[neongreen]

I can already hear the cries of protest.
"why would you waste your time and money?"
"Bro, just buy a new bike"
"dude nobody rides 26ers"
I hear you, it's just that I'm going to ignore you.

After building my kid a balance bike, I think I'd like to continue to improve my abilities by building a new frame for my old 26er. It's a 2007 Diamondback response. This was my first MTB, I rode the piss out of it in college. The frame is built to withstand the apocalypse. It's seen new components about 3 times over, and I enjoy the bike. It's now my backup bike and my main bike is a 2014 Santa Cruz Bronson. Yeah, I ride old bikes. I'm kinda cheap.

Above is the rough geometry of the Diamonback Response (not sagged). My complaints with this bike are mainly that it does feel a bit cramped, I have the seat slid all the way back on the rails. Very ccasionally this leads to wheelies on climbs. I have a short stem on it, maybe 40 mm because my weight feels too far forward with a longer stem, and as a rule I just don't prefer long stems.

So here's what I'm finally getting at. I'd like to take all the components (except maybe the 30mm fixed seat post) and swap them over to a frame of more modern geometry. The purpose of this is three-fold. One, to modernize my hardtail. Two, as previously stated, serve as a learning exercise in framebuilding. Three, to perform as an A-B comparison to demonstrate the differences in old vs new geometry on an aggressive hardtail.

Currently the fork is set at 110mm travel, I'd put it back to the original 160mm. The diagram below (sagged fork) shows of course the geometry, but also the fit in the form of the seat-handlebar-BB relationship I use on my Bronson.

So now that I've explained myself, this is where I'm looking for feedback. How does this geometry look? Am I going too short on the chainstays? I know that this should be determined somehow by Center Of Gravity (using fuzzy math?) but it seems a bit like a black art to me. Also I think the front center is about as long as I can reasonably get it unless I start getting really crazy with the seat tube angle. I'd love to hear your thoughts.

 

[SteveJewels]

Good on you! Cool project.

Good luck!

 

[pvd]

Looks in the ballpark.

1. BB is a bit low. Bring it up to 300-305. 300 if you are on 165mm cranks. (I've moved my bb's up slightly since 2017 and my wife isn't an aggressive rider)
2. CS is probably good at your height and with that STA
3. if you use BikeCad, you may be able to squeak a little more length on the front end.
4. Do a lot more detailing on that 'print'. The hours you spend now will save you a lot of heartache later.

For example, this is the kind of detail you are looking for. I'm showing a 2017 bike that I made for my wife that is close to your size.

PVD Glamorous Glennis | Peter Verdone Designs

 

[thisnameisvalid]

I don't really know anything about designing frames, but the only thing that stood out to me is the 405mm chainstays. My 27.5" hardtail has a similar front centre to yours with 420mm chainstays and a ~74 degree seat tube angle at sag and I find them a little short (my sweet spot would probably be 425). My thinking is that you could consider having slightly longer chainstays (maybe 5-10mm?) to put a little more weight over the front wheel?

 

[jsudar]

I’m with PVD— raise your BB up a bit. I run at 305 with 170 cranks. If your local trails are very smooth—like no rocks to hit your pedals on, you’ll maybe be okay where you’re at, but if you’ve got rocks, raise it up.
I think your chainstay length is good. If you’re gonna run 26ers may as well have a short, poppy rear end.
With your steeper seat angle, a little more reach wouldn’t hurt. Keep in mind, I don’t know your body measurements.
I’d do a trail measurement on your drawing too. It’s a useful number to have.

 

[OldSchoolMBer]

For example, this is the kind of detail you are looking for. I'm showing a 2017 bike that I made for my wife that is close to your size.

PVD Glamorous Glennis | Peter Verdone Designs

Are you using Solidworks for that drawing?

 

[pvd]

Are you using Solidworks for that drawing?

That's BikeCad. That's one of the most important tools in the bike designers tool box. It does almost everything you could want until you start real part design.

