100 vs 120 Rear Suspension (XC build)

I am building up a new full-sus XC racer around a FM936 Frame (Very progressive).
Racing on mostly technical natural trails with some enduro style descents.

I am going to set it up with 100f & 100r for the time being and upgrade to 110 or 120f 35 Sid when I find a good deal.

So at the end it will be 120f & 100r.

The bike has a relatively low BB (45mm drop) so I bought a 165x40 rear shock running at 100+mm thinking because the sag distance is less it would mean a higher BB on the shorter travel shock.

Is this correct thinking?

A benefit of having more travel as its my one and only MTB and ride pretty much everything on it.

Would returning the shock and replacing with a 42.5 (115mm) rear travel be a huge game changer? Will this impact BB height?

It’s my first full sus would appreciate your views


The longer shock will actually give you a slightly higher BB. By my calculations, going to a 42.5 stroke length will give you about 106mm of rear travel, not 115mm. The BB would only be raised by maybe 3mm, less at sag. So not really significant geo changes (edit: my bad, if you’re sticking with a 165mm shock length the BB would be in the same spot, or a bit lower like you said if you choose to run more sag).

I’d stick with what you have. It’s what the frame was designed for and I don’t think the extra minimal travel will matter.

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Also a 120mm fork will raise the BB quite a bit, and slacken the head tube to 66*


The 165*40, 42.5 and 45 shocks are identical. You don’t need to replace anything, just open the air can and snip the travel spacer off.

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Really? it is etched on the shaft (40mm).

How did you calculate rear travel? The frame is designed to do both 120mm and 100mm - with great reports of it running 120mm. (NS Synonym TR1 is basically the same frame but 120)

TBH I am unsure if 45mm is considered a ‘too low’ bb drop - this is my main reason of thinking of changing the shock. I can return it for free for a 42.5mm one.

Just printing. Your sag marks will be wrong after the snip tho. My shock has no sag marks so I don’t care.

I have the NS synonym RC, which I extended to 120. It has dual Lockout so there is literally no loss.

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Thats Great! Thank you! This gives me some flexibility incase I ever want to change :slight_smile: How did you find the NS RC at 100mm - how did you find the bb height in relation to pedal strikes? I always run 165mm pedals but have Rally XC pedals which are pretty hefty.

Also what shock are you running - i understand the rear triangle is different on your frame.

It might be worth having a look through this thread: Carbonda fm936 Thoughts

Loads of experience there about the FM936, including (if I remember correctly) knowledge about putting a longer shock on. Again, if I remember correctly, there are some subtle differences between the FM936 and the NS Synonym at the back end which impacts how much travel the FM936 can take without hitting the frame.


Yes - it can take up to 42.5 w/o hitting the frame. I am just debating between 40 and 42.5 but its as simple as removing a token then I don’t need to choose between the two options of shock :slight_smile: I suppose i will witness myself the pros and cons. I’d like it to feel more firm - and because I couldn’t source a cheap remote lockout shock, i’ll likely keep it in 1 setting for the most part, maybe less travel would mean less bobbing?

It’s not exact because it depends a bit on the linkage design, but a 40mm shock that gives 100mm rear travel means you’re getting 2.5mm of travel for every mm of shock length. So if you increase the shock stroke by 2.5mm that leads to 2.5*2.5=6mm of additional travel

Makes me think that 2.5mm change may not be the same level of difference when comparing a 100mm shock to 120mm shock…

Is that an open mold frame?

I think if they are genuinely Enduro descents then you’d not regret the extra rear travel. You will get to the bottom (possibly) faster, and (probably) fresher.

If it has a lockout then you won’t notice the difference. I can’t speak to the efficiency of the frame/suspension design, but I guess it probably has pretty similar leverage/anti squat either way and in my opinion will be a more confident and adaptable bike at longer travel.

If I remember rightly you just built up a new HT too didn’t you?

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There’s not really such a thing as a 100mm shock or a 120mm shock. There’s just stroke length and leverage ratio of the frame. One frame might give 120mm of rear travel with a 40mm shock stroke (3:1 leverage ratio) and another might give 100mm with the same stroke (2.5:1 ratio).

Apologies if you understand this and I’m missing your point.

A fox float 3 pos whatever the newish one is. Actually I snipped a 5mm spacer - left the 2.5 in. My RC comes with a 37.5 stock, and uses a 42.5 for 120mm. I know it works as the trail model is identical frame.

I have a deviate highlander for big mountain riding, so I just ride this for racing, marathon, and easier stuff.

Pedal strikes are mostly about technique honestly, but yeah - I run 165s (because mine is bad)

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I’m sure I read recently about a different linkage for the 120mm version?

Guess it depends on the suspension layout as to the effect of shock change?

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I decided life’s too short so I bought a full-sus frame! The geo on the NS synonym seems perfect but I can’t justify buying the NS at this moment of time.

I ride with the enduro-bros on my Thursday spins - Its mostly natural stuff, slow, tech & steep - rather than jumps/big drops (most of it is rollable but very steep like 90 degrees). This is where the short reach of my current bike (407) and the head tube (69) really struggles. Also traction is an issue and I hear full Sus helps with this.

Well it sounds like I can take out a spacer to run either 40 or 42.5 on my 165x40 sidluxe - so I guess that solves my problem. If i bottom out a lot I will move up.

Yes sorry - I mean 100 vs 120 of travel.

From looking up I think there is a 1:4 ratio.

37.5 = 102, 40 = 108 and 42.5 = 115

That makes sense. The specs in the link were different. That’s a 2.7:1 ratio, pretty normal for XC suspension.

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No, the Synonyms are identical RC and TR, just a higher grade of layup for the RC.

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