[guypie]

Well no one has posted in the framebuilding forum for 5 days, so I figured I would take one for the team and share my current build. This is my second frame build and I decided to jump in the deep end and go for a full sus. I used linkage x3 to design the suspension layout, its a single pivot because that's the easiest option.

The general idea was to keep financial investment to a minimum so I am using 26" wheels and straight gauge 4130 because thats what I had lying around. I am learning as I go so may as well keep it cheap and cheerful! The frame is designed to take 27.5" wheels too so if I decide its a winner I can try to find a fork and borrow a wheelset off one of my other bikes.

I don't currently have a proper jig because I have yet to decide on a design, so I used 3d printed lugs/standoffs on a 18mm mdf backing board for the front triangle. This is how I made my first bike too, it works pretty well as a one off type deal, but definitely need to make a jig for future builds

I went with 44mm tube for the downtube since the pivot point is mounted in it. I cut the pivot point out at the same time as the bottom bracket cope, I would do this after welding the front triangle next time. Things have changed a bit over the course of the build as at the start I didn't have a functional vertical mill, now I do.

I put in a BTR style head tube brace

The geometry of the bike is pretty much just standard DH bike overall, nothing really worth mentioning.

And thats the front triangle mostly complete, overlaid on the scale drawing. Still needs shock pivots fabricated and welded on and the bearing pockets machine to final dimension. I am waiting for some inserts for the boring bar to arrive for that. I did make a boring bar out of silver steel (i think they call it 01 or W1 in america?) but I wasn't happy with the surface finish I was getting for a bearing housing. I probably messed up the tool geometry a bit.

So next up I needed a rear triangle jig. After watching a video of BTR fabricating a rear triangle for their pinner and seeing it was basically just box section with fixturing points I figured I could do something similar.

This is what I came up with. It could do with being thicker walled section, but this is what was hanging around in the scrap pile. There's 2 mounting points at the shock eye to get the most travel with both 27.5" and 26" wheels before the rear tyre runs into the seat tube. It will be 190mm with 27.5" and 200 with 26" wheels. Its running boost spacing because everything I have is boost so this may as well be too. I had to brace and then skim the faces at the main pivot end as the box section was flared by about 2mm there.

Anyway, that's about as far as I have gotten. Should get a bit more done over the holiday period hopefully.

 

[JSGN]

Looking forward to see it assemble. The frame looks amazing as of now.

 

[Bikesaredumb]

I'm on board with the Second Frame is Bouncy Frame club. I know everybody loves the early BTR x brace, but I really dig the tube style like you did. 🤘

 

[a_j_p]

What is the down tube? 1.75"x0.049", or what 44mm straight gauge did you end up using for this? I assume your shock mounts land on it as well?

And is that a 35mm tube for the main pivot?

 

[guypie]

What is the down tube? 1.75"x0.049", or what 44mm straight gauge did you end up using for this? I assume your shock mounts land on it as well?

And is that a 35mm tube for the main pivot?

Its 1.75"x.035, yes shock eye mount will be welded to it as well. Will probably make them quite long to spread the load to avoid oil canning. Main pivot when finish machined will be 35mm ID for a 6202 bearing, the housing I made on the lathe to 39mm because the hole in the frame that was supposed to be 38mm (1.5") got a bit wallowed out by a wobbly holesaw and I wasn't happy with the fitup.

 

[a_j_p]

Its 1.75"x.035, yes shock eye mount will be welded to it as well. Will probably make them quite long to spread the load to avoid oil canning. Main pivot when finish machined will be 35mm ID for a 6202 bearing, the housing I made on the lathe to 39mm because the hole in the frame that was supposed to be 38mm (1.5") got a bit wallowed out by a wobbly holesaw and I wasn't happy with the fitup.

Rock on. I am currently in process of a similar-ish frame executed a bit differently. I could not find 1.75"x0.035" tubing to save my life which is why I ask... (Though I did eventually find a source for 44.5x0.95mm.)

Excited to see your progress.

 

[93M500]

Looking sharp, the sizing is perfect, nice traditional clean look.

 

[guypie]

A bit of an update here. Rear triangle is mostly complete, just needs a brace for the disc brake tab. I'm undecided on if I will put a seat stay brace in, there's a bit of a clearance issue to put it outside the rear triangle and an engineering challenge to put it inside the rear triangle. Any suggestions/input on if it is required or not would be appreciated. The seat stays are 5/8"x.049 and the chain stays are 7/8"x.035. The bridge connecting the sides is also 7/8"x.035.

Sorry if the scales offend, just keeping track of things. The general goal was to keep the frame under 4kg, not too stressed about weight though. I think I am on target at the moment.

In place on the bike. Looking pretty good I think!

Made a little fixture for mounting IS tabs. Good practice with the milling machine, slowly getting more familiar with it but there's still a huge amount to learn.

Will pop a 1/2" tube brace under the tab. Might put one on the other side too, undecided at the moment. Apart from that I'm just waiting to a couple of profiles to come back from the laser cutter for the shock mount on the downtube. After that theres a couple of bits that need turning on the lathe and cable routing, painting etc.

 

[a_j_p]

Looks great. Glad to see it coming along so well.

I'm undecided on if I will put a seat stay brace in, there's a bit of a clearance issue to put it outside the rear triangle and an engineering challenge to put it inside the rear triangle

I don't know what space constraints you actually have, but if you do decide to add a bridge... some bar stock, a little internal drilling/lathe work on the ends, and some quick milling with some v blocks and aluminum scrap makes a thin bridge relatively easily.

