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Perfect Geometry 4 a FS 29er?.... class project help

hbracca

Chimp
Jun 4, 2008
48
0
Eugene, OR
Next term in my CNC machining class i will need a project and I wanted to get started on the design/drawings. My teacher used to be the manufacturing engineer at Burley (local Eugene, OR frame builders) and thought a bike would be a cool project (him and I geek out together most free time during class). Also, one of the guys in my class is a veteran welder and is willing to help me out as long as we make 1 for him also.

I was thinking of making a 80-100mm FS 29er with the pivot around the BB like an Arrow DSS3 or a Cove G-Spot. I will prob build it around a RS reba 29 and a RS monarch 4.2

I am very new to the 29er family/cult as I have only done a couple rides on a friends monocog rigid. I enjoy the 29er quite a bit, but with bad wrists (rock climbing) and a DH background, I would prefer something a little more plush that would still give me the torture and simplicity of a true SS setup.

I was hoping some of you could help me out with angles, lengths, etc. I have been told that it is a bit difficult to get the correct "feel" with standard 26 in geometry. On the monocog I felt very high, like I was on top of the bike, not "in it". I would imagine that this has something to do with BB height to wheelbase ratio with a bit of steep head angle, but like I said, I'm prety new to this whole 29in thing.

My current 26in AM bike is set up with a 23 tt, 12.25 bb, 16.5 stays, and a 69ish ha @ 32 talas. Would it be possible to stay with a pretty similar "feel"?

Thanks for the help!
 

MMcG

Ride till you puke!
Dec 10, 2002
15,457
12
Burlington, Connecticut
Why the concentric bb pivot? Lenzsport used to use that style but has since changed.

I wonder if you could replicate the suspension design of a Schwinn Rocket 88 without infringing on any of dw-link or Schwinn patents. That was a suspension design ahead of its time IMO.

A few things - you can do a longer travel 29er now. Forks available and they feature better offset for longer travel capabilities.

A few things to worry about IMO are:

1) Keeping the front end from getting too tall - maybe you can accomplish this by using a 1.5 headtube and a headset with the spacers to run a 1 and 1/8th inch steerer tube.

2) chainstay length. Hard to keep em shortish because of tire to seat tube issues. For example, most 29er fullies feature chainstay lengths ranging from about 17.5 (Lenzsport Lunchbox) all the way up to 18.5" (New Salsa Big Mama). Most fall in the 18 to 18.2" range though.

Cool thread here!

I hope Alex Morgan (bcd) will chime in on this thread as he's been able to build his carbon DH frames with pretty short chainstays - probably the shortest I've read about to date.

Let's keep it active and constructive Monkeys!!!
 

hbracca

Chimp
Jun 4, 2008
48
0
Eugene, OR
Why the concentric bb pivot? Lenzsport used to use that style but has since changed.
I wanted to run horizontal dropouts/tentioners instead of a derailer style (I hate chain slap and my SS DJ bike is so maintainence free) That pivot point would make the axle path up and a bit forward, that might be why they got rid of it, when the whole VPP thing said how bad that was, but I beleive Santa Cruz has since retracted that view. I rode a DSS3 for a season and axle path didn't annoy me at all.

A few things to worry about IMO are:

1) Keeping the front end from getting too tall - maybe you can accomplish this by using a 1.5 headtube and a headset with the spacers to run a 1 and 1/8th inch steerer tube.

2) chainstay length. Hard to keep em shortish because of tire to seat tube issues. For example, most 29er fullies feature chainstay lengths ranging from about 17.5 (Lenzsport Lunchbox) all the way up to 18.5" (New Salsa Big Mama). Most fall in the 18 to 18.2" range though.
1. I was thinking about using an integrated headset or a zero stack, but the 1.5 interface is burly

2. chainstay length brings up another question, what is a good average tire clearance. I've heard of a few 2.4 29er tires, but how common are those in the rear? I only run a 2.35 in the rear of my DH bike.

thanks MMcG, I was hoping you would chime in on this.
 

