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Discussion Starter · #1 ·
just wondering, would you say that having a laterally stiff rear end to a full suspension bike with a 4 bar design, comes from the size/design of the chainstays, or the size/design of the suspension rockers? See i'm not really having an issue with lateral flex on the rear end of my motobecane comp ds, b/c going downhill through the gnarly technical stuff, it stays pointed where i want it to go, but i'm wondering if i build a new rocker for it, keeping the design same, but doing away with some of the weight saving milling, and maybe adding a sleeve and bolt between the rockers (between the down tube pivot, and the seat stay pivot), would help stiffen things up even more....see i dont really have much else to compare my bike to, as i haven't ridden any turners, or konas or ellsworths ya know....maybe i'm just wanting to modify something that doesn't need it...i mean the chain stays on my bike are identical to those on the transition coverts....what do you guys think? will engineering and building a new, burlier rocker for my bike, make it any more laterally stiff?
 

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Elitest thrill junkie
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The short answer is that you probably won't see a lot of difference.

The long answer is that there are a myriad of factors that go into it, and you have to design the bike accordingly. You can't just say a single pivot is flexy, or that a multi-link bike is rigid, because there are examples of both situations with both bikes.

The foes is a good bike to describe some of the factors. It's a single pivot bike, and there's a scissor-linkage that connects the main frame to the swingarm, but it doesn't change the shock rate or affect the wheelpath, it's sole purpose is to add rigidity to the rear end. Then there are the relatively short chain and seat stays, the asymetrical design around the BB that further shortens the required lenths (chainstay, and allows for a shorter bottom bracket shell), the fact that the bottom bracket and main pivot mount are machined from one solid piece of aluminum, the big main pivot, and so on.

I have a turner 6-pack, and to make that adequately stiff it was designed with bushings in the pivots to control the lateral loads that are imposed on pivots, and because bike pivots do not rotate very much, bushings are very ideal (and far more laterally rigid than bearings). There are also bearing systems that are rigid, but 95% of the manufacturers out there do not use them. Other good features include the fact that the shock mount, main pivot, and bottom bracket are again machined from one big chunk of aluminum. On the other hand, the linakages on this bike are fairly long, and that does contribute to some flex, but again, compared to most bikes it's very rigid. The suspension members are pretty beefy.

There are other factors, but these are a few for you to think about. If you were to change one, would it really make a huge difference?
 

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www.derbyrims.com
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Yes, but consider handling balance too

snobrder5 said:
just wondering, would you say that having a laterally stiff rear end to a full suspension bike with a 4 bar design, comes from the size/design of the chainstays, or the size/design of the suspension rockers? See i'm not really having an issue with lateral flex on the rear end of my motobecane comp ds, b/c going downhill through the gnarly technical stuff, it stays pointed where i want it to go, but i'm wondering if i build a new rocker for it, keeping the design same, but doing away with some of the weight saving milling, and maybe adding a sleeve and bolt between the rockers (between the down tube pivot, and the seat stay pivot), would help stiffen things up even more....see i dont really have much else to compare my bike to, as i haven't ridden any turners, or konas or ellsworths ya know....maybe i'm just wanting to modify something that doesn't need it...i mean the chain stays on my bike are identical to those on the transition coverts....what do you guys think? will engineering and building a new, burlier rocker for my bike, make it any more laterally stiff?
Yes, after linkage geometry design differences of the various four-bar suspensions (floating axle or not), the upper link and rear link stiffness influences the lateral flex of the four-bar rear suspension more than the lower swing arms. "Burlier" upper and rear links will increase lateral stiffness.

(This flex issue has been an interesting topic lately for me. The rest of this below is my own analysis - opinion - of suspension flex factors in handling balance.)

But for handling balance reasons before trying to stiffen the rear end lateral flex any more, be sure your rear wheel's lateral flex is not already stiffer than the front wheel's lateral flex.

As I noticed years ago in my amateur car racing suspension tuning days the same is with bikes also, that a firmer front suspension than rear is actually easier to handle speed (with the exceptions of extremely rear wheel weighted, or front- or four-wheel drive). The same goes for motorcycles and mountain bikes.

Although we are statically positioned on a mountain bike with near 2/3 rear weight bias, when riding while hard cornering and braking where handling balance is most apparent we distribute more weight onto the front wheel for best traction and cornering control.

