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Sub 40lb Custom Gearboxx Bike

HaveFaith

Monkey
Mar 11, 2006
339
0
Administrator edit: Updates and full build pictures are available starting here:

http://www.ridemonkey.com/forums/showthread.php?p=2907166#post2907166


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I have posted a few things over in the gearbox bike thread saying I have been designing and getting ready to build a dh race bike utilizing the g-boxx2 and making it sub 40lb for a full build with my buddy Ryan. I figured its at a point that people could check it out, parts are being made and we are getting ready to start building tooling.I feel that 40lb is the threshold where if you can make a gearbox bike this light, it is going to be easier to accept by the general DH race public. Super low CG and unsprung weight are big bonuses and I have a feeling this thing is going to be super snappy and ride much lighter that it is. The specs are as follows:

HA: 65.5
Eff. Top Tube: 22.38
BB height: 13.75
Travel: 7.25
Chain stay: 17.38
Wheel base:45.25 (full droop)
Target frame weight: 7lbs with hardware, no shock (optimized for 165lb rider)
Integrated seat mast

Ive designed this bike a little less slack, but with a decent length top tube (for me) to try and keep the wheelbase a bit longer for stability at speed, but also the rider CG further forward, to natural weight the front end for cornering. The rear end is 4130 steel and the front 7005 AL. I did this to stiffen up the rear end, as in the given volume, you can make the stiffness to weight much better with steel. If you have any questions, let me know. Here are some pics:




 

Hougham

Monkey
Mar 28, 2007
375
1
Very nice model. Always fun to have a go at the frame thing. I hope it works for you. It does look like it may flex, You may be better going for larger bore thinner wall tubing if it does. What’s leverage ratio across the travel? I suspect all that milled parts of the steel rear end would be hell to make. You would be talking hundreds just for the side mounts for the swing arm on the box.
 

HaveFaith

Monkey
Mar 11, 2006
339
0
Very nice model. Always fun to have a go at the frame thing. I hope it works for you. It does look like it may flex, You may be better going for larger bore thinner wall tubing if it does. What’s leverage ratio across the travel? I suspect all that milled parts of the steel rear end would be hell to make. You would be talking hundreds just for the side mounts for the swing arm on the box.
Sorry, I forgot to list the leverage ratio: 3" stroke shock for 2.42:1 ratio

The milled parts are already be doing as I type, not quite as bad as you would think! (having connections and asking favors helps)

From my FEA stuff, its actually quite a bit stiffer than you would think. The rear end is steel, which is 3 times the stiffness of AL, and while the seatstays are pretty small, the chainstays arent much smaller than a standard AL tube --> Stiffer there. Ive already purchased the tubing and its here at the shop. The Headtube will be 1.5" so that makes the other stuff look pretty thin as well.

Thanks for the comments.
 

HaveFaith

Monkey
Mar 11, 2006
339
0
Nicely done.

Could be a good candidate for a split-pivot?

Good luck with the build!
I was actually considering this route, but did not because, while at this point it is just for fun, I didnt want to step on DW's or anyone elses toes.
 

karpi

Monkey
Apr 17, 2006
905
0
Santiasco, Chile
turn the shock around! like's promesing! try to get the chainstay shorter. Is it a dh frame? if so, could you snuck a bit more travel into it? just my 2 cents
 

no skid marks

Monkey
Jan 15, 2006
2,513
27
ACT Australia
I'd love to see a Horst type pivot on the back,to get a more rearward axle path(it is a DH bike?). Looks great for a trail bike.
I like the big shock(stroke).
Still dubious about the weight,but who cares. Would hate to see you compromise strength to reach a target weight though.
A long cockpit(top tube)will not help weigh the front,quite the opposite.
I'd go with copied(or slightly tweaked) geo you know you like.
Good on you for doing this project,even the manufacturers can't really get it done.:rolleyes::cool:
 

HaveFaith

Monkey
Mar 11, 2006
339
0
Make a gearbox that ins't a single pivot, and I will be impressed.
There isn't too much to gain going that route (besides complexity and weight). With the high forward pivot, the wheel path is partially rearward and mostly vertical. The concentric pivot and output drive actually keeps a pretty consistent squat curve, especially compared to a standard single pivot design. In fact, the only main downside is braking and since the rear end is traction limited as it is, I would make that trade.

The geo numbers are actually very close to a Sunn radical, with a bit less travel. Ive kept the travel down to 7.25 because of our local terrain. This bike will be fast for the california scene, there isnt enough super rough, high speed stuff (MSA) to warrant the trade off in low speed handling and pedaling. 65.5 HA is pretty far off from a trail bike geo #, and the front end isn't long compared to a trail bike, just a bit longer than the current trend in DH bikes.

I will post pics as the build comes along.
 

