Help with gravel bike fit

My present road bike is a 56 cm BMC Roadmachine 01 (stack 586, reach 390, TTeff 556, stem 110 mm)

My present gravel bike is a 56 cm Specialized Diverge Comp Carbon 2X GRX (stack 610, reach 392, TTeff 573, stem 90 mm). Both bikes have 172.5 mm cranks and 42 cm handle bars (flared on the Diverge). The top of my saddle is 78 cm from the middle of the crank along the seat tube. Both bikes have set back seat posts.

Note that Specialize measures the stack to the bottom of the stem, not the top of the head tube- the actual stack is probably 555. But I have the stem turned up and the bike uses a hover bar which raises the overall stack a bit more.

Both bikes fit me really well. However, I find that on long gravel rides over about 40-50 miles, my back starts to tighten up on the Diverge. I think it is because I do not get out of the saddle on gravel and I could use a little more stack (I am old and losing flexibility). I also find that at times I wouldn’t mind a bit more tire and on rough trails I have experienced occasional pedal and crank strikes due to the low BB on the Diverge.

I am looking at the Lauf Seigla and Otso Waheela C as replacements for the Diverge. I was able to briefly ride both a medium and large Seigla. The medium (stack 564, reach 394, and TTeff 563) felt great size wise, but the ride was short and I’m concerned with the lower stack that on long rides I will have the same issue with my back tightening up. Unfortunately, on the large (stack 593, reach 405, TTeff 584) I felt stretched out. The medium Seigla is spec’d with 172.5 cranks and 42 cm handlebars (which is what I normally ride), while the large is 175 and 44 cm.

I will not be able to test ride the Waheela C, hence this post. The reach on the large Waheela is only 3 mm longer than the Diverge, while the TTeff is 1 cm longer. I’m thinking the Waheela would be a better overall fit compared to either Lauf. I would be able to get a higher stack which would help on longer gravel rides and have the ability to run wider tires when I am going to be on rough trail.

If I go for the Waheela, should I use a zero offset seat post, or would I stick with the offset post? How would a zero offset affect overall ride feel?

Looking to see if others confirm what I am thinking, because Otso doesn’t have the best return policy and I don’t want to be hit with some crazy “restocking” fee.

I can also mention that I recently rode a 56 cm Salsa Cutthroat- stack 619, reach 385, TTeff 56- and I felt really cramped on the bike. It was spec’d with an 8 cm stem. I was also not thrilled with the PF bottom bracket.

Any input would be appreciated.

I tried to insert a image capture from Geometry Geeks, but I am not sure if it worked.

Good instincts trying to compare the stack and reach taking into account the stem, stem angle, etc. I’d start by taking that to the next level and either busting out your high school trigonometry skills or finding an online geometry calculator that will give you handlebar X and Y (or stack and reach at the centre of the handlebar clamping area) so you can really compare apples to apples. A true calculation should include head angle, stem angle, stem length, height of the mandatory headset cover/spacers, optional spacers, and height of the stem clamp. There are a few sites I’ve seen out there that will do that but I can’t remember which ones, sorry (I’m tracking mine in a spreadsheet I wrote).

Also remember that different handlebars have different reaches, and different shifters have different reaches as well, especially if you’ve got different brands on different bikes. I don’t factor those numbers into my math because they can change pretty drastically depending where you mount the shifters on the bars and on whether the bars are rolled more forward or back from one bike to the next. Nearly impossible to quantify unless you’re using identical parts and measuring angles, so I don’t bother. However, I do think it’s helpful to know if the reach on one set of bars and/or shifters is 5-10mm different than the other as that can affect things - it’s just you have to include some guesswork as well as the numbers.

If you want to know if your saddle position is truly identical across all bikes you’ll need to measure not just saddle height but also saddle setback relative to the bottom bracket. There are fancy tools for this but you can get a pretty decent DIY measurement with a long builder’s level and a tape measure. Make sure the bike is level on the ground (or if you want to do it in a workstand just make sure the centre of your wheel hubs are level with each other), then use the level to get a vertical line down from the tip of your saddle and use the tape measure to see how far that line is behind the centre of your BB spindle (on Shimano this is easier on the non drive side because of the open crank end).

Maybe don’t mess with saddle position if you’ve already been professionally fitted on both bikes as there are lots of reasons you might not want the same saddle position relative to BB from one bike to the next, but if it’s all a DIY job then it’s usually a pretty good starting place to replicate this setup across all bikes.

Then you’re finally ready to compare your numbers (and to look at how you’d likely end up setting up a new bike based on an online geo chart).

FWIW here’s my N=1:
Compared to my road race bike my racey gravel bike has handlebar XY at 13mm higher stack and 21mm shorter reach and it’s not long enough for me. Stem is already as long as I’m willing to go on that bike and it’s nearly 4 years old so I’m looking to replace it for something where I can get more like +13 and -10. In the meantime I’ve had to increase the saddle setback to get a similar length but I’m aware that it compromises my pedaling a bit.