Notice the rather small dimensioned rear swingarm of dirt motorcycles allows much rear wheel flex to match the long travel relatively flexy forks for bump compliance compared to stiffer shorter travel street motorcycles on smooth pavement.

If the rear is not stiffer than the front, the rear will follow the front in corners and braking. A laterally stiffer rear end will chatter on bumps and loose traction before a flexier front suspension looses grip, so the rider then chases the rear's sideways direction by steering way off line. A stiffer front suspension may chatter before the rear but the rider has more feel of the limits and directional control of the steering in front.

There's a handling balance to flex, also with vertical suspension firmness, having a bias towards a stiffer (and firmer) front end due to greater cornering loads when braking into corners than we can ever coast or accelerate to load the rear as much.
 

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Elitest thrill junkie
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derby said:
Although we are statically positioned on a mountain bike with near 2/3 rear weight bias, when riding while hard cornering and braking where handling balance is most apparent we distribute more weight onto the front wheel for best traction and cornering control.

Notice the rather small dimensioned rear swingarm of dirt motorcycles allows much rear wheel flex to match the long travel relatively flexy forks for bump compliance compared to stiffer shorter travel street motorcycles on smooth pavement.
Wow, that's some far out stuff.

No, I don't think we're biasing way more weight front, if we were, those relatively weak 30lb springs in our forks would bottom out. Yeah, there's braking obviously, but most people aren't ON the brakes in the turns, especially if they want to go fast.

No, I don't notice that the rear swingarms on moto bikes are "small" or flexy. Sounds like a crock of crap.
 

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Jayem said:
Wow, that's some far out stuff.

No, I don't think we're biasing way more weight front, if we were, those relatively weak 30lb springs in our forks would bottom out. Yeah, there's braking obviously, but most people aren't ON the brakes in the turns, especially if they want to go fast.

No, I don't notice that the rear swingarms on moto bikes are "small" or flexy. Sounds like a crock of crap.
On my bike when I'm pushing the limits, the rear becomes very light when braking and entering harder corners nearly lifting the rear off the ground sometimes while my fork dives deep into travel. And I continue trailing (feathering) my brakes most of the way around the corner to help keep weight on the front through the turn for best front tire grip until I can pedal out of the turn. I notice the top motocrossers and GP riders corner this way, I learned from them this style 20 years ago. I used to win most of the time from my braking and cornering style compared to those who braked earlier and quit braking when entering the turn. Only for dog-leg turns should you quit braking before turn in. Even so a stiffer and firmer front end than rear is easier to handle with a slower style.

And if you think high-end dirt bike suspension isn't flexier than a high-end street bike it's hilarious to me! (I guess I should have qualified that, a 250 Ninja suspension probably does flex as much as a top 450 dirt bike. But I assumed similar performance class.)

I'm interested in very fine-tuning. I know from my racing days that a 1/8 inch difference on a rear sway-bar made the difference of winning and struggling.

Like a said it's my opinion based on lots of winning experience. I value yours too. Thanks for your considerate words.
 

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derby said:
And if you think high-end dirt bike suspension isn't flexier than a high-end street bike it's hilarious to me! (I guess I should have qualified that, a 250 Ninja suspension probably does flex as much as a top 450 dirt bike. But I assumed similar performance class.)

I'm interested in very fine-tuning. I know from my racing days that a 1/8 inch difference on a rear sway-bar made the difference of winning and struggling.

Like a said it's my opinion based on lots of winning experience. I value yours too. Thanks for your considerate words.
Sure, just like a hardtail is stiffer than any FS bike, but I'm lost in trying to explain what that proves. So a mx bike is not as stiff as a street bike, isn't that because they don't want to put the weight on the bike and the requirement for 14" of suspension travel means that it would require extensive reinforcement to bring it to that level, and that of course would crazy heavy.

Is a rally car as stiff as an F1 racer? No, but they still design rally car chassis to be as rigid as possible for a variety of reasons, pretty much all of which are established as bettering the handling and performance. In fact, I can look at pictures of much older Mx bikes that had much smaller (flexier) suspension members. Hmm...it seems to be difficult to find an industry that has moved away from "flex". Probably because flex=fatigue and fatigue=failure and loss of precision handling and inefficient suspension action.

You've fallen off the deep end.
 

