[rbx]

If cc pivot bikes suppose to be bad pedalers then why do people report that their BMW bikes pedals so well?(its grossly a highly placed CC bike)

Is it because of the high pivot jackshaft design that creates a downward tangent force to the axle path(up and to the rear) wich keeps the bike from squating?

Or is because the pivot is closer to the rider C.G which diminishes the moment(c.g x distance to the pivot) therefore squating less then a lower placed pivot?

 

[zedro]

Or is because the pivot is closer to the rider C.G ....therefore squating less then a lower placed pivot?

the 8-ball is pointing in that direction.

this bike was all about pivot point vs. CG for a few reasons....

 

[dw]

Whats your benchmark for pedaling performance RBX?

dw

 

[zedro]

Whats your benchmark for pedaling performance RBX?

dw

uh oh...more cryptic responses from dave

 

[rbx]

Whats your benchmark for pedaling performance RBX?

dw

Demo-9 would be a good comparaison

 

[zedro]

Demo-9 would be a good comparaison

well, that wont react pedalling through the bumps like the BMW would, thats for sure.

 

[Rik]

Neither accelerate like a hardtail

 

[Sir_Crackien]

hey DW that is the best pedaling suspension design out there. i do mean without a stable platform shock and the make it far to every other company you cannot chose your own design because we all know its a good pedaling design

 

[Duzitall]

hey DW that is the best pedaling suspension design out there. i do mean without a stable platform shock and the make it far to every other company you cannot chose your own design because we all know its a good pedaling design

Say what?

Chris

 

[dw]

Yeah, I didnt understand that either...



Anyways, its quite simple. There is no "majic voodoo" associated with any of the rear suspension on bicycles and motorcycles that are sold today, and there never will be.

All are analzed the same way, using the same principles of physics.

Lets compare 3 concentric pivot bikes quickly. Zedro's bike, a BMW, and a Rotec.
Without getting into diagrams and equations, heres how it breaks down. (becuase Ive already posted this here 100 times in the past, so if you are really interested in themath, use the search function)

Zedro's bike will have more anti squat than the BMW, which has more anti squat than the Rotec.

Now lets compare the bikes above to a couple of popular bikes on the market. Say a 2004 SGS DH, and a 2004 Turner DHR. The SGS DH and Zedro's bike both have the highest amounts of anti squat in the begining of the travel of all of the bikes. The DHR will be next, then the Brooklyn, then the Rotec. The Demo 9 falls in between the SGS and the Turner.

Anti squat itself is not a gage of pedaling performance, you need to consider other factors like leverage rate progression, and damper characteristics. Throw pedal feedback concerns on top of this, and then try to put it all together so it is packagable into a lightweight and production friendly layout. A more refined squat curve will allow the rider in the end to run less compression damping and less spring in some cases than a less refined design, and therefore allows for better traction with increased acceleration performance.


Just remember this, you can have a blast on any bike!

dw

 

[vitox]

say dave, is it impossible or unfeasible to make a standard test (involving dynos and what not) for pedalling performance?
im thinking some form of benchmark as you said.

 

[dw]

It could be done, but the thing is, not all bikes are designed using the same "standard" for lack of a better term. It would be more dificult to compare an XC bike to a DH bike, because the DH bike is designed to accelerate on the flats ond down grades, where the XC bike is designed to accelerate on hills and some flats. You would need automation and your data would be massive. One thing to consider is that for a lot of bicycle companies, where they most likely dont have an engineer on staff, let alone a competent suspension dynamicist, it becomes a nearly impossible task to figure out the parameters, build such a machine, and then use the machine to get useable data.

Id love to build such a rig if someone wanted to pay me the $$$$ to do it. It would be one hell of a complex machine. Like an MTS tester with a whole crapload of add-ons!

Dave

 

[vitox]

well what i meant was a machine that could be made to benchmark frames, but that would require some sort of industry standard.

just as you point out xc bikes have different design priorities than dh bikes, but maybe the industry or a few companies could agree on a "set" of properties to test for and also on different ways to weigh those parameters so as to reflect the different types of bikes.

sounds awful complex anyway but maybe one day it can be done, its been done in other places anyway. hopefully with enough brainpower and funding a way can be devised to summarize performance in a few simple and meaningful tests that dont require a millionary investment in machinery to test for.

 

[dw]

The thing is Vitox, if the industry were to come to an accord on a set of priorities, then it would mean that guys like me, who have done all kinds of analysis and testing to define a set of design parameters would have to share thm with the rest of the industry. This could begin to negate the advantages that the people pushing technology enjoy now. I personally would not want to share that information.

