First Full Suspension. Single Pivot Enduro Build Log and Questions

evan.trem · February 20, 2024, 3:36am

First Build Log post after lurking for a while working on the design. I’ve got a few frames under my belt now and I’m looking to step up to the next level.

This one will be my first full suspension frame, my second TIG welded frame, and my 4th frame overall.

The bike is a 170/170 mullet single pivot with a 230x65 shock. Design-wise I’m taking a lot of cues from Starling along with a couple of build logs from this forum.

I’m looking for feedback on kinematics and tube sizing, as well as general ideas of course. I want to nail all those details down so I can pull the trigger on the design. I will post some pics of CAD and Linkage.

Tubes:
DT: 1.5x.035 (38.1x.9mm) single bend 4130 tube (Bend by Bike Fab Supply)
TT: 1.375x.035 (34.9x.9mm) 4130 tube
ST: 1.375x.035 (34.9x.9mm) 4130 tube
CS: Zero Uno S-Bend MTB MPV190B6022 (Bike Fab Supply)
SS and Shock Upright: .75x.035 (19x.9mm) 4130 tube
Shock Doubler Plate: 1.5x.035 (38.1x.9mm) 4130 tube
Main Pivot Tabbed Box: .06 (1.5mm) thick 4130 plate
Pivot Yoke Tube: 1.25x.065 (31.8x1.65mm) 4130 tube
Pivot Yoke Arms: .375 (9.52mm) thick 4130 plate
Dropouts: Paragon UDH
BB: T47
HT: 44x110mm

evan.trem · February 20, 2024, 3:43am

Linkage Screenshots:

evan.trem · February 20, 2024, 4:51am

Looking for some feedback from the hivemind on tube sizing. It is always difficult for me to know how think is too thin… is everything too heavy etc. I’m about 180lb fully kitted and this will be an “enduro” bike so I’m trying to size everything accordingly. On the other hand, I’m not doing any New World Disorder type huck-to-flats anymore.

Specific worries I have with tube sizing:

Is a 1.5x.035 downtube thick enough with full suspension?
Will making the doubler plate for the shock mount out of 1.5x.035 be thick enough?
Is .75x.035 strong enough for my chainstays?
Should I go with straight gauge 1.375x.035 top tube or could I get away with butted?
Should my rear triangle be more… triangulated? Do I need a 3rd tube off of the pivot, kind of like the Bitchn Bikes Jerk?
How thick should my shock mount rails be?

Pi_bikes · February 20, 2024, 5:18am

Looks pretty good to me. I recon longer chainstays would be nice (opinion)

Is a 1.5x.035 downtube thick enough with full suspension?

I have 2 full squishes with 1.5x035 down tubes. One of them has the shock landing on the downtube like that, the other has a link back to the seat tube so the force is more constrained. No issues with either so far, It really depends on how/what you ride I think. I can get a little bit of wind up out of the bike in hard cornering, If you like big jumps and shralping berms maybe go up a diameter or two, if you are a normie then it will be probably be fine. For reference I weigh a similar weight, go medium fast and do medium sized jumps.

Will making the doubler plate for the shock mount out of 1.5x.035 be thick enough?

Doubler plate will be fine I think, my first full squish has a 1.75x035 downtube and no doubler plate and no issues so far.

Is .75x.035 strong enough for my chainstays?

I think so, my current daily driver has 3/4 cs and 5/8 ss both 035 and no issues so far. I do think another solid link between the 2 sides of the swingarm would be a good idea. Wife bike only has 1 link as pictured and you get quite a lot of flex under corner loading that I am not a fan of. If there is space between the seat tube and tyre at bottom out I have found a seat stay bridge to work out well, otherwise have a look at starling’s bolt on bracing plate on the front of the uprights

Should I go with straight gauge 1.375x.035 top tube or could I get away with butted?

Butted would probably be fine, I have used 1.25x035 so I imagine a butted 9/6/9 would probably be at least equal in strength, and stronger at the HAZ which seems to be the most common failure point.

Should my rear triangle be more… triangulated? Do I need a 3rd tube off of the pivot, kind of like the Bitchn Bikes Jerk?

I don’t think it’s needed. Maybe if you like how it looks and are planning on going big?

How thick should my shock mount rails be?

I have been using 4mm 304 stainless, I know others have gone significantly thinner with 4130 but no one offers lasercut 4130 where I live. Also, stainless doesn’t go rusty so thats nice.

crowe-molybdenum · February 20, 2024, 8:23am

I, as one independent person, would say it’s not enough tube for that suspension layout. obviously people do build bikes like this and they do seem to go fine, the ones I built broke and i’ve seen others broken too.