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Discussion Starter · #7 ·
ok so you're saying that i'm better off with a stiff front end, than a stiff rear end.....well the marzocchi drop off i'm running really has no flex to it, so i guess i'm fine then..that is, if my rear suspension is flexing at all....maybe i just have to ride a really high end bike that is known to not flex, to figure out if mine if overly flexy...which like i said i dont think it is, b/c it tracks through the rough stuff no problem, and it handles just as well as my kona hardtail did....with the exception of the back end not bouncing around through the rough stuff like a hardtail does...i'm still debating on the rocker link...i traced out my current rocker, and located the mounting holes, figured out distances and angles between then, and i might just work around that, maybe add some flow to it, take out the 3" travel hole and just leave the 4 and 5" ones...maybe add those braces connecting the two...not really sure yet...

also, do rockers need to be heat treated? or can i just grab a chunk of 6061 from the local metal supply, and go to town..? honestly i can do everything i need to make a functional rocker with a drill press and a bandsaw....
 

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snobrder5 said:
ok so you're saying that i'm better off with a stiff front end, than a stiff rear end.....well the marzocchi drop off i'm running really has no flex to it, so i guess i'm fine then..that is, if my rear suspension is flexing at all....maybe i just have to ride a really high end bike that is known to not flex, to figure out if mine if overly flexy...which like i said i dont think it is, b/c it tracks through the rough stuff no problem, and it handles just as well as my kona hardtail did....with the exception of the back end not bouncing around through the rough stuff like a hardtail does...i'm still debating on the rocker link...i traced out my current rocker, and located the mounting holes, figured out distances and angles between then, and i might just work around that, maybe add some flow to it, take out the 3" travel hole and just leave the 4 and 5" ones...maybe add those braces connecting the two...not really sure yet...

also, do rockers need to be heat treated? or can i just grab a chunk of 6061 from the local metal supply, and go to town..? honestly i can do everything i need to make a functional rocker with a drill press and a bandsaw....
More laterall stiffness is always good, and I'm not aware of any sports that have proven otherwise. The rocker link thing is probably not where you should waste your energy. Again if it's a "long" rocker, going to something more beefy will help, but the amount that it has to be reinforced may not make it worth it. Take for example single pivot bikes with elevated chainstays, these are bikes like the heckler, bullit, and many others. No amount of reinforcing on the swingarm actually makes them very "stiff", because the entire rear swingarm acts like a lever and lateral deflections on the trail can fairly easily flex it. This is why Foes and Yeti and a few others use a reinforcing swing-linkage that greatly reinforces the rear triangle in terms of stiffness. So in this sense, sometimes going to a "more beefy" part may require something so beefy that it will be very detrimental.

Once again, how rigid are the pivots on your bike? If they're the skateboard-type bearings that pretty much every manufacturer uses, it's going to be futile to make this "upgrade" because your pivots will flex a great deal.

My point is that many things have to come together on a bike to ensure it is laterally rigid, this of course improves handling, allows the suspension to do it's job better due to less binding on off-camber impacts, but it's not something that can be easily addressed after the fact.

It would be a good experience to ride a "stiffer" bike possibly, as you usually have no idea what your missing until you try the "next level", but on the other hand your bike probably works fine and while the rear end may not be the stiffest, it may be better to save your pennies for a ventana or a foes or something similer that is built with this consideration in mind. You may not get anywhere near the same stiffness despite upgrading the rockers.
 

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conjoinicorned
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Jayem said:
Wow, that's some far out stuff.

No, I don't think we're biasing way more weight front, if we were, those relatively weak 30lb springs in our forks would bottom out. Yeah, there's braking obviously, but most people aren't ON the brakes in the turns, especially if they want to go fast.

No, I don't notice that the rear swingarms on moto bikes are "small" or flexy. Sounds like a crock of crap.
you are (of course) welcome to whatever opinion....but i really believe that we bias weight to the front of a bike in steep descending and hard cornering. why do you think ~80% front braking is the ideal, if we didn't have any weight bias up front?

i have a bike with very stiff Alu. front triangle, and relatively "flexy" steel rear end. it is my favorite bike in rough/off camber, as the rear end follows the terrain to a degree and i've never had another bike equal in hard cornering, which again i attribute to the "flexy" rear end.

derby brings up some interesting points IMO....but i will agree that bikes should be as stiff as reasonable!
 