Dave

 

[Kornphlake]

One thing to consider is that for a lot of bicycle companies, where they most likely dont have an engineer on staff, let alone a competent suspension dynamicist.
Dave

You said it again, Aztech bikes aren't designed by an engineer, you're such a hater. :eviltongu

Seriously though where do you get the idea that other companies don't have engineers who do any kind of analysis? I'm not doubting you, actually I have a similar opinion, but you don't strike me as the kind of guy to make a statement like that unless you have some experience or reason to believe it. Do you care to share your reasoning behind making such a statement as the one above.

 

[dw]

Its just plain fact Kornphplake. A lot of bike companies dont have on staff engineers. A lot do. Most of the larger companies have some sort of engineering staff. The easisest way for you to find out who has what is to make some phone calls. In this day, factories do a good deal of "design" work on frames, and there is a pretty good database of what tubes people use for different application, so you dont necessarily need an engineer on staff to build a nice bike. Also, having a degree doesnt imply competency, and there are alos a lot of competent people without degrees.

Suspension dynamics, especially for HPVs though, is a very specialized field.

dw

 

[zedro]

Lets compare 3 concentric pivot bikes quickly. Zedro's bike, a BMW, and a Rotec....

dw

dude, my bike isnt concentric!!!!

actually i found it funny how many people though i was using two chains with a concentric jackshaft...guess my clever chainguide return pulley mounted on the swingarm fools even the cleverest of engineers

look closely at the chain circuit. Btw, mounting the pulley here reduces lower chainline extension; its at its longest in the middle of the travel, and shortens by 5-8mm at full extension or full compression. And of course theres no chainguide arm to bend or cause clearance problems with the tire, especially since the high pivots need a high return pulley to save the derailler from a thrashing.

And for a production model, i'm thinking a nice clear E13 drivetrain cover would go nicely with an E13 bashguard....

 

[dw]

Instead of editing, ill just add to my last post here. A lot of good bikes have been built through good ol trial and error. A lot of good bikes have been built when the designer uses an alternate version of physics to analyze a system, adn by luck things turn out to be closer to good than bad. Not very many bicycles that I am aware of have been developed starting with a baseline understanding of chain driven human powered vehicle suspension systems. I have talked with a lot of people in the bicycle industry about suspenison, and 95% of the time, the concepts that I discuss are totally alien to everyone. Ive talked with some very competent motorcycell suspension dynamicists though, and we are eye to eye, so I am confident that it is just a very time consuming subject that a lot of companies have not had the resources to fully explore and understand YET.

Dave

 

[dw]

dude, my bike isnt concentric!!!!

actually i found it funny how many people though i was using two chains with a concentric jackshaft...guess my clever chainguide return pulley mounted on the swingarm fools even the cleverest of engineers

look closely at the chain circuit. Btw, mounting the pulley here reduces lower chainline extension; its at its longest in the middle of the travel, and shortens by 5-8mm at full extension or full compression. And of course theres no chainguide arm to bend or cause clearance problems with the tire, especially since the high pivots need a high return pulley to save the derailler from a thrashing.

And for a production model, i'm thinking a nice clear E13 drivetrain cover would go nicely with an E13 bashguard....

I could have sworn you said originally you designed it so you could move that pulley around and mount it to the main frame or the swingarm to play with stuff? I guess either way, the anti squat picture will be similar, which means more than the brooklyn frame, but your pedal feedback will change. The whole analysis picture changes when you mount the idler on the swingarm instead of the frame.

id like to ride it.

 

[tmx]

dude, my bike isnt concentric!!!!

are you sure? dw said it was.

 

[zedro]

I could have sworn you said originally you designed it so you could move that pulley around and mount it to the main frame or the swingarm to play with stuff?

id like to ride it.

the original system i designed for it has never been used before by anyone as far as i know, and basically the pulley was in the same general location but will change positions throughout the travel (ie. a variable chainline). Unfortunatly the mechanism i originally designed and had fabricated to do this wasent exactly refined enough for the task, so i went with this alternative i designed in to try out first. I also made a concentric and jackshaft setup possible, but never implemented.

The current setup works so well with pedalling however that i ditched plan A (which is kinda ironic since its really what started this whole project in the first place).

if you see me i'd be happy to have you try it....although i am 6'2" eh...

