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I find the Trek FX Sport 4 to be comfortable but much less fun to ride than the Specialized Sirrus 4! Unfortunately, on the Sirrus I feel like I'm arching my back and want to reach further forward. Moving the seat back helps, but it doesn't resolve the issue. The Sirrus has a 32mm higher stack and 14mm shorter reach, but otherwise has a nearly identical geometry (link to full geometry comparison below) Sirrus vs FX Geometry Comparison Can a professional bike fit compensate for these 10-30mm differences and make the Sirrus as comfortable as the FX Sport? I assume that adding a stem several cm longer can address the problem?

For context, I am looking for a flat bar road bike for use on group rides on paved roads. I have chronic lower back pain that prevents me from using drop bar bikes. I'm honestly not sure why the Sirrus feels so much more fun, but it just feels like I'm more "connected" to the Sirrus. The FX Sport ride quality feels... Dull? Maybe it's because the Sirrus has narrower tires vs the FX's gravel-like tires? Regardless, I would like to know your thoughts on the geometry question.

P. S. I have tried smaller sizes, they do feel less comfortable for both the FX and Sirrus.

Update: I spoke with Specialized support, and they said that I can get a bike shop to swap a part in the suspension stem system to reduce the stack by 15mm. Specifically, the bike comes with a 15mm rise headset cap for the future-shock stem, and it can be replaced with a 0mm rise headset cap.

full geometry chart Full 99spokes.com spec and geometry comparison

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    Not knowledgeable enough about geometry, but there's a likelyhood that "compensating for deviations" will make the Sirrus closer to the FX in terms of behaviour. That being said, the tires play a big role in the behavior, you should compare the two bikes with the same tires before drawing conclusions on the geometry. "Dull but comfy" is something you can get by having wider tires at lower pressure (given that the FX 10mm wider tires, it's a possibility).
    – Rеnаud
    Commented Oct 2, 2023 at 7:02
  • (also, the FX Sport has a carbon frame, while the Sirrus has an aluminium one — which is also a reason of the better comfort — if you have lower backpain, I strongly recommend you to avoid aluminium seatposts (or if aluminium, suspended, but no telescopic suspension) - in my case, a good carbon seatpost makes the difference between backpain and no backpain in long rides)
    – Rеnаud
    Commented Oct 2, 2023 at 7:46
  • I worry that I might buy the FX and put on nice tires, only to find out it still doesn't feel (subjectively) fun. Then I'd be stuck with an expensive bike I don't use or a significant restocking fee. In terms of seat posts, I plan to use a Cancreek suspension seatpost!
    – Peachy
    Commented Oct 2, 2023 at 8:03
  • The concern is valid, and it's very hard to predict how a bike will feel when a single component is modified, for example the canecreek seat (that seem to be among the best ones — on my side, I went for a Canyon VCLS 2.0) post can also totally cancel the comfort advantage of the FX. Also, another point is that you may look for antagonizing characteristics: stiff bikes feel "very connected" to the road but are uncomfortable. If you afterwards look to improve comfort, you'll loose in "road connexion". Lighter wheels also have a significant impact on the liveliness of a bike.
    – Rеnаud
    Commented Oct 2, 2023 at 10:29
  • What are the seat tube angles on both bikes? That difference in reach but with almost identical top tube lengths implies a different seat tube angle. The higher stack and shorter reach on the Sirrus likely means that is has a less-upright seat tube. Moving the saddle even further back is going to put your weight even farther behind the bottom bracket to get the reach you want.
    – Andrew Henle
    Commented Oct 2, 2023 at 10:37

3 Answers 3

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It’s difficult to make definitive statements without seeing the OP in person. Also, bike fit is complex. However, here’s my attempt.

First, in general, you don’t want to push the saddle back to change the reach to the handlebars. Bike fit is difficult because if you change one parameter, one or more other parameters may change. With your saddle position, you should get the correct saddle height (approximately is fine), then you set the fore-aft position, then you separately think about the handlebar’s horizontal and vertical positions.