This is my SolidWorks drawing:

New Possibilities with Additive Construction | Peter Verdone Designs

www.peterverdone.com

 

[AdamR83]

I've actually built a 26" hardtail fairly recently with very close geo to what you are proposing... a bit higher BB (more like 310, I hate pedal strikes, especially when climbing), but 65 static head, 75 seat (though I have the saddle down all the time), 406 CS (singlespeed so dictated by gearing), 430 reach (short torso), and front centre ended up about 725.

It generally rides well, but one thing I noticed is that its hard to weight the front wheel properly when descending and turning (or I'm just crap), which is one thing a slack head angle seems to demand to prevent washing out.

I think on this bike it is perhaps due to the ratio of CS to front centre with the short stays. I could throw some numbers out and compare them with 'real' manufacturers bikes, but I'm typing on my phone so its a pain, and I'm sure you can do such simple sums!

The latest bike I built ended up with the same front centre length, but 13mm longer stays (SS again - an extra pair of chain links) and it feels a bit more balanced. I definitely feel more confident that the front is going to go where I tell it to, without being so conscious of leaning right over the front.

Hope that helps and makes some sort of sense...

 

[OldSchoolMBer]

That's BikeCad. That's one of the most important tools in the bike designers tool box. It does almost everything you could want until you start real part design.

This is my SolidWorks drawing:

New Possibilities with Additive Construction | Peter Verdone Designs

www.peterverdone.com

That's cool, nice drawings! Do you use FEA in the frame/parts design?

 

[neongreen]

I've actually built a 26" hardtail fairly recently with very close geo to what you are proposing... a bit higher BB (more like 310, I hate pedal strikes, especially when climbing), but 65 static head, 75 seat (though I have the saddle down all the time), 406 CS (singlespeed so dictated by gearing), 430 reach (short torso), and front centre ended up about 725.

It generally rides well, but one thing I noticed is that its hard to weight the front wheel properly when descending and turning (or I'm just crap), which is one thing a slack head angle seems to demand to prevent washing out.

I think on this bike it is perhaps due to the ratio of CS to front centre with the short stays. I could throw some numbers out and compare them with 'real' manufacturers bikes, but I'm typing on my phone so its a pain, and I'm sure you can do such simple sums!

The latest bike I built ended up with the same front centre length, but 13mm longer stays (SS again - an extra pair of chain links) and it feels a bit more balanced. I definitely feel more confident that the front is going to go where I tell it to, without being so conscious of leaning right over the front.

Hope that helps and makes some sort of sense...

This is really helpful info. It's not the end of the world if my first full size frame design isn't exactly everything I want it to be, but just thinking about where my butt will sit in relation to the rear axle made me think I could stand to lengthen the rear a bit.

 

[neongreen]

Looks in the ballpark.

1. BB is a bit low. Bring it up to 300-305. 300 if you are on 165mm cranks. (I've moved my bb's up slightly since 2017 and my wife isn't an aggressive rider)
2. CS is probably good at your height and with that STA
3. if you use BikeCad, you may be able to squeak a little more length on the front end.
4. Do a lot more detailing on that 'print'. The hours you spend now will save you a lot of heartache later.

For example, this is the kind of detail you are looking for. I'm showing a 2017 bike that I made for my wife that is close to your size.

PVD Glamorous Glennis | Peter Verdone Designs

1. Fair enough, I want to run it low but I'm not trying to determine the bleeding edge of what I can get away with. I run 170mm cranks.
2. I'm thinking I might add 10 mm to the CS.
3. I've played around with bike CAD and wish I had a real version of it, but that $500 license is kinda hard to swallow. I suppose I could spend the time in the web version to optimize that point.
4. I'm doing the modeling in Fusion 360. The model needs a lot of work before I make a real print. I still have to learn how to do that as Solidworks is my native language (though I haven't spoken it in a decade).

 

[pvd]

2. I'm thinking I might add 10 mm to the CS.

I'm 5'10 and am using 410-415mm chainstays in the modern context. I figure that's a bit on the long side for you. 405-410mm makes a lot of sense for you as you are working with 559 wheels and it's not a big challenge. I'm doing 622 wheels which is a lot harder.

 

[AdamR83]

3. I've played around with bike CAD and wish I had a real version of it, but that $500 license is kinda hard to swallow. I suppose I could spend the time in the web version to optimize that point.