MMcG

Ride till you puke!
Dec 10, 2002
15,457
12
Burlington, Connecticut
Sorry I missed your desire to have it be SS capable - in that case a concentric pivot like what is/was used on the G-spot, and what Kona currently uses on the Kowan would make the most sense. I think Lenzsport still uses this type of design on his FS single speed - I think he calls it the Milk Money or something along those lines. It is more of an XC Full suspension single speed.

It sounds like you are leaning towards a 29er dirt jumper/slope style type of bike - am I on the right track? Do you think 29" wheels lend themselves well to that type of application? Or will this be more of a 29er FS trail bike?

If you only plan to run 2.35 in the rear you'll have multiple tire choices. I don't think it is so much the chainstay clearance, but it is the clearance between seat tube and rear wheel when the rear wheel cycles through its travel that is the tough part about a FS 29er design.

This is a cool thread. If it gets more traffic, I'll sticky it soon.

Cheers,

Mark
 

hbracca

Chimp
Jun 4, 2008
48
0
Eugene, OR
It sounds like you are leaning towards a 29er dirt jumper/slope style type of bike - am I on the right track? Do you think 29" wheels lend themselves well to that type of application? Or will this be more of a 29er FS trail bike?
I already have a good Hardtail DJ bike with 26's and I'm not really into the whole slopestyle type of thing. I just really like to go fast on trails. I can do that on my DH bike, and even though it is light (for a DH bike), I would still have to push it a ton. I was thinking more along the lines of a lightweight FS trail bike that I still felt comfortable descending on. It would be great for training, but really the only reason I ride up a trail is because eventually you get to come back down.
 

MMcG

Ride till you puke!
Dec 10, 2002
15,457
12
Burlington, Connecticut
So do you want this fully to have gears? Were you thinking of running something like a Saint rear derailleur on it or something? Or do you want it to be FS and Single speed?

I'd say try and build it around the new Fox 120 or the new Reba with Maxle, also 120mm - and both with I think 46mm of fork offset. Perhaps do a search on Lenzsport Lunchbox for some inspiration on how to do it and keep the chainstays fairly short.
 

Guitar Ted

Monkey
Aug 21, 2006
305
0
Waterloo, IA
My advice would be to take a hard look at the Lenz products and see what Devin is up to there. In fact, if you give him a phone call, and explain what you are up to, I think he just might give you some golden tips. He's a one man operation, so don't e-mail him. The phone is best.

Anyway, Mark is correct on the concentric pivot Lenz being the "Milk Money"



The chainstay issue is a bit over rated in my humble opinion. I know that many here will poo-poo that notion, but in my experiences 18 inch long chainstays actually help you climb on a 29"er As for Salsa's Big Mama, I found it to be the easiest 29"er to wheelie since my original Karate Monkey. Chainstays are just part of the overall equation, something to be concerned about, yes. However, 18 inch long chain stays are not a deal breaker in my mind. It'll make your job far easier too.

Mark is correct about the forks too. Longer offsets make 29"ers steer far better than they used to. You can get 26"er-ish handling if you want it. If you stay 100mm or less up front, I'd consider a 71-72 degree head angle. If you go with the 120mm options, (I agree- it's the better choice right now), you could back that head angle off to 69-71 degrees and still have a nice handling bike.

You haven't mentioned your material of choice. Aluminum? If so, I think you might have a bit more of a challenge unless you can form it yourself somehow. Devin does his own forming on his Lenz creations, which allows him freedoms that you might not be able to take advantage of with straight tubing. Something to consider.



Here's a typical "faux bar" design in aluminum to check out in 29"er guise. You can see the efforts made to lower the standover and keep the head tube area low.



Here's a KHS Tucson and you can see the integrated headset, lowered top tube, and pretty straight forward 4 inch travel design. Again, this is a big wheeler. This is par for the course with 29"ers. Most are aluminum and have some sort of formed tubing.

