[Apps]

Hey, guys this is a first time post for me but I’ve been around for a while reading some of the entries that have to do with suspension technology, pivot placement, frame design, etc.

I had talked with Steve (Socket or S or theflyinfatman) for a while about some geometry’s and different setups for a gearbox bike. I through together a couple of different design starting of course with a composite structure (very similar to the Lahar) and moving towards something I could improve upon with different iterations.

Note that although this may look similar to the basics of a Brooklyn race link that was only inspiration for how to get the shock out of the way of the gearbox. And as awesome and smooth the gearbox has been in my very short period of testing it was a huge pain to get a suitable motion ratio as well as appropriate anti-squat characteristics with it. Look for the next one to gear drive the gbox right off the cranks (the next one may not happen that soon as being a college student defines a lack of money).

The following is the result of our talks back and forth and my ideas for what a gearbox bike might be in order to attain reasonable anti-squat curves for a slight range of sag settings, reasonable motion ratios, reasonable packaging (very hard), decent bump performance, what I thought was an appropriate axle path, and with iterations fairly low weight.

Given that I choose this bike as somewhat of a test mule I set it up with geometry that would suit me and my riding style and also gave it the tank bike feature of being virtually indestructible. Every tube size is over done and I know the end result is heavy but try not to lean on that to much as the goal here was not weight.

I used a sram i-9 as my gearbox of choice based on price and gear range as you may have noticed.

Anyway geo and tube sizing is listed below (along with other normal specifications)

Head Angle: 67
Seat Angle: 71
Top Tube (Horizontal): 22.75
Wheelbase: 43.55
BB Height: 14.2
Chain Stay: 16.75
Head Tube Length: 4.5
Axle to Crown: 22.52
BB center to top center of head tube: 27.86

Tubing (All 4130 Steel unless otherwise noted):
Head Tube: No idea but its around .080 thick
Seat Tube, Top Tube, Lower Brace Tube: 1.25 x .058
Down Tube: 1.5 x .058
Pivot Tube: 1.625 x .095
Swing-arm: 1.5 x .75 x .049 with ¼ Stainless dropout tabs (that don’t drop out)
Gearbox Mount: .065 Stainless 303 sheet

So get to the pictures below and let me know what you all think. I’ll be very curious to see what Steve has to say as well (hint give me some feeback Socket). Obviously my component choices were on a budget too as you’ll see below.

Link to the pictures:
http://s195.photobucket.com/albums/z184/Apps21/

 

[Bulldog]

Good, bad or indifferent no one can take away the fact that you built your own bike! And a rare gearbox bike at that! Nice work man, seriously.

 

[jonKranked]

ok just to be the technical police, I don't know if I would consider an internally geared hub to be a gearbox.

REGARDLESS! that's an AWESOME looking bike. Great build quality on the frame too. what does it weigh in at? I know that wasn't a goal, but I'm just curious to see where you netted out.

 

[Apps]

Haven't weighed it up yet actually. I just know its heavy

 

[ohio]

ok just to be the technical police, I don't know if I would consider an internally geared hub to be a gearbox.

Why not? An internally geared hub is a gearbox located at the rear axle. G-boxx 1 is based on a rohloff, as were many other designs. Numerous others are based on the Nexus. In my opinion, nexus and rohloff are both more elegant approaches to gearboxes than the G-boxx 2.

Apps, nice work! How's she ride? That's what matters a whole lot more than the opinions of RMers. Next question is what would you do different next time?

 

[konastab01]

God thing about the rohloffs is that they just work and you dont need to mess about with them musch to get them right.

 

[jonKranked]

Why not? An internally geared hub is a gearbox located at the rear axle. G-boxx 1 is based on a rohloff, as were many other designs. Numerous others are based on the Nexus. In my opinion, nexus and rohloff are both more elegant approaches to gearboxes than the G-boxx 2.

Apps, nice work! How's she ride? That's what matters a whole lot more than the opinions of RMers. Next question is what would you do different next time?