I swing between 85 and 90kg dry; so kitted up, ill be a little bit more.

I’ve only built* 3 full suspension steel bikes; *all using someone else’s rear triangle/ swingarm, or suspension design (yeti/trek etc) my total frame build count is somewhere around ~70

the first two full suspension bikes I built with downtubes this size both failed at the downtube/shock mount (which was laid out like yours) , one catastrophically, one gave heaps of warning. both big enduro bikes, one while riding grade 5/6 trails in New Zealand’s Wairoa gorge, one riding a grade 3/4 flow trail in Christchurch adventure park.

my latest (current) bike has 30mm more reach than your design above, and is half a degree slacker, it only has 140mm travel on each end with a 320mm bb height unsagged. and ive been riding it at least as hard without trouble for nearly 4 years, is as or more demanding terrain.

this current bike uses a 44.5mm OD downtube with a 0.9mm wall, and the shock doesn’t mount to the downtube. there are also no other things on the downtube, braze ons, bottle bosses etc. NB:you will need to find a wizard to bend this tube if you decide to use it.

one eye of the shock is attached to the rocker, the other is attached to the swingarm A Lā trek fuel EX 2015. ( the actual bikes front triangle is on the right in this photo )

both the broken bikes had doubler plates as you describe.

I’ve seen a small number of other bikes also of that basic layout, from popular brands, also with that tube, also with a doubler plate, which have also failed there.

Robertt · February 20, 2024, 9:13am

Paul Aston has 38 0.9/0.6/0.9 reynolds dt. So should be strong enought. One of the first pictures on instagram, have a look. Not sure, but I think Starling also is using butted DT?

Luniz82 · February 20, 2024, 1:24pm

The Starling uses this exact same DT, it’s the longest they have (850mm)

liberationfab · February 20, 2024, 2:44pm

One thing I’m noticing from this CAD render - do you have (or want to have) adequate fork crown clearance? It looks like you might need another bend in the DT to accommodate that.

Daniel_Y · February 20, 2024, 3:40pm

Awesome CAD skills. The bike is basically done!

Is a 1.5x.035 downtube thick enough with full suspension?

I think so, especailly if you are reinforcing it

Is .75x.035 strong enough for my chainstays?

Strong enough? yes. Stiff enough? maybe not. Since the rear end is moving around, you might need more stiffness than a hardtail (which use .75x.035)

Should I go with straight gauge 1.375x.035 top tube or could I get away with butted?

Both SG and butted will be strong enough, especially if you use a 9-6-9 butted TT. The butted tube will have more torsional flex, which is a good thing IMO.

How thick should my shock mount rails be?

Total guess, but based on bike designs I have seen, I think .125 would be fine.

evan.trem · February 20, 2024, 4:10pm

@liberationfab uh oh . I think you are right.

I had a double bend DT in there originally but I kept running into issues fitting a 230mm shock. I’m planning on having Andrew at Bike Fab Supply do the bending so I’m limited by what his Toob Bender can do.

This is exactly why I posted before buying everything

Daniel_Y · February 20, 2024, 4:30pm

The double bend is really difficult to achieve with a 230CLR. You may need to do single bend + gusset so it does not look goofy:

You probably already know, but for the lurkers, this is how to call out the knob clearance: 2023-2024 36mm User Specifications | Bike Help Center | FOX

TDB · February 20, 2024, 5:42pm

I’m not entirely sure but I think Starling use a custom 853 tube. They are 38.1 but I believe sans the thin section. A tube that Reynolds make for motorsport applications, or so I heard.

I wouldn’t mount a shock to a butted downtube myself.

Aston’s spec list gave cause to raise an eyebrow. That guy breaks everything he touches and somehow he’s still ragging on an 8/5/8 top tube…and that bent down tube… I’ve never seen such a thing from Reynolds and 853 is notoriously difficult to bend (even by the ££££ guys).

Not here to say it can’t be done. Just watch out. Use plain gauge and make the welding/ brazing of the shock mount and gusset the best of the whole frame.

Luniz82 · February 20, 2024, 5:47pm

I can’t recall exactly where it was but n an interview, Joe McEwan said that the 850mm DT is effectively the limit on how long he can build a custom geometry bike.