Second, you want the Trek FX Sport’s riding position, but the reach and stack at the handlebar are 14 and 30mm lower than on the Specialized. We can change stem length to adjust reach. This will not affect the handling of the bike unless you go to a really short or really long stem. We can change the number of spacers to adjust stem height. So, we would ideally need to know how many spacers are on the Trek. It may not be possible to lower the Specialized’s handlebar position by a full 30mm if it didn’t have enough spacers under the stem.

Third, I know you said you tried the Sirrus in a size smaller. However, you would really want to have the bike store replicate your desired position on the smaller Sirrus. That means changing saddle height and setback, plus changing the handlebar position. I suspect they didn’t do this. It would be much better if you got them to.

Fourth, ideally, we want our backs to be as straight as possible. It is true that some people do favor a more rounded lower back. The goal isn’t necessarily not to do this at all, but you want your back as straight as possible. I think your intuition is correct on this score.

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Just a note, that doesn't appear in the answers and might deserve more than a comment: reach and stack are measurements of the frame, not the bike. The space between the stem and the top of head tube (from where the stack is measured), the length of stem and its angle and the the rise, width and sweep of the handlebar will all impact the relative position of the handle bar to the bottom bracket/saddle.

So yes, by changing the stem, you can get the handle bar at the same position (relative to the saddle) and compensate for the difference in stack and reach. The issue here is that there is not enough info to be sure that you can just do it based on the direct comparison of the reach and the stack. For example, the Trek handlebar has a rise of 15mm, while the Specialized seems to have none (hard to say based on the picture). Assuming the other parameters would be equal (which I can't verify), the height difference between the handlebars of the two bikes would be 16mm, not 31mm.

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  • If this is true, this would be great news! According to a conversation I just had with Specialized support, the stack of the Sirrus can be reduced by 15mm by replacing a part in the suspension stem.
    – Peachy
    Commented Oct 3, 2023 at 21:44
  • If the support is responsive, you can ask them the different measurements of the stem/handlebar. Trek has them on the product page (stem and handlebar differ depending on the size of the frame). That would allow you to compute (vs guessing) the specs of the parts you'd need to replicate the position.
    – Rеnаud
    Commented Oct 4, 2023 at 4:29
  • According to Specialized, the handlebar on the Sirrus does not have any rise! Now that the difference in stack and reach is really only about 15mm, I'm more optimistic. I also contacted a bike fitter, who said there's a lot leeway to change geometry of Sirrus and FX Sport bikes.
    – Peachy
    Commented Oct 4, 2023 at 17:07
  • @Peachy about the justification for the bounty, to complete: the question has in fact several sub-questions. If it's only about getting the handlebar at the right position, as long as you can find stems and handlebars that match your requirements, the limit is the availability of parts. But you then you have the handling sub-question, and it's there that it get hard, partly because it's not a science and there's a subjective component. For example: it's likely that installing a longer stem will make the bike more stable, which some consider boring.
    – Rеnаud
    Commented Oct 5, 2023 at 5:58
  • For the bounty, I'm asking about getting the handlebar in the right position.
    – Peachy
    Commented Oct 7, 2023 at 3:23
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As i can't comment yet die to lack of reputation points, ill try to answer with what information from the comments and ur question, and a picture of the bike you used can help us examine your concern, so please provide it if possible. The sirrus has more stack but less reach, this make your position far more upright and the angle formed between ur torso and arms much smaller than the fx sport. I think this is what causes your back to arch and you felt like you want to reach forward, but this is also what makes riding the sirrus more "fun" as it is more responsive and you have higher center of gravity, meanwhile the FX sport provides you with more stability and control esp. Over high speed because of the lower center of gravity and you feel less agile.

To compensate for the sirrus you can replace your stem with a longer one, maybe 20mm more of what you have currently, but by doing so you'll lose a bit of your agility. Another thing is to consider the handlebar. You can tweak the angle to change the reach and stack, check how you can change the stack and reach by a bit by rotating your handlebar, also you can read this to understand more about it

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  • Yes i already saw the bike from their own website, but it would be better to see "your" bike with your settup and fitted already so i can see real differences on your setup between those 2 begaise the image in their web can differ from yours (saddle height, etc.)
    – Ravidas K
    Commented Oct 3, 2023 at 1:22

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