I have another gravel bike that’s a bit more adventure and ultra-endurance focused and compared to my road race bike the bar XY is 25mm higher stack and 21mm shorter reach, and that one doesn’t feel too short despite being the same length as the ‘cramped’ racier gravel bike.

All three bikes have similar enough handlebars and shifters that I consider them broadly equal. So for me it seems like I can raise my stack and shorten my reach at roughly a 1:1 ratio (for at least the first 25-30mm). I’m sure I’ve read somewhere that it’s more commonly a 1:2 ratio but I can’t recall where that was or even which way round was the 1 and which was the 2.

I know that’s not a direct answer to your questions but it’s how I would go about making real comparisons if I was thinking about committing to a bike sight unseen, which is effectively what you’re looking at with the Otso.

Oh and when it comes time to figure out how much seatpost setback you need that’s another mix of trig and guesswork. Assuming you have a saddle position you want to replicate and it’s currently clamped square in the middle of the clamping area on the saddle rails, then any bike with the same seatpost angle will require the same seatpost setback to end up clamping in roughly the same spot.

If the angle differs then it’s about solving for an isosceles triangle that has two legs at your saddle height and an acute angle that is the difference in the seat tube angles between the two bikes. Assuming your stated saddle height of 780mm that’s going to be a difference of 6.8mm for every 0.5 degrees of seat tube angle change.

So from the Diverge to the Otso you’re going 1 degree slacker which would put the centre of the clamping area 13.6mm further back compared to the BB if you used a seatpost with the same amount of setback. If you’ve got 13.6mm of useable clamping space behind your current saddle clamp and in front of the limit mark on the saddle rails then you can use the same seatpost setback on the Otso.

I’m pretty sure I’ve oversimplified that a bit because it’s arcs and not triangles so that calculation isn’t strictly accurate, but it should be close enough to work out which seatpost setback you’re going to need, especially when the differences between angles are within a degree or two.

Wow- that is WAY more complicated than I thought it was going to be! However- great input and it is much appreciated.

I think the bottom line is 13.6mm more posterior positioning of the saddle compared to the Diverge- and I definitely do not have that much available clamping space with my offset post. So- I would need a zero offset seat post to correct for the change in saddle in relationship to the BB if I were to opt for a Waheela.

I have never purchased a bike without riding it, and I am sort of getting the vibe that is still a good idea as all this geometry stuff is only part of the equation.

Again- thanks for the insight on frame fit calculations.

I hope you don’t mind me reaching out to you for a little more advice.

I have the identical saddle on both of my bikes. I measured the saddle set back on my BMC and Diverge- from the saddle tip to the center of the BB is 7 cm on both bikes.

I also measured from the tip of the saddle to the center of the handle bars and that measures 55 cm on both bikes.

The BMC has an 11 cm -12.5 degree stem and the Diverge has a 9 cm +6 degree stem.

The stack on the BMC to the top of the stem is 696 and to the top of the stem on the Diverge is 650. On the Diverge with the stem turned up I believe that adds about 2 cm of stack and the bike has a Specialized Hover bar which raises if another 1.5 cm- so total stack on the Diverge would be 685 (and I lose stack on the BMC with the stem inverted at -12.5 degrees, so I am assuming they are close).

I am not accounting for the changes in reach that the additional stack produces, but it would seem that I’ve done this with saddle position as both bikes have identical saddle set back and identical distances from the saddle tip to the center of the handlebars. The BMC has Di2 12 speed and the Diverge GRX 800 shifters, while the bikes I am considering are SRAM Rival. I am sure there is a difference in the geometry of the handle bars, but that might be too much information.

The part I am confused on is how to apply the above dimensions to the bikes I am considering. Would you be able to provide any additional insight? The one area I think would be most critical would be how to achieve the appropriate saddle set back (7 cm), as this will determine overall reach and stem length. Would one of the bicycles geometry be better than another to get the dimensions correct? Would one bike definitely not work?

OK I had a stab at putting your numbers into my excel tool, but I need to caveat all of this by saying I’m not a bike fitter and my entire experience has been about tracking my own fit changes and purchase options. One of my bike fitters said my tool is good and I managed to predict my Retul fit spec for my Tarmac exactly, plus I’ve used it to help spec a custom titanium frame that I’m happy with, but another fitter who is a more advanced research scientist type said there’s an issue with my calculations. I suspect it’s just that I’ve done the angles like a carpenter rather than a scientist and so have sometimes gone the opposite way round a circle than is the accepted norm, but I don’t know for sure. I’m also no excel expert so had to look up how to write most of the trig formulas and I can’t even make total sense of them looking back now. But the tool works for my bikes so might work for yours too.

Someone like @mcneese.chad is maybe better positioned to comment here and I note he often links to Bike Insights which can be helpful.

Also the website I mentioned that allows you to include stem, spacers, bar reach, etc. is Bike Geo Calc and I’d trust it over my spreadsheet if in doubt. It’s a bit confusing at first but is a really robust tool. You need to click on the different buttons at the bottom to show/hide different measurements, then you can click each measurement itself to change the number. That’s just the default bike so you’d need to start by copying over your bike’s geometry chart before you can start playing around with it.