I guess I'm more looking at it from a design standpoint... if I were designing a transmission that would be built into a wheel of a bike that the most off-road travel it would see is a gravel path, I would have a substantially different set of evaluation criteria than I would if I was designing a transmission that wasn't going to be built into a wheel and would be used almost exclusively for DH/all mountain riding. But I digress, I don't want to derail this thread anymore than I already have.

I too am interested in what else you would put into the next updated/upgraded version of the frame.

Was this your first attempt at building a frame? Or first attempt at a full suspension rig?

In terms of the drivetrain and suspension, had you given any consideration to incorporating the drivetrain and suspension any further? IE having a intermediary step from the output of the transmission to the main pivot of the swingarm to avoid chain growth (between the swingarm pivot and the rear wheel)? Somewhat like the jackshaft that's on the Racelink, but integrating the transmission.

 

[Apps]

Oh, the old how would you do it different trick. Well like any good designer (jokingly including myself of course) I am working on that right now. The next design will incorporate a virtual pivot so that more closely control the anti-squat curves throughout suspension travel. In that regard I'm digging on the idea behind the dw link and I think it is a genius design incorporated very consistent anti-squat curves for multiple variations for chainlines which of course varies your force based IC (which of course I don't have to deal with because of the gearbox).

The next one will probably be aluminum with a ton of weight savings and also use a gear driven gearbox or internal hub because tensioning two chains at once appropriately can be quite complex.

I did consider using the output of the gearbox or a jackshaft as the pivot point but I needed the chain to provide just a very slight amount of help towards my anti-squat profile. Also 3 chains would have been a major, major nightmare and to top it off the gearbox is wide enough as it is so placing chainstays on the gearbox axle would have been a pain. Not to mention the torque mounts for the gearbox that keep it spinning instead of the axle would get quite complex.

Also next time I would like to get a shorter swingarm length in there if I was to do a single pivot again so that it might achieve an even more rearward axle path. Getting the weight further down would also be nice. Don't get me wrong its actually right between my knees the whole time but it could be lower.

The ride is awesome. Nothing slows it down. Plowing through the biggest of rock gardens is smoother than I ever could have imagined. Pedaling performance has been outstanding too. It doesn't duck and dive when I'm hammering on it and doesn't seem to have any pedal feedback either. Braking performance is also outstanding (and also tunable via the length of the link) with a tiny bit of squat to balance out the little bit of dive I get from the fork.

Keep the questions coming. What do you all think I should have done different?

Oh, yea thanks for all the compliments.

 

[Apps]

On another note I love the gearbox other than its weight and the idea of single ring with an idler going to a rear derailleurs has crossed my mind recently but it would be against my policy of making drive components unsprung.

 

[jonKranked]

This has more to do with the SRAM i-9 than your particular design, but can you shift while coasting?

 

[f0ggy]

thank you for this inspiration! I've always wanted to make my own frame, what are you studying right now and what have you been studying to get up to where you are like is the brake-squat ect information that you have acquired on your own or school. Thanks for your tips cant wait for your next alu bike should be rad!

 

[ohio]

The ride is awesome.

Great to hear!

As for what I would recommend? Well, I do think the DW-link is brilliant and does all of the right things. In practice, however, I think it's an incremental gain over a well-placed single pivot, ESPECIALLY when you have the luxury of fixed cog/sprocket size and a long swingarm. Both of these facilitate closer to optimal anti-squat characteristics. Sounds like you're pretty into suspension dynamics... pick up a copy of Foale's Motorcycle Handling and Chassis design and it's basically a step by step guide for designing chain driven single pivots (with a healthy intro to more complex designs).

Given that, I would stick with the single pivot, and if you can try to get both the gearbox and the pivot lower and further forward to lengthen the swingarm and lower center of gravity as long as you're still maintaining sufficient anti-squat (depends on cog/sprocket size). That has the added benefit of giving you pretty good braking characteristics without need for a floater.

Finally, on structure I think Aluminum is a wise choice for the front triangle especially because it will be easier to machine parts for the gearbox, but don't be afraid to stick with steel for the rear. Also, trangulating the swingarm at the shock linkage and offsetting the linkage attachment from inline with the rear axle and main pivot will give you a stiffer rear end, especially as the swingarm get longer.