I have done this and then placed the rear wheel + tire on the frame, let all the air out of the shock and used a strap between the wheel and upper seat tube to hold everything in fully compressed state while fitting/tacking the bridge in a spot with decent clearances.

There's also the option suggested to me recently of dropping the bridge below the chainstays like the nukeproof mega shown below:

 

[Thylacine]

Did you calculate the suspension kinematics before you started?

 

[93M500]

Looking very nice!

 

[guypie]

Did you calculate the suspension kinematics before you started?

I had a bit of a play around on linkage x3 until I found a balance that I thought would be ok. We will see if it is or not once the bike is finished!

I want to use this bike baseline for future builds, ride it and find out what needs changing for the next bike. I would imagine it will probably be fine, but not optimal. I did some comparisons with some of the orange single pivots and it is fairly similar. I have not ridden any of those personally though.

 

[guypie]

I decided to go with the dropped seat stay brace. That wheel is a 27.5 with 2.4" tyre, clearance is pretty marginal but this bike will probably end up staying 26", I just want the option to be there.

Tig brazed on some cable guides, a minor mistake here was to put the derailleur cable guides on top of the chain stay. Not really a biggie but reduces the space between the chain and the stay which probably makes chain slap worse, not to mention needing to put slap tape or some sort of protection over the cable making cable replacements more difficult.

On assembly I found a clearance issue that I completely missed

That's a 30 tooth chainring, I will be able to go up to a 32 but that's about it. Not really a big deal for me cause I don't race high speed DH trails, if speeds get that high I'm not pushing to go faster. Would probably be an issue for a race bike I imagine. The colour scheme is currently grey-green 2k primer. Anyway, got it all assembled and took it for a spin up the local trails.

It ride really well! I was a little worried as there's a fair amount of pedal kickback but I didn't notice it unless I was at the large end of the cassette. Very smooth suspension, though lacking in mid stroke support a bit. Might be able to resolve that with shock adjustments. I did find that I need to run fairly low sag (25%) I think due to the linear nature of the single pivot. The local trails are pretty smooth but performance on big hits and medium chunk is very good, though I don't think it will deal well with high frequency chatter. Don't really have much of that on these trails.
The anti squat worked very well too, pedal bob is minimal when seated. In saying that the climbs were not easy, though I suspect that is because the bike weighs in at just over 17kg which is not unexpected for a DH intended bike. Also the seat post I have is too short to get full leg extension so that doesn't help either!
All in all I am very happy with the result, it needs a few minor changes but the overall design seems solid and useable for what I was intending. It will be getting a pretty paint job after a few more test runs to make sure further mods aren't required and I will post an update then.

 

[Bikesaredumb]

I REALLY like this! Looking forward to seeing it in pretty colors. Good work!

 

[guypie]

Just realised I never updated this with pictures so here's a few...

'

The paint job is a 3 tone fade going from mazda starlight silver > GM 1988 corvette gunmetal grey metallic > Black with a 2k clear over the top. This was my first attempt at a fade and I am pretty happy with how it came out, was easier than I expected to get a decent looking result.

I ended up finding a boxxer WC and a fox van rc which perform much better than what was on the bike previously. I thought the coil would be a problem being linear with the very linear leverage rate but it actually works fine. Might be an issue for someone sending huge jumps and drops but it's fine for me. I actually use this bike in the way that most people use an enduro bike because it pedals pretty well. It is a little heavy but not excessively so at 17kg.
The yellow doodad on the chain stay is a chain slap prevention thing that I drew and 3d printed, it has some inner tube siliconed to the inside to keep it quiet. Works mint, I will probably print one it a more subtle colour at some point.

The main take away points/things I would do different next time from a design perspective are:
1 - Shorter head tube. its hard to find double crown forks that will take a 160mm head tube and it limits your options for bar height. in future I would probably do a 125mm with zero stack headset for a bike like this.

2 - Longer shock. I used 216x63 because I had one. A 240x75 would perform better for a bike of this travel, be less likely to need custom valving and much easier to find a suitable replacement if yours sucks/breaks etc.

3 - Shorter seat tube. Could have gotten away with a shorter one. I still have space for a 180mm dropper, but more drop is more good IMO.

4 - 26 is dead. Wow it is getting hard to find decent tyres in 26, I could only find 2.3" tyres locally when I really wanted 2.4". Fortunately this bike can fit 27.5" wheels but the main problem comes back to problem 1 - finding a DC fork to take a 160mm head tube. It might end up with a 190mm Zeb one day and end up looking a lot more like the long travel enduro bike that it actually is.

5 - Longer chain stays. I recon another 10-15mm on the chain stay length would be nice for moving the weight bias forwards a little. That one would probably be hotly contested by some, but that is my personal preference.

6 - The cable routing is a mess, more thought needs to go into this on future bikes

Anyways I have had a lot of fun with this project and while there's a few things I would do different if I were to do it again, I am super happy with the bike I have ended up with. It rides great and I am having a lot of fun with it. And learning means future bikes will be better than this one too, always a good thing. Next on the agenda is a trail full squish for my wife. I have also made a ridiculous hardtail thing to try out super short chain stays and see how I like them since building this bike, so I might post up some more bits and bobs on that build a bit later.

 

[JSGN]

Its beautiful m8! Looks amazing!

 

[romulin]

Thanks for the updates and thoughts.
Awesome looking rig, you are pretty skilled too!
Keep us posted

Odoslané z M1 pomocou Tapatalku

 

[Bikesaredumb]

That fade is so slick I didn't even notice it a first. I read the description and had to go look again. I dig the absolute hell out of this. Really nice job.