 

[Kornphlake]

Instead of editing, ill just add to my last post here. A lot of good bikes have been built through good ol trial and error. A lot of good bikes have been built when the designer uses an alternate version of physics to analyze a system, adn by luck things turn out to be closer to good than bad. Not very many bicycles that I am aware of have been developed starting with a baseline understanding of chain driven human powered vehicle suspension systems. I have talked with a lot of people in the bicycle industry about suspenison, and 95% of the time, the concepts that I discuss are totally alien to everyone. Ive talked with some very competent motorcycell suspension dynamicists though, and we are eye to eye, so I am confident that it is just a very time consuming subject that a lot of companies have not had the resources to fully explore and understand YET.

Dave

Okay this makes sense, I know there isn't really a textbook on bicycle suspension, or any suspension for that matter, just good old kinematics and a those who either understand it or don't. I'm one who only understands when someone explains things to me, I don't have the fundamental knowledge to create my own theories or prove the designs made by trial and error. I guess by your first post I got the idea you were implying that trial and error isn't engineering.

 

[EVRAC]

My take on the main difference between a bike like a Rotec or a G-Spot vs. BMW is the chainring size. With a large chainring on a cc pivot bike a triangle is formed (rear axle, pivot point, upper chain-chainring contact point). Stomping on the pedals uses that leverage to compress the rear sus. With the bmw, the distance from the pivot to the chain contact point (tangent point) is smaller, therefore less leverage and less bob. That's why normal single-pivots (Giant DH, Bullits etc.) have the pivots as close as possible to the chain tangent point. Look at the Honda, it's chain is very close to the pivot.

 

[zedro]

I guess by your first post I got the idea you were implying that trial and error isn't engineering.

that's one of my fundamental design concepts for the bike, although i wasent taking shots in the dark, but rather leaving a range of adjustment based on certain key parameters and concepts, test them out and then work backwards to establish "formulae". Basically i didnt want to commit to strict theoretical numbers and a fixed system, only to be stuck with a very time consuming mistake that i cant afford to fix.

 

[dw]

are you sure? dw said it was.

hey hey hey ! see, i was right after all! well sort of. heh (i have to admit though, for a moment I was not totally sure I wasnt losing my mind there! )

 

[-BB-]

hey DW that is the best pedaling suspension design out there. i do mean without a stable platform shock and the make it far to every other company you cannot chose your own design because we all know its a good pedaling design

My brain hurts from reading that...

 

[dw]

My take on the main difference between a bike like a Rotec or a G-Spot vs. BMW is the chainring size. With a large chainring on a cc pivot bike a triangle is formed (rear axle, pivot point, upper chain-chainring contact point). Stomping on the pedals uses that leverage to compress the rear sus. With the bmw, the distance from the pivot to the chain contact point (tangent point) is smaller, therefore less leverage and less bob. That's why normal single-pivots (Giant DH, Bullits etc.) have the pivots as close as possible to the chain tangent point. Look at the Honda, it's chain is very close to the pivot.

Well, thats one of the parts that is important, but equally if not more important is the fact that the TMX pivot is higher. There are 2 forces that combine to create anti squat. Driving force (the force of traction pushing through your wheel, through your suspension, on the frame) and chain pull force. Driving force is ALWAYS the higher amount of force.

dw

 

[dw]

I guess by your first post I got the idea you were implying that trial and error isn't engineering.

Engineering is all trial and error really, its just trial and error using mathematics and aplied science as a guide instead of blind faith. I think thats what seperates the two.

 

[Kornphlake]

that's one of my fundamental design concepts for the bike, although i wasent taking shots in the dark, but rather leaving a range of adjustment based on certain key parameters and concepts, test them out and then work backwards to establish "formulae". Basically i didnt want to commit to strict theoretical numbers and a fixed system, only to be stuck with a very time consuming mistake that i cant afford to fix.

That's exactly what we see the big name companies doing from one model year to the next is slightly moving a pivot placement, using a different stroke shock, a longer linkage plate... Your bike just allows you to make those changes without having to cut more metal. You did have a ball park estimate of where you wanted things to be, maybe more like an infield sized estimate and that's really about as close as the average designer can get without a bit of trial and error. The design value to design effort ratio is roughly exponential in that the initial design effort adds a lot of value, but toward the end your working months to split hairs in the final product. In the final stages of a design it's faster to do prototyping and test by moving things around physically than to try and crunch numbers, create models or run simulations.

That said, I'd consider both the guy who takes the initial design from concept to alpha prototype just as much of an engineer as the guy who takes an alpha prototype, drills it full of holes and fabricates different parts tinker toy fashion to get it to a final version.

 

[zedro]

Driving force is ALWAYS the higher amount of force.

dw

exept maybe the bike that was designed the previous year at my school, which had a pivot higher than a BB7 but didnt run any pulley system whatsoever

these guys didnt know jack about bikes tho, and got a friend and Balfa to design and build the whole thing, yet they won the design award anyway (the faculty later realized they got burned and wont even mention the project ever happened at all, and it kinda screwed up my chances to win the award the next year ). Not surprisingly, i've never seen the bike on the trails.

