That will work pretty well I reckon. Hit the SEND button!
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I don’t want to hijack this thread, but I could use some geometry advice for a few bikes. It’s my first time building for others and I want to make sure my geo makes sense.
This bike is for a 5’ tall woman rider with a 27.25" inseam. She wants a 130mm travel hardcore hardtail on 27.5 wheels.
Questions:
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Is the 30mm BB drop enough? She will be using 155mm cranks, and I’d like to keep the BB as low as possible. Bringing it much lower starts to bring the stack up more than I’d like.
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Does the HTA and trail look ok? She really wants to be more confident descending than her Marlin currently allows. She’s a really strong rider, but I don’t want her fighting the front end.
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How does the FC:RC ratio look? Currently, it’s sitting at 1.65, but I could bring that up with shorter chainstays.
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Any issues jumping out?
Thanks so much!
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Model the fork so you can be sure it wont hit frame.
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I would say at least another 20 mm of BB drop is very manageable. I’m running a 320 BB height with 175’s — for 145mm ground clearance — and that’s considered a tall position. You’ve currently got 170 mm from pedal spindle to ground on 155’s.
HTA and trail looks fine, about standard for a ‘hardcore hard-tail’ these days. A 74 STA is on the slack side but that’s often a personal preference thing.
@Neuhaus_Metalworks and @Daniel_Y have commented elsewhere on this forum that they’ve built smaller bikes with shorter rear ends that the test riders said it felt harsh. A 420 R-C is already kinda short so I’d leave that.
I would try to dimension the dropper post you plan to use. On a more aggressive bike like this that’s something I’d wanna max out. Looks like you could get another 20mm of travel there. And not sure how picky you want us to be to be but that TT needs to be moved down the ST a bit. That will impact water bottle clearance so I’d probable model them in too.
Will be a very fun bike!
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I’m not her size, but I can say that I wish my 27.5 mtb had more than 30mm drop and I run 170 cranks. When I first got the frame I surveyed Bike Insights and 30mm drop was on the low end for 27.5 bikes. It’s easy to find bikes with 45mm drop and you are running much shorter cranks.
Are you sure about your axle to crown measurement? I think a fox 34 set to 130 in 27.5 wheel size would be 521mm unsagged. With 26mm sag (21%) you’d be at 495mm? I expect Rock Shox to be in the same ballpark. Double check the numbers, I’m going by memory.
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I am guessing these are all unsagged dimensions? I recommend dimensioning axle-to-crown like this:
This is our design:
We tried 65HTA with 130mm fork, but the feedback we got was that the bike was too imbalanced. That is why we settled 66HTA with 120mm fork.
You can afford to drop the BB more. However, I wouldn’t get carried away. Small bikes have more weight on the front wheel (rear-front-ratio), so the bike is already very planted. Having a higher BB allows you to loft the front wheel easier.
I wouldn’t go shorter on the chainstay, you get diminishing returns. Some people want to pop off every feature. Others are happy to take the B line. What riding style does your client have?
Questions:
- What bar width?
- What bars?
- What stem?
- What headtube?
- What fork?
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Daniel, is that for a “27.5” 120mm fork with unsagged dimensions? It seems to match the Fox numbers. I’d love to get your opinion on using sagged vs unsagged in drawings.
My personal take is based on her wanting more descending confidence.
42mm fork and 65 deg HTA. 430 chainstay and call it done.
I would happily run the 30mm drop. with the slightly longer wheelbase of my numbers above getting to low in the pendulm makes it harder to change direction.
That’s my approach anyway.
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Assuming normal trails without a ton of rocks to smash pedals into, you can do at least 20mm more BB drop, if not more. Run a Flatforce stem if needed if handlebar height is an issue.
Agreed with others that the fork crown clearance may be a problem.
I’d whack a degree or degree and a half off the HTA which will also help with your stack problem.
She’s short enough that a super-v style frame with no toptube would be worth considering, IMO. Most people aren’t willing to go down that rabbit hole, though.
-Walt
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Thats an unsagged fork.
My preference is to show drawings and fixtures unsagged:
- the numbers are easier to validate (you can see the nominal fork dimensions at a glance)
- the vast majority of mainstream brands list unsagged geometry
For me, the biggest reason is that when the bike is climbing, the geometry is unsagged, so it matches the geo chart. Climgin is when all the bike fit, seated pedaling fit, butt to bar reach, weight distribution, wheel flop etc… really matters.
On the topic of HTA, I would really want to know the bar width before making the call. We put a lot of the 5’4 and below riders on 720mm bars, and they really like it. Try moving around the contact patch with a 65HTA with 720mm bars, it really sucks.
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How hardcore of a hardcore hardtail does your custy want? Below is an enduro race frame I just wrapped up for someone who’s 5’ / 153cm tall.
130mm fox 36, 160-175mm dropper, 27.5x2.6. Drawing is with a normal stem, but we’ve got a flat force -22° for it in the mail.
I don’t seem to have the finalized drawing in my phone but we dropped the bb down to 310mm.
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So a bit more information @Daniel_Y
Customer is a bit of a stretch haha. My SO wants to shred a bit harder than her bike allows lol
We used her current bike as a comparison tool and mocked up some of the changes. She’s currently on 720mm bars with 50mm setback and a 50mm stem. She feels very comfortable with that bar width. Currently designing around a BFS 105mm HT and a Rockshox Recon RL.
@anon91558591 When things were shifted to match my current design’s cockpit layout, she wanted a bit more stack, which I’ll achieve by dropping the BB. If it turns out to be too much with her current stem/bar combo, we’ll spring for the Flatforce. I like the idea of a Super V, but pictures of it freaked her out.