Anyway, this should be interesting. Thanks for starting the thread.
 

hbracca

Chimp
Jun 4, 2008
48
0
Eugene, OR
So do you want this fully to have gears? Were you thinking of running something like a Saint rear derailleur on it or something? Or do you want it to be FS and Single speed?
I was thinking I would make the dropouts with interchangable pieces, kind of like a mixture of the dillinger and the All Mtn Proto for Soul (maybe 12x135 maxle to), but with a tensioner built in (hard to desribe). Ive been working on them for a couple other proto bikes from Soul and they are starting to look good so I will probably steal the idea for my own project.

At first I will probably try a Sram I-motion 3spd internal hub, but eventually wanted to go to SS on a 12x135, that's why I wanted the concentric pivot and the versatility.

I'd say try and build it around the new Fox 120 or the new Reba with Maxle, also 120mm - and both with I think 46mm of fork offset. Perhaps do a search on Lenzsport Lunchbox for some inspiration on how to do it and keep the chainstays fairly short.
I looked around for the specs on the Reba (I like the Maxle idea) on Srams website it says 80-100 for the 29er which is OK, but 120 would be nice. Beside the 46mm offset, does anyone have the axle to crown measurement?


My advice would be to take a hard look at the Lenz products and see what Devin is up to there. In fact, if you give him a phone call, and explain what you are up to, I think he just might give you some golden tips. He's a one man operation, so don't e-mail him. The phone is best.

Anyway, Mark is correct on the concentric pivot Lenz being the "Milk Money"



The chainstay issue is a bit over rated in my humble opinion. I know that many here will poo-poo that notion, but in my experiences 18 inch long chainstays actually help you climb on a 29"er As for Salsa's Big Mama, I found it to be the easiest 29"er to wheelie since my original Karate Monkey. Chainstays are just part of the overall equation, something to be concerned about, yes. However, 18 inch long chain stays are not a deal breaker in my mind. It'll make your job far easier too.

Mark is correct about the forks too. Longer offsets make 29"ers steer far better than they used to. You can get 26"er-ish handling if you want it. If you stay 100mm or less up front, I'd consider a 71-72 degree head angle. If you go with the 120mm options, (I agree- it's the better choice right now), you could back that head angle off to 69-71 degrees and still have a nice handling bike.

You haven't mentioned your material of choice. Aluminum? If so, I think you might have a bit more of a challenge unless you can form it yourself somehow. Devin does his own forming on his Lenz creations, which allows him freedoms that you might not be able to take advantage of with straight tubing. Something to consider.



Here's a typical "faux bar" design in aluminum to check out in 29"er guise. You can see the efforts made to lower the standover and keep the head tube area low.



Here's a KHS Tucson and you can see the integrated headset, lowered top tube, and pretty straight forward 4 inch travel design. Again, this is a big wheeler. This is par for the course with 29"ers. Most are aluminum and have some sort of formed tubing.

Anyway, this should be interesting. Thanks for starting the thread.


Thanks guys, I think I'm starting get some of this nailed down. I started doing some sketches and I'm pretty sure I could get the chainstays down around 17.5 without running into to big of a problem, maybe shorter, but I want good mud clearance. Would anyone have the exact dims for a 29in 2.4 tire (true outer dia. and width).

I would be using Aluminum, prob 6061, but that is a ways down the road. probably straight gauge so I could bend it a bit without causing to many issues.

-HB
 

MMcG

Ride till you puke!
Dec 10, 2002
15,457
12
Burlington, Connecticut
I think building it around a 12mm thru axle in the rear is a GREAT idea. If you could get 17.5" chainstays that would be a great number to work with IMO.

I need to get my ass on a few FS 29ers to try them out. Problem is they are few and far between here in the Northeast. Maybe I can talk the owner of a local shop who carries Kona to let me take a little spin or two on the Hei Hei they have in the shop.

If I can - I'll report back to you.