Good luck. I never got as far as you... I'm jealous.

 

[dirtdigger]

Haven't weighed it up yet actually. I just know its heavy

yeah using those heavy wall tubes is guna make it well heavy, go to .049 for the main tubes and even .035 for the top tube.

sweet build!

 

[bikemonkey]

congratulations on a great build. it looks really nice. I am already looking forward to your next bike.

you could make that chain tensioner redundant. as mentioned before you could have a concentric design, it is hard to get it right, you already pointed out the with of the hub as a mayor problem. what bbshell with are you using, maybe you can get away with a 100 mm with bottom bracket and a 150 mm spaced hub.

you could have a lawhill design like with equal upper and lower singarms, if the both axels (hub/gearbox and wheel) are placed equally you will have no growth in the chain lenght.

or you look at mx bike, that are not concentric and they have no chain tensioner. they do have pulleys placed on the right places, I think that the honda dh machine was this way too.

nice work you have done !!!!! please post more images

 

[dhiflyer]

Gearbox Bikes

 

[jonKranked]

was that honda a proto of the rumored production version?


wtf is up with that seat?!!?

 

[Stray_cat]

This has more to do with the SRAM i-9 than your particular design, but can you shift while coasting?

Yes. It's a plantary transmission. I tested one a while ago (not in an MTB enviroment) and was pretty impressed with the system.

 

[xy9ine]

was that honda a proto of the rumored production version?

one of the early protos (with the cvt tranny).

 

[offtheedge]

nice work.

 

[no skid marks]

I think you're mad not persueing and refining your current design, you've done a great job that doesn't appear to need to many tweaks, you've acknowledged it's great performance, Why not stick with the design. The shock and box could both easily be lower, and now you have a model to look at.
Great job,well done.
Johncranked, stop being a tosser, it's a ****ing gearbox. Is chocolate not chocolate when it's shaped like an egg?
It's gears in a box, it is a gearbox.

 

[Steve M]

Wow, that looks pretty cool! I only vaguely remember talking to you about this, was it like a year ago that we were chatting about it, and was it here or on Farkin? (I have way too many PMs to read through and find them sorry!)

Anyway looks the goods mate, pretty impressed! Am I right in thinking that you've gone for a 1:1 final drive ratio and zero chain extension just after the sag point?

 

[Apps]

Right on the money Steve. We talked around November, December time last year. I think at that point I was still on the composite boat. I made some test pieces and just didn't like the way things were going to have to be for me to get everything to work, especially for a first bike.

No Skid Marks:
I see your point and I certainly will consider sticking with this type of design. On the next one it would be awesome to get the box lower and an even further rearward axle path. Not to mention weight. I do like the idea of a steel rear and Al front triangle as I think I've seen a few times recently with Have Faith's bike and the balfa stuff. Some days my mind floats about the possibilities for a 4 bar setup and maybe I get a little ahead of myself without realizing that even now I have accomplished the almost all of the same objectives I'm looking at for a 4 bar with a single pivot.

Jon:
Don't know what your talking about on the seat deal. It''s just one of my old seats(heavy and comfortable).

On another note I'm mate blacking it as we speak and of course got rid of about every sticker. I'll post up some more pics of the beast on Monday when I get a chance.

Again guys thanks for all the praise and even though Steve doesn't remember it he was a huge help in the design iterations for this bike.

Casey

 

[Steve M]

Yeah cool, bits and pieces of the conversation are drifting back to me, I think it must have been in my Farkin inbox somewhere. Cool to see it all come to fruition! Have you done any anti-squat calculation or just estimated it all?

 

[Apps]

I did geometric calcs based on a proposed cg location. This of course doesn't account for the fact that the rider is constantly altering this location throughout the ride even in the the y direction. So in my opinion its an estimation at best

 

[Steve M]

Yeah I think that's all you can really do unless you actually instrumented and measured a rider on some kind of dyno/wind trainer thing to see where the COM moved to. IMO it should give you a pretty good output anyway.