On bending: you can apparently get a 631 tube, bend it and then have it heat treated by reynolds to get a bent 853 tube

TDB · February 20, 2024, 6:00pm

Yeah Reynolds will do that on request. They work with a fab shop nearby who can bend the tube and then send it back for heat treating. Haven’t done so myself but I did enquire at one time. Still seems like that DT on Aston’s bike must be longer than 850 no? Perhaps it’s right at the limit with 500mm reach, might just stretch.

Don’t want to derail, perhaps people who’ve done the butted down tube and shock mount can be more help.

My point was just that I wouldn’t advise it and have always avoided it after having heard of and seen failures there.

Design looks great to me!

Daniel_Y · February 21, 2024, 2:24am

Thanks for the inside info. I agree, a butted downtube seems a bit sketchy, especially in a production bike meant to be ridden hard. We have had water bottle bosses crack downtubes because of the thin butt (which is why we reinforce them).

@evan.trem my two cents about the butted downtube:
I always err on the side of caution for prototypes. To me it’s better to start with something that works than to shoot for perfect and risk having to start over. You can always iterate and lighten the frame or tune stiffness on V2.

evan.trem · February 21, 2024, 5:58am

Thanks everyone. Lots of good input. I’m leaning towards 1.5x.049 on the downtube now.

I’m messing around with the downtube to incorporate proper knob clearance per the Fox drawing. It’s looking a lot like @Daniel_Y reference photo with a small guesset.

Anyone found good knob clearance drawings for either a Fox 38 or Zeb? I know it won’t be that different from a 36 but I would like to be extra sure.

I’m going to do a bolt on plate to join the rear triangle halves, similar to Starling’s design.

DEVLINCC · February 21, 2024, 6:01am

I get 850mm non heat treated butted tube from BFS who custom bends it for me. Strength is not the lack of here. The non heat treated tube is plenty strong enough. It’s stiffness that will prevent fatigue BUT… I wouldn’t ever design a bike with the shock loading the weakest point of the structure, which is what that design does. I know plenty of bikes both in prototype and in production and they are going ok. Just is not an optimal solution in my eyes. I wouldn’t go with a butted tube and just accept that this design is inherently a heavier structure to cope with the bending loads.

evan.trem · February 25, 2024, 4:48am

Quick update on my design.

Added a re-enforcement plate to join the two halves of the rear triangle. It is bolt on.
Added a chainstay re-enforcement plate behind the yoke tube. (I have space between the tire and I think I need all the rear end stiffness I can get).
Added a brake gusset.
Added tube plugs to join the upright tubes to the main pivot.

I’ve bumped up the tube wall thickness on a few tubes in bold below:

Tubes:
DT: 1.5x.049 (38.1x1.25mm) single bend 4130 tube (Bend by Bike Fab Supply)
TT: 1.375x.035 (34.9x.9mm) 4130 tube
ST: 1.375x.035 (34.9x.9mm) 4130 tube
CS: Zero Uno S-Bend MTB MPV190B6022 (Bike Fab Supply)
SS and Shock Upright: .75x.049 (19x1.25mm) 4130 tube
Shock Doubler Plate: 1.5x.049 (38.1x.9mm) 4130 tube
Main Pivot Tabbed Box: .06 (1.5mm) thick 4130 plate
Pivot Yoke Tube: 1.25x.065 (31.8x1.65mm) 4130 tube
Pivot Yoke Arms: .375 (9.52mm) thick 4130 plate
Dropouts: Paragon UDH
BB: T47
HT: 44x110mm

I appreciate all the advice so far. It is very helpful to get perspective from other builders. I agree this design is not the most optimal from a loading and kinematics standpoint. I think a lot of my considerations for this build is keeping the complexity and fabrication time to a minimum, along with following in the footsteps of other known designs. I hope this will allow me to move forward quickly with minimal time spent on kinematics design, FEA, CNC Machining, and other things that might take up time and derail my project.

I think I’m close to solidifying the frame design. The next big step is designing a rear triangle weld jig. I have seen some others just use their standard jig, but I think with all of the independent pieces involved I will need to fab something to fixture and locate all of the rear triangle components. I’ve seen some other promising designs based on aluminum extrusion, but any advise on a rear triangle weld jig would be helpful.

evan.trem · February 25, 2024, 4:55am

Detail Pics:

Pi_bikes · February 25, 2024, 6:28am

I made a jig for one of my rear triangles. Just a couple of bits of box section welded together and holes in the correct places with turned standoffs

It worked well