Based on her feedback, I steepened the ST by half a degree and also pushed the front out a bit.
She’d really like to stick with the 130mm fork, but is open to dropping it to 120. She’s used to an 80mm coil fork that is way too stiff for her, so anything will be a game changer.
@wzrd Thanks for sharing that print. We do most of our riding around the midwest, so it doesn’t need to be quite as hardcore as that haha. Most of our riding has a lot of punch climbs and techy descents. The goal is a bike that can balance both confidently with room to stretch its legs during trips further east or out west.
Here’s an updated drawing. Thanks for all your help, everyone!
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Looks good to me. I think the geo with the lower BB is much better.
Why a 105mm headtube? not a 100mm headtube? Exactly which headtube are you thinking of using?
People like to nitpick 1-2deg of HTA or 10mm of travel (including me). But IMO the difference between a well-designed bike is all the annoying little details:
- Bottle clearance
- how much of a DT bend you will need to clear the fork crowns
- standover clearance
- shifters hitting the toptube
- bottle space
- dropper insertion
- Fork damping and airspring
Going to a 120mm fork mitigates the first 5 of those issues. I suggest looking into all those design constraints before making a decision on fork travel.
For lighter riders, I recommend looking at the MRP Raven. If you ask, MRP will build up a 120mm 27.5 fork. The fork damping is on the light side and the separate negative air spring allows you to tune it extra soft (if that is what the rider needs). It’s a pain to set up though.
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Total brain fart on my end, it’s 102 not 105. This is the headtube.
I’ll try modeling both versions out to solve for everything else before hitting send. Thanks a lot for the fork recommendation. I think she’ll have to stick with a more budget option for now and consider the upgrade down the road.
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If you can find one of the old RS dual-air forks they are the best that ever existed for light riders. Very tuneable via the negative spring (and even travel adjustable if you’re clever) and light enough damping for small folks.
-Walt
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I love this game!
Here are my recommendations:
- 120mm fork to get her hands lower. You can ride gnarly trails on a 120mm.
- Flatforce 44 stem to get her hands lower
- SQ Lab 16º bars to maximize front center
- At least a 150mm dropper. The drawing shows a OneUp 150mm.
- 90mm head tube to get her hands lower. You can buy a 47mm straight tube. That’ll give you the most room on the tube to place the HT and DT.
- Check to make sure you don’t need a bent DT. You’re likely right on the cusp of needing one.
- Since this is custom, spend the time to model whatever handlebars you plan to use so you can display where her hands will actually be in space to compare it to her current bike. Using frame stack and reach as a reference isn’t useful.
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Hopefully, this is the final drawing!
Changes:
- 120mm Reba fork, bumping us down to a 42mm offset. Fusion model shows no clearance issues
- SQ Labs 16deg and a cheaper Flatforce alternative. -30deg, 50mm
- BB dropped to 50mm
- 150mm KS Rage dropper
- 65.5deg HTA
This gives me:
- 367mm BB to Grip X axis
- 635.6mm BB to Grip Y axis
- 116mm trail
@Daniel_Y My concern at this point is whether the trail is too much. She has a strong upper body, but I dont want it to be unbearable. A 66deg HTA puts it at 112, and a 66.5 HTA puts it at 109mm.
Thanks again!
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…and GO! If it isn’t quite perfect you can build another one with the tweaks. There is a point when you have to pull the trigger and just send it. We obsess over the numbers but all the suggestions above are going to work.
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This was our design evolution for our small bikes:
Two initial prototypes:
- 65HTA 130mm fork 415CS 75STA 415RE 35mm stem
- 66HTA 120mm fork 415CS 75STA 390RE 45mm stem
We converged on this geometry for the third:
- 66HTA 120mm fork 420CS 74STA 390RE 45mm stem
We tested these bikes with more than 5 different riders <5’4. Our main tester Dana (5’3) is pretty strong and skilled, she can regularly get top 3 QOM’s on strava descents on her hardtail. Everyone preferred the 66HTA bikes. The terrain was the Bay Area (steepish, good grip) and Southern California (less steep, rocky, loose).
This is my opinion:
Women and men ride mountain bikes differently. Even though Dana is a very skilled rider and easily hangs on my wheel, the assumptions of how and what average 5’10 male riders like did not apply. There are nuances like: upper body strength, the weight of the fork-wheel-tire assembly, the handlebar width, and how the dropper post clamps to the rails which do not translate 1:1 to the “shrink it” model. We learned this through testing, and those test bikes were not cheap (they were not customer bikes to eliminate that bias).
In Dana’s words, the slack bike was fun when moving fast, but it was not fun to ride. She was always fighting wheel flop and front wheel slide on flat loose corners. She felt the bike was very fatiguing.
I am not saying that slack bikes are a no-go for small bike designs. If you look at @wzrd design:
I think the design is a very complete package, the long reach and chainstays will put more weight on the front wheel. The slack HTA brings down the stack. It’s overkill for rolling southern California terrain, but I’m sure it’s super fun on the steeps.
Instead of projecting my expectations of how a bike should ride and its geometry to your rider, these are some questions you could ask to determine the question of 65.5, 66, 67 HTA and BB drop:
- On climbs does your back wheel spin first, or your front wheel lift off?
- On the technical climb, you encounter a root/rock step. Do you struggle more lifting the front wheel over, or getting the back wheel over?
- Do you prefer climbing or descending?
- How do you feel about flat loose corners?
- How do you feel about steep downhill switchbacks?
- When you approach a rock garden, do you tend to straight line it, or pick your way around?
- What aspect of descending do you want to improve? (examples: drops, jumps, rock gardens, root nests, cornering (flat and berms), steep cutes, flow
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