P.T.W

Monkey
May 6, 2007
600
0
christchurch nz
The milled parts are already be doing as I type, not quite as bad as you would think! (having connections and asking favors helps)
Not sure with the idea of having alot of machined steel parts.
Its actually comparably heavier than fabricating the same part.Its quite common for some machined alloy race car parts(uprights mainly)to be replaced with fabricated steel parts to save weight an cost for the same stiffness.

nice work tho!:clapping:
 

no skid marks

Monkey
Jan 15, 2006
2,513
27
ACT Australia
Huh??? can you explain how a "horst" pivot would achieve a more rearward axle path?
By having the Horst pivot a lot lower than the rear axle,the chain stay would extend through it's travel. the down side would be the need for a chain tensioner.
As for having to remove the chain for a flat,just patch the tube with the wheel in the frame,it's rare that a puncture is unfixable. To tyre would be a different story.
 

S.K.C.

Turbo Monkey
Feb 28, 2005
4,104
25
Pa. / North Jersey
Sounds like you've definitely got the right idea for shooting below the 40lbs. mark, but I wonder if the current g-boxx technology needs to be reduced in size and weight before that can happen...
 

HaveFaith

Monkey
Mar 11, 2006
339
0
Good luck with your work. I don't want to be a negative nancy but there are a number of things that really frighten me from what I see.
Please share, I encourage all comments!

As for chain tensioning, Im going pretty low tech at this point, going to make something similar to the old school bmx horizontal dropout tensioners. Ill be running tubeless so fixing flats will not be a major issue.

For machining the steel parts versus fab, at this point in production, its actually easier for me to spend the money and have components machined, then pin down our welder for more time than is necessary to build a fab part (he is volunteering after work to help out). Plus, I wanted to keep the tolerances fairly high at the component level, so that it wont be as much of a pain to keep everything aligned and straight at the final weld stage. You can see at the lower shock mount that I opted to go for a fabbed box section to get the stiffness and lower mounting, machining that section out would have just been a big waste of material for me. I definitely understand the optimizing with welded sheet pieces for saving weight and gaining stiffness, ive used these methods for various projects at work.

For the ones worried about weight of the frame and breakage. Ive optimised this design for a rider around 165lbs. Would it break if a 225lb rider thrashed the thing? Yes. That is the beauty of designing a custom frame for a given use. I intend this thing to live a race season but dont expect anything more. Just as a formula one gearbox only needs to last 4 races, designs on the edge for weight reasons trade reliability at a certain point. Dont get me wrong, Im not intending this to break, but when it does Ive designed it to be non-catastrophic.
 

HaveFaith

Monkey
Mar 11, 2006
339
0
Also, all of the pivot points other than the main pivot on the g-boxx will be running IGUS polymer bearings for simplicity, weight and stiffness.
 
Apr 16, 2006
392
0
Golden, CO
Make a gearbox that ins't a single pivot, and I will be impressed.
Haha people are so caught up in marketing, its definately paying off.

Good job Have Faith. Could you have saved weight by going the "geared hub in the frame" route or is the Gboxx optimized (lighter) for this by letting you avoid the double chainring and extra chains. Awsome to see the frame down that low on weight and good use of steel for the rear. Are you welding/milling/physically fabricating any of this yourself or are you just designing and giving the commands? Either way have fun riding it, I can't wait to ride something I build.
 

HaveFaith

Monkey
Mar 11, 2006
339
0
Haha people are so caught up in marketing, its definately paying off.

Good job Have Faith. Could you have saved weight by going the "geared hub in the frame" route or is the Gboxx optimized (lighter) for this by letting you avoid the double chainring and extra chains. Awsome to see the frame down that low on weight and good use of steel for the rear. Are you welding/milling/physically fabricating any of this yourself or are you just designing and giving the commands? Either way have fun riding it, I can't wait to ride something I build.
Im active in the fab, with my buddy Ryan who is the co-conspirator on this project doing a good amount of actual production work. I went with the Gboxx because it can be a stressed member and the standard is already established for mounting purposes. This idea of ours came up in our heads in January. If I had been planning for a longer time, then maybe I would have gone a different route, but this seemed easiest for us to make a gearbox bike the quickest. Thanks for the kind words.

Here are some pics of stuff being made. First is the upper gboxx mount after being waterjetted and getting ready to be sent out to machining. Second is of the bell crank after waterjet and a piece after machining in the pocketing.


 

HaveFaith

Monkey
Mar 11, 2006
339
0
That look like good quality machining. What program did you use to draw the parts up in? I wish that we had a CNC machine at work. We may get one this summer. I'll keep my fingers crossed.
I used solidworks 07, though I just got 08. Its nice and easy to use, even for those not super technically minded. I also use ProE for other projects, but this was easiest in SW. The bellcrank was actually machined by hand to save a bit of money, while the more complicated stuff will be CNCed.
 

NY_Star

Turbo Monkey
I used solidworks 07, though I just got 08. Its nice and easy to use, even for those not super technically minded. I also use ProE for other projects, but this was easiest in SW. The bellcrank was actually machined by hand to save a bit of money, while the more complicated stuff will be CNCed.
I use autodesk Inventor 9. At work now we have 2 south bend lathes and 2 bridgeport mills. How did you do the radii on the hand mill?
 

buildyourown

Turbo Monkey
Feb 9, 2004
4,837
0
South Seattle
Cool project. I too see some issues, but since you've already started, you will learn them yourself. Is the front end steel or alum? Thru axle? There is a reason you never see horizontal drops on a DH bike.
Where did you get the gear box from? I'd love to get my hands on one.
 