GT would be a great guy to PM about tire width and whatnot as he's done a lot more tire testing than I have for his blog and for www.twentynineinches.com

Cheers,

Mark
 

hbracca

Chimp
Jun 4, 2008
48
0
Eugene, OR
Any updates/further thoughts on this?
I started working on the solidworks model, but the weather has been so nice, it's been moving slow. Im thinking 17.5 chainstays, about 100mm of travel pivoting around the BB, with some style of tensioning dropouts. Not sure on head angle yet, but I want to build it around the Reba 29er 120mm with Maxle.

What do you guys think about having waterbottle cage access (important or not). I'm not used to having one, but I'm also used to shuttle runs and at the most 10 miles in the saddle with a camelback.
 

MMcG

Ride till you puke!
Dec 10, 2002
15,457
12
Burlington, Connecticut
I'd say try and find a spot for a cage if possible, but don't bother if it has to be on the downtube or some whacky spot. It would be good not only for water bottles, but also for batteries for lights for night riding.
 

hbracca

Chimp
Jun 4, 2008
48
0
Eugene, OR
After you finish up in solid works - will you have access to the raw materials to move to building the frame?
I have access to all of the 6061 for machining that I want from the college, I have a welder lined up, and the college also has everything for heat treat, the difficult part will be tubing, but I have a few places locally that build in 6061 so I should be able to dig up some proper tubing.
 

ÆX

Turbo Monkey
Sep 8, 2001
4,920
17
NM
I started working on the solidworks model, but the weather has been so nice, it's been moving slow. Im thinking 17.5 chainstays, about 100mm of travel pivoting around the BB, with some style of tensioning dropouts. Not sure on head angle yet, but I want to build it around the Reba 29er 120mm with Maxle.

What do you guys think about having waterbottle cage access (important or not). I'm not used to having one, but I'm also used to shuttle runs and at the most 10 miles in the saddle with a camelback.
if you use one front chain ring use a two chain setup then the high pivot will
allow you to run super short cs w/o the wheel running into everything.

it will still be concentric feel just higher up.
 

hbracca

Chimp
Jun 4, 2008
48
0
Eugene, OR
Any updates? Progress? Further thinking?
I've been in Whistler for the last few days so I haven't really had a chance to get much farther on it.... super fun trip 3 days of riding and no injuries:biggrin:

BCD,
I think I'm going to stay away from the 2 chain setup, but that would allow for some nice short stays.
 

Demeter

Chimp
Aug 20, 2008
1
0
Moscow
Hello here!

Sorry if i picked the wrong thread, but i have the similiar case.
With my fellow friends we are trying to design a FS 29er for ourselves. Both of us have height of around 192cm, so frame size gonna be around 23", and frame will be built from titanium.

I've bought a Linkage to model suspension, but it did not make me a suspension designer ;) I've uploaded few trash prototypes to its web library under brand "PetBike".
Example:

or

Btw are there any articles to read about this program? For example what leverage ratio graph is good and why...

The ultimate goal is to kill dozen of rabbits with one shot.

1) Geometry shall be suitable for aggressive cross-country and trails.
2) Tire 29x2.2" shall fit with at least 10mm clearance
3) It shall be 100/100mm suspension (We own RS Reba 29" atm) convertable to 120/120.
4) Horst link type preferable
5) It shall be possible to mount Hamax Child Seat.
6) It shall be possible to put big bag inside the front triangle of the frame.
and so on
7) It shall use any wide spread air rear shock 165/38.

Any advices on angles, geometry, etc. are highly welcome.

PS Sorry for my poor english, bears around me don't speak it ;)
 
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MMcG

Ride till you puke!
Dec 10, 2002
15,457
12
Burlington, Connecticut
The first one looks like it is a Titus Racer X copy and the second one looks like a Cannondale Scalpel copy.

It'd be cool to see the second one done as the first one has already been done.
 

Guitar Ted

Monkey
Aug 21, 2006
305
0
Waterloo, IA
Here is my two cents on those two designs, Demeter.

#1: Lower center of gravity, triangulated rocker link that should resist lateral movement better. Lets you design with most any seat tube length and allows for better standover height.