 

[ssaddict]

That said, I'd consider both the guy who takes the initial design from concept to alpha prototype just as much of an engineer as the guy who takes an alpha prototype, drills it full of holes and fabricates different parts tinker toy fashion to get it to a final version.

If were talking HT's here it's not a huge deal, unless your trying to squeeze grams out of a frame. But for FS designs you better damn well run your designs through some CAD or your going to have to do one of a few things:

Overbuild everything, use customers as test subject's,
or spend years in testing and never actually get the product out.

Cause you really can't do the same level of stress testing and analis any other way.

My tinker toys always broke.

 

[zedro]

If were talking HT's here it's not a huge deal, unless your trying to squeeze grams out of a frame. But for FS designs you better damn well run your designs through some CAD or your going to have to do one of a few things:

Overbuild everything, use customers as test subject's,
or spend years in testing and never actually get the product out.

Cause you really can't do the same level of stress testing and analis any other way.

My tinker toys always broke.

IMO, the CAD part is more important for the drivetrain and suspension stuff, as well as clearances than it is for the structural part. Look at the BMW Linkbike or Balfas, those arent refined FEA analysed designs and no 3D modelling was used. As long as you use simple structures (instead of complicated monocoque and yoke designs etc), you could keep things very analogous to simpler HT designs.

 

[Kornphlake]

Right, you run numbers to make sure things will be strong enough, but you get to a point where you don't really know what the numbers mean untill you tinker around with it. For pedal kickback for example, you can calculate how much pedal kickback you'll have in some quantifiable units but you don't know how much is acceptable until you build up a bike and ride it to see if you can notice it or not, then it's an optomization game from there. The same could be said for structural analyisis, there's a point where you just don't really know what the variables are, I can calculate the impact force of a 200lb rider hucking off a 8' roof to flat but I don't know how that will affect the structure of the frame if the rider lands rear wheel first, how far on the rear wheel, tweaked out a little bit, and such. Hopefully the safety factor would account for variables like that but you really don't know until you have done some testing. Like DW points out the two really do go hand in hand, I'd consider a design that was built in CAD and used some FEA and such along with thourough testing to be superior to the homegrown tinker toy bike but computers and analysis can only take you so far.

 

[dw]

As I pondered this thread and recent events while insulating my garage, I thought it might be better off if I posted the following:

Just so that there is no confusion, everything I have said here about bicycles/ motorcycles and engineering is in direct response to Kornphlake's question, and has NOTHING to do with any of the frames or brands associated or mentioned in this thread. Anything mentioned was used specifically for comparative purposes, and no actual performance data is given.

Its rough when you cant have a technical discussion on the good ol' interweb without a disclaimer...

Maybe some of that paranoia is wearing off on me..?

 

[Kornphlake]

You're such a push over.

 

[zedro]

Its rough when you cant have a technical discussion on the good ol' interweb without a disclaimer...

Maybe some of that paranoia is wearing off on me..?

i think you should have a disclaimer in your signature.

do you happen to live in a glass house?

 

[dw]

exept maybe the bike that was designed the previous year at my school, which had a pivot higher than a BB7 but didnt run any pulley system whatsoever

these guys didnt know jack about bikes tho, and got a friend and Balfa to design and build the whole thing, yet they won the design award anyway (the faculty later realized they got burned and wont even mention the project ever happened at all, and it kinda screwed up my chances to win the award the next year ). Not surprisingly, i've never seen the bike on the trails.

heh, that bike sounds pretty wild, even on that one though, driving force will be higher than chain pull force. You could have a single pivot bike with a pivot 1 million kilomenters above the rear axle and a 450mm chainstay length, with a standard gearing, and drving force effect will be higher still. Its just one of those universal constants.

Dave

 

[dw]

hhahaha

you guys crack me up

maybe im getting soft in my old(er) age

 

[zedro]

heh, that bike sounds pretty wild, even on that one though, driving force will be higher than chain pull force. You could have a single pivot bike with a pivot 1 million kilomenters above the rear axle and a 450mm chainstay length, with a standard gearing, and drving force effect will be higher still. Its just one of those universal constants.

Dave

yeah i know...but try to take a big drop and see what the physical constants do to your ankles....

 

[Kornphlake]

hhahaha

you guys crack me up

maybe im getting soft in my old(er) age

Are you wearing a dust mask? I'll bet you're getting loopy from inhaling fiberglass. Really we're not that funny.