HaveFaith

Monkey
Mar 11, 2006
339
0
It's radii. The plural of radius is radii.
Use the correct size tool and finish your machine pass in the corner pretty much. Not the most accurate way of doing it, but its good enough for the piece.

For the horizontal dropout, the axle is going to be located pretty much at the front, so the bolt can get the most material to bite, just had to add a little bit of adjustability so that we can tension the chain. I agree its not ideal, but it was the easiest way to tension the thing at this point!
 

HaveFaith

Monkey
Mar 11, 2006
339
0
Cool project. I too see some issues, but since you've already started, you will learn them yourself. Is the front end steel or alum? Thru axle? There is a reason you never see horizontal drops on a DH bike.
Where did you get the gear box from? I'd love to get my hands on one.
The front end is 7005 AL. Didnt want to invest in tooling for heat treat, thats why its not 6061. Hard to find plate stock for machine pieces out of 7005 though! Rear end is TA 12mm x 150mm. Using a standard hub converted to single speed.

We bought the gear boxes from g-boxx in germany. Freaking dollar to euro conversion!!!!
 

buildyourown

Turbo Monkey
Feb 9, 2004
4,837
0
South Seattle
For the horizontal dropout, the axle is going to be located pretty much at the front, so the bolt can get the most material to bite, just had to add a little bit of adjustability so that we can tension the chain. I agree its not ideal, but it was the easiest way to tension the thing at this point!
You will need to use some sort of chain tug. Otherwise, the wheel will slip and cock in the dropouts.
7005 alum frames are heat treated too. Sending out 6061 is pretty cheap. My guy will do a frame for $100.
 

HaveFaith

Monkey
Mar 11, 2006
339
0
You will need to use some sort of chain tug. Otherwise, the wheel will slip and cock in the dropouts.
7005 alum frames are heat treated too. Sending out 6061 is pretty cheap. My guy will do a frame for $100.
Yeah, I posted previously that I was looking to use a bmx style tensioner on both sides. Cheap and low-tech, but should work.
The 7005 needs to be aged true, but its not as aggressive as the 6061 heat treat as far as temps go (from what I have read). I was more worried about the frame distorting during a high temp heat treat that the 6061 would go through. Am I operating under the wrong understanding?
 
Apr 16, 2006
392
0
Golden, CO
Im active in the fab, with my buddy Ryan who is the co-conspirator on this project doing a good amount of actual production work. I went with the Gboxx because it can be a stressed member and the standard is already established for mounting purposes. This idea of ours came up in our heads in January. If I had been planning for a longer time, then maybe I would have gone a different route, but this seemed easiest for us to make a gearbox bike the quickest. Thanks for the kind words.

Here are some pics of stuff being made. First is the upper gboxx mount after being waterjetted and getting ready to be sent out to machining. Second is of the bell crank after waterjet and a piece after machining in the pocketing.
Thanks, Yea I forgot the usefullness of the gboxx's case as a structural element, like an engine in a motorcycle.
Quick Q on the parts you got waterjetted. Did you know the person doing this for you or did you have to get quotes from a private company with a waterjet machine? If so was it easy to get a quote on such a low quantity of parts, and if you don't mind me asking, what was the cost? I'm currently designing something that will have parts that can be waterjetted and I'm just trying to test the waters.

Good job with the frame, Its seeming like more and more people are making their own stuff recently with the advent of software like solidworks (sure has helped me) and cnc becoming cheaper. I'll be making something soon myself, stating with a HT just to get my welding on mitered tubes down pat. Also good job on posting this, try and keep up with the tread, we could use more step by step threads on this topic, and you arent exactly going down a paved road with your design, so it will be cool to see all the problems and how you solved them. I'm hoping to make an "open source" style thread in the next year were ill try and post everything on my own DH frame as well.
 

buildyourown

Turbo Monkey
Feb 9, 2004
4,837
0
South Seattle
Yeah, I posted previously that I was looking to use a bmx style tensioner on both sides. Cheap and low-tech, but should work.
The 7005 needs to be aged true, but its not as aggressive as the 6061 heat treat as far as temps go (from what I have read). I was more worried about the frame distorting during a high temp heat treat that the 6061 would go through. Am I operating under the wrong understanding?
Honestly, I've never built any 7005 weldments. I've done them out of 6061 and haven't had too many issues with distortion.
I do know that 7005 bikes are artificially aged and solution heat treated. If you only do natural aging, I assume you are sacrificing some strength. If you design accommodates the lower strength, then you should be golden.
7005 is pretty rare outside of bikes. I've never been able to source the billet. (and I'm a machinist). I've only ever seen it sold through bike building companies.