#2: Higher center of gravity, barely any triangulation on the shock linkage, (Note: a Scalpel has no pivots on the chainstay, so it resists lateral movement there) The design forces you to use a longer seat tube, and complicates standover issues.

That's just what my impressions are. I'd go with the first of these two designs.
 

ByStickel

Chimp
Nov 8, 2007
38
0
WNC (via nj,ca,tx,in,&va)
Number 1 is a lot more appealing to me for ergonomic (nad-gonomics?) reasons. The Scalpel/early FSR designs had overly long seat tubes, which function fine, but are hard to take, aesthetically.

The lateral stiffness will really be determined by the execution of the link and it's pivots. It could be acceptable, or a flexy mess, depending. The best way to reduce flex is lateral triangulation, but it doesn't look like the current link will allow for much of that. Look at Trek's new linkages for ways to make them stiffer, and at the oblong tube Ellsworth bonds between their rocker arms.

I have to respectfully disagree about the importance of a low CG. When we ride, we have a 100-250lb mass sitting atop a 27lb chassis. When we make quick changes in direction, that 250lb doesn't do a lot of moving. Instead, the wheels steer the bike out from under the rider to initiate lean. The bottom of your wheels move most and most quickly, and the contact point between your saddle and arse moves little. During cornering, and most riding, a very low CG would hurt you more than a high one. That's one reason people suggest lighter wheels, so if you put the mass of the shock down around axle height, well...

And how much weight are we really talking about here? Locating a small hollow shock 8" higher isn't going to make a big difference. Running a saddle bag with multi-tool and tube would have to make a much greater impact than shock placement.

People always talk about how great it is to have a low CG on a bicycle, but I think the effect is mainly psychological.

Could be wrong. Would love to have a discussion on it.
 
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MMcG

Ride till you puke!
Dec 10, 2002
15,457
12
Burlington, Connecticut
Hey Steve,

Do you think one could achieve a good performing shortish-stayed FS 29er using a design like Schwinn used with their Rocket 88 series bikes?

I'm curious to get your thoughts on that one.
 

Guitar Ted

Monkey
Aug 21, 2006
305
0
Waterloo, IA
Steve, here's my thoughts on the center of gravity debates:

The "importance" of it is certainly not fixed yet, that I've seen, but there is something to it.

Here's the deal, if you consider the wheels contact patch the pivot point, and any deviance from perpendicular to the ground while riding, weight on the frame becomes somewhat of an issue. Obviously the further from the pivot point weight is placed, the more energy it will take to counter act that weights movement to one side or the other, and of course to bring it back to verticle. This is, as you say, a minimal concern, in the short term. But cumulatively, this weight higher up on the bike can become more of an issue over longer rides. At least I believe it could.

Take a gross example, a touring bike. Panniers are generally mounted lower than higher for this reason.

Does having a few grams lower on a bike make that big of a difference to most trail riders? Probably not, but in some cases, I believe it does. And if it makes a minimal difference to the positive, and you can do it, why not do it?

Just my thoughts on the matter.
 

ByStickel

Chimp
Nov 8, 2007
38
0
WNC (via nj,ca,tx,in,&va)
We should do a blind taste test to determine the importance of CG... but of course we won't.

Without it, it's all just talk, but I like to talk more than I care about accuracy, so...

I think you want ultimate stability with a touring bike. Maneuverability is not it's thing. A mountain bike needs instability to make quick changes in course. Most of that movement happens low to the ground as you make small deviations from your intended course in order to avoid roots, rocks, etc. Watch a bike weave through a slalom and you'll see that the rider moves a lot less than the axles do. I do know that people equate a low shock position with maneuverability.

Mark,
(-edit- I should read before I reply )

The Rocket 88 would be pretty difficult to do properly in a 29er. That's a lot of crap to get behind the seat tube. What it has going for it is that it has initial chain growth for good pedaling feel, but since the bottom link is so short, that chain growth diminishes quickly, reducing pedal kick-back during rapid compression.

I think that I-drive has more potential, despite the poor review of GT's 29er FS. Seems like poor execution.
 
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