My spreadsheet with your numbers in it as best I can tell is here. The forum won’t let me upload here so that’s a link to my google drive and I’ve made the file publicly available. Should be read-only so you’ll need to download a copy to save changes. Some things worth noting:

  • This was written first and foremost to see what it would do if I changed stems and spacers around on a bike I already owned - that’s all the variables in the red/orangey section. I found the green section with absolute measurements difficult to interpret at a glance so I added the yellow section which just shows the difference compared to the bike listed at the top.
  • BMC’s manual seems to indicate stack and reach goes to the top of the mandatory headset cap, but the drawing here shows that cap being excluded from the measurements. I’ve guessed that it is 20mm tall which is about average for a chunky profiled one. Double check and edit for yourself as required.
  • “Stem Stack” is the height of the stem on the steerer tube - the calculation takes the midpoint of this measurement in order to find the location of the middle of the handlebar clamp rather than the bottom of it. 42mm is whatever Ritchey stem I was using when I first made this - change it to whatever is right for you.
  • I don’t know if you’re running any spacers on the BMC or the Diverge so I’ve said ‘0’. Adjust as required.
  • I found this post suggesting that the Diverge geo chart goes to the top of the small futureshock cover - if you have the large one on or additional spacers you’ll need to adjust.
  • There’s no place to account for your hover bar here, but as you say, that should be straighforward because it raises the stack completely vertically so won’t affect reach with complicated angles.
  • I don’t think the Saddle to Bar Drop section is particularly useful here. I added it when building up a fixed gear bike with a different crank length and therefore different saddle height and was trying to understand that aspect of it. Was easier to leave it than delete it though and can sometimes be interesting to know.
  • There’s no place to account for different shaped bars or shifters here. I think 12s Shimano road is a few mm longer than GRX800 but I can’t remember how much - 3-5mm? No idea how Rival compares either. The BMC and Diverge both have 70mm reach bars but that doesn’t mean much if the shifters aren’t in the exact same place and bars rotated to exactly the angle the manufacturer spec’ed, etc. as I mentioned before.
  • I didn’t look up the height of the mandatory headset cap on any of your prospective purchases. That number is a guess. Some searching online will probably help you find a more realistic number.
  • Once you get the numbers right for your existing bikes you can start to see what kind of stem/spacer setup will be required to get similar handlebar XY on the other bikes. Of course this also assumes you know what fit you’re trying to achieve - if I were you I would be trying out a few different stem and spacer combos on the Diverge and taking notes about how each setup feels (far right of the spreadsheet) before committing to a new frame. I’ve got versions of this saved with 10+ different lines including the exact same bike but with different stem and spacer configurations so I can keep notes on my experiences with each and see how they compare to each other.

In terms of seatpost choice, I’ve always just gone to this triangle calculator and put in the difference between seatpost angles as ‘Angle C’ plus my seatpost height for both ‘Side a’ and ‘Side b’. That tells you the distance between the middle of identical offset seatpost clamps at two different angles. In your case the BMC will be the ‘steepest’ or closest to 90 degrees, meaning it will be the furthest forward. All the smaller numbers (or slacker seatposts) on the other bikes will be further back compared to the BMC, with the triangle calculator telling you how far.
Then a setback seatpost will be Xmm further back again, so in the case of the BMC versus the Diverge you’re going from 15mm setback to 20mm setback, which means the centre of the clamp on the Diverge should be the distance shown in the triangle calculator PLUS 5mm to account for the extra setback over the BMC. (I think that’s 9.5mm for the angle plus 5mm setback so the clamp should be 14.5mm further back on the rails on your Diverge compared to your BMC to achieve the same saddle position relative to BB - or to phrase it the other way round the saddle on the Diverge has to be slid 14.5mm further forward in the clamp compared to the BMC to get the same effective position.)

I hope that helps. If not, hopefully a real bike fitter will be along soon to tell you all of that in a much simpler and clearer way which is more relevant to your questions!

Thanks so much! That spread sheet is AMAZING. I do have 35 mm of spacers on the BMC. And I think the overall total stack on the Diverge is actually about 40 cm lower than I stated if you correctly measure from the bottom of the stem.
With each of the comparison bikes on a 10 cm stem it would seem that there is only a small difference in overall reach between the bikes with the Waheela being 3 mm shorter than the BMC, the medium Lauf 12 mm shorter, and the large Lauf 5.5 mm longer. The difference the program gives for the Diverge is 8.5 mm shorter. I will play around with the spread sheet a little more, but I think any of the bikes would be acceptable size wise. I will also check the geometry triangle.

However, there is always that unknown about actually sitting on the frame and going for a 10-15 mile ride.

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Thanks for the tag. I have seen this but not had enough time to do a proper read of the full text. Still digging back into regular life after vacation. Seems to be some good discussion and points above, so I will hold off any real work here unless it’s needed.

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