Pop Brent an email and explain you are just one guy making frames for yourself in your shed, so struggle to justify the full license fee, and that you read on his forum that he can offer reduced pricing for this circumstance... in my experience he will come back to you with a very attractive price.

Edit: Bike cad pro license renting ?

 

[OldSchoolMBer]

4. I'm doing the modeling in Fusion 360. The model needs a lot of work before I make a real print. I still have to learn how to do that as Solidworks is my native language (though I haven't spoken it in a decade).

How do you like Fusion 360? Most of my experience is with Solidworks too but don't have access to it anymore

 

[a_j_p]

Fusion, BikeCAD, SOLIDWORKS... I don't think it matters much for your first frame - whatever you do you'll learn what works and what does not for you. if you decide to continue with more frames then you can shift/adapt/add software as needed. I agree with PVD that details early prevent heartache later, but everyone has their own workflow... I can't remember the last time I opted for a 2D drawing over a reference sketch and a 3D model but that's just me.

Have fun and learn as you go and keep us posted.

 

[dr.welby]

It generally rides well, but one thing I noticed is that its hard to weight the front wheel properly when descending and turning (or I'm just crap), which is one thing a slack head angle seems to demand to prevent washing out.

Since it's an easy swap, what about trying out a slightly longer stem, or lower the bars in general? One thing I noticed once I started doing really long frames is that too short a position was cramping my ability to weight the front wheel. With the longer front end I didn't need a short stem to hang my butt off the end of the bike.

 

[dr.welby]

2. I'm thinking I might add 10 mm to the CS.

I say go short so you can really see the difference. I don't see the point of building a custom frame that's within a finger width of an existing bike.

 

[OldSchoolMBer]

Fusion, BikeCAD, SOLIDWORKS... I don't think it matters much for your first frame - whatever you do you'll learn what works and what does not for you. if you decide to continue with more frames then you can shift/adapt/add software as needed. I agree with PVD that details early prevent heartache later, but everyone has their own workflow... I can't remember the last time I opted for a 2D drawing over a reference sketch and a 3D model but that's just me.

Have fun and learn as you go and keep us posted.

I hear ya and agree but I'm not building frames. It was for drawing up parts for something unrelated. The nice thing about Solidworks and Fusion is the FEA capability

 

[natzoo]

To the OP:
I'm happy to see someone using Fusion360. Great start. I actually find fusion way easier than Solidworks. Fusion drawings have some issues... but for a bike... it's good enough.

My entire 3D CAD model always starts out as a base sketch, just like yours:

A few comments about short chainstays:

Chainstay length is a means to end: finding the right weight balance and riding characteristic for you. How you ride, where you ride, what you like to ride is all subjective and relative. That may mean your chainstays are slammed, or it may mean it is long. That is up to you to discover for yourself. Experimenting is part of the fun. Sliding dropouts offer 20mm of adjustment so you can find out first hand what works best for you.

If you want my opinion on your design: A bike is not just a HTA and short chainstays. It is the sum of it's components. 26in wheels are fun and playful and they will never match a 29 to 27.5 in grip and roll over. You should design a bike that plays to the strengths of the parts that you have. Think: rally car, not monster truck. Something to get loose and wild, pop and jump, and hit the pump track.

• I would design something around a 130mm fork, to keep the front end firm, responsive, and poppy
• I would have a long headtube for a tall stack, so you can lift the front wheel easily
• I have no experience with 26in wheels and slack headtube angles, so I don't have an informed opinion. 65 is probably a good starting point
• I would stay around 420mm chainstays, with sliders so you can try between 410 and 430mm
• I would keep the BBheight around 310-320mm (unsagged), to help the bike pop up.
• a 450mm reach looks about right for someone with a 700mm saddle height

More general advice: I believe a lot of people fall into the trap of extema seeking. It is easy to maximize/minimize parameters on a bike and call it "optimal". It is a much harder exercise to find a balanced design.

 

[neongreen]

Thanks to all for the input, there are a few differing opinions (all well thought out, IMO) which is to be expected. I'll have to mull it over a bit. I have time, as I need to plan out my jig as well.