the seat stays closer to the tyre, so that it can move further towards the seat tube and take that 45mm shock.

So if my theory is correct, the FM936 should have more travel. Exactly like the NS Synonym, only that you can't run a 45mm shock.

Which would mean:

37.5mm = 100mm
40.0mm = 107mm
42.5mm = 113mm
45.0mm = 120mm (theoretical, not possible)
June 16, 2021, 02:49:37 AM 		1
Carbonda FM 1003 / Flybike FM 1266 180mm "Super Enduro" So, after the FM 1266 has been on the Flybike website for a while (and changed from a Santa Cruz clone to something more original), Carbonda has now announced the same frame called "FM 1003" for end of 2021.

For geo and render pic, see link below
http://www.carbonda.com/news/136.html

It uses a four bar design with a large one-piece rear triangle, making it look more like a single pivot. I've done a quick analysis with Linkage X3 and the results are promising. Very nice linear progression thoughout the travel and 100 per cent anti squat at sag (32t).

Anti rise seems a bit high though. In theory, this means that the rear end won't extend under braking, keeping it low (which is a good thing). But also the suspension may get stiffer and lose traction when braking over a bumby surface.

Can't say if this is a copy of another bike. If not, then this is an impressive new development.
July 19, 2021, 12:58:37 AM 		3
Re: single pivot or Horst link As mbarronj said, it depends on the design. Anti squat however isn't necessarily a factor. Both designs can have very similar anti squat curves, starting high and going low throughout the travel. VPP on the other hand will usually look a bit different, with anti squat values staying on one level or even increasing throughout the travel. Whether or not that's a good thing, probably can't be universally answered. It always depends. And that's also true for all the other numbers, curves and values. Generally, a single pivot flex-stay design will allow less fine tuning of the suspension characteristics, but be lighter and simpler. Horst link or twin link (like VPP) designs both use a virtual, moving pivot point and offer plenty of tunability. The question is, if you need that on a short travel bike. That's why you'll often find single pivot designs on bikes with 100-130mm of travel.

Back to the anti squat: I've attached the anti squat curves of the LCFS958 and the FM1001 as well as the famed FM936 for reference, all with 30t and 32t chain ring options and the same center of gravity at 1050mm. You can see that the horst link design doesn't have any advantage there. It does offer a more linear anti rise at around 50% though, whereas the other two have higher anti rise at about 100%, falling thoughout the travel.
July 27, 2021, 03:08:44 AM 		2
Re: single pivot or Horst link
Quote from: ASB77 on July 27, 2021, 08:50:18 AM
I appreciate the replies.  Forgive me for being simple but I don't quite understand the charts and would love a bit of explanation on what is going on at a given point.

No problem, that's what a community is for :)

If you have general questions about what anti squat is, read this:
https://www.pinkbike.com/news/definitions-what-is-anti-squat.html

In short: When you accelerate by pushing into the pedals, your body's inertia will make the rear suspension squat. But at the same time, the chain pulls on the rear wheel, usually having the opposite effect of extending the rear suspension. That is called "anti squat". The percentage says how much of the inertia-squat is countered by the chain pull. 100% (in theory) means no movement in the rear suspension at all.

Yet, there are many things that play a role here. Two are probably the most important: Center of gravity and chain ring size. Generally, longer legs = higher COG = more squatting (leverage) = lower anti squat percentage. So "your" anti squat can be a bit higher or lower than the one in the graphs, but the curve will look the same. With chain rings, smaller ones have higher anti squat.

The rear cogs also play a role. Depending on the size of the chain ring, anti squat can vary greatly throughout the gears. The graphs show blue areas where all the gear's anti squat curves go through, from 10t to 50t.

Now, it's considered optimal to have around 100% anti squat at SAG point, since that's the travel you're in when pedaling. I've marked that point in all the graphs below. All the bikes here have falling anti squat curves, meaning that anti squat is lower when the bike is deeper in it's travel. Some bikes like the Propain Tyee (VPP) actually have a slightly rising anti squat curve. That can have the advantage that on steep climbs, when your weight is all the way back and the rear suspension is more compressed, you still have lots of anti squat. With bikes that have a falling curve, you may need to lean forward more, decompressing the rear end.

Looking at the graphs, the FM936 with 32t and the FM1001 with 30t should have the best pedaling characteristics. They both have about 95 percent anti squat at SAG in any gear. With the LCFS958, the anti squat with 32t is a bit low, and with a 30t it changes quite a lot with every gear.

But that's just what the graphs suggest. In the end, there are many other factors apart from those numbers. Where your COG is, how "clean" your pedaling technique is, how you weight your bike etc. ... So I'd take this with a pinch of salt. They're all not bad. I'd probably go with the Carbonda FM1001 though, but mostly because everyone seems to love the brand ;)
July 28, 2021, 01:44:00 AM 		4