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ferday said:
you are (of course) welcome to whatever opinion....but i really believe that we bias weight to the front of a bike in steep descending and hard cornering. why do you think ~80% front braking is the ideal, if we didn't have any weight bias up front?

i have a bike with very stiff Alu. front triangle, and relatively "flexy" steel rear end. it is my favorite bike in rough/off camber, as the rear end follows the terrain to a degree and i've never had another bike equal in hard cornering, which again i attribute to the "flexy" rear end.

derby brings up some interesting points IMO....but i will agree that bikes should be as stiff as reasonable!
Do you think your rear end is flexy because it's made out of steel?

Did you see my post about turns and the fact that I was talking about situations other than the weight bias caused by braking?

Do you use full-brake power every time you are using the brakes (for full 80% weight shift), or do you use something less than max braking most of the time?

How much front-brake do you use IN turns (not before, but while turning).
 

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conjoinicorned
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Jayem said:
What makes you think your rear end is flexy just because it's made of steel?
LOL

i didn't say that. i said the rear end is relatively flexy, and it happens to be made from steel. the "flex" is part of the design, material choice in this case was designed so there would be some "flex".

Did you see my post about turns and the fact that I was talking about situations other than the weight bias caused by braking?
you are of course correct, the 80% weight shift is the maximum. even without braking, i find often significant front weight bias in corners, especially if there is any bank/berm.
i only wanted to say that derby has some interesting points....but again i agree with you bikes should be as stiff as reasonably possible.
 

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Discussion Starter · #12 ·
alright guys thanks again for all the info you're providing...kinda gives some perspective to my ponderings...here's another question....how much of being able to hold a line through the rough stuff, is made up of rider skill, and how much is because of the bike....i.e. would steve peat be able to blast through the same rockgarden (and stay on course) on say a santa cruz superlight, as he would his on his v10....like on the technical trails here in fla, and the occasional out of state trip, how much difference will i REALLY notice....i guess this might be hard to answer unless you know the trails i'm talking about, like santos, and markham...but let me know what you guys think...thanks again!
 

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snobrder5 said:
alright guys thanks again for all the info you're providing...kinda gives some perspective to my ponderings...here's another question....how much of being able to hold a line through the rough stuff, is made up of rider skill, and how much is because of the bike....i.e. would steve peat be able to blast through the same rockgarden (and stay on course) on say a santa cruz superlight, as he would his on his v10....like on the technical trails here in fla, and the occasional out of state trip, how much difference will i REALLY notice....i guess this might be hard to answer unless you know the trails i'm talking about, like santos, and markham...but let me know what you guys think...thanks again!
The terrain would have to be pretty crazy for you to "not be able to hold a line" on a "flexy" bike. Sure, the flex and suspension binding DOES come into this, but if you're talking about steve pete or any other very experienced rider (such as myself), they can usually do it on virtually any bike. As an example, I had a marzocchi shiver SC, which was an EXTREMELY flexy front fork, but I took the bike down wicked rock gardens and through all sorts of other stuff. Did it cause me to take lines that I didn't want to? Sure. Did I crash because of it? Not often if at all. Did I go through rock gardens with much more confidence and precision when I had a stiffer bike (front and rear)? Yes. The benefits are there, but it doesn't mean that steve pete couldn't go through a rock garden. Now, it may mean that said rider can't pick the line they want and that may cause them to be slower and not win, but that's a little different situation. As an example of a more direct situation, I found that it was much harder to choose the line I wanted on off-camber stuff when I had a bike with a flexy rear end. If you've ever tried to ride many off-camber lines with relatively flexy and rigid bikes, you'd notice a huge difference with the rigid one, as you have to fight the flexy one a lot more to keep it from going downhill in the direction it wants, and sometimes you can't help it.

Only in the most extreme situations would you see a big difference in terms of crashing or not crashing IMO, but that doesn't mean that there aren't big differences besides that. The overall precision, ability of the suspension to absorb vertically, and other factors do make a decent difference. It's kind of like looking back at the stuff that was ridden on 6 and 7" "downhill" bikes back in the day. Has the terrain gotten that much crazier? No, I can point out plenty of examples from back then. Have the riders been able to ride the crazy terrain with much more confidence and control? Yep. That's kind of the difference. It's not like it'd be impossible one way or the other.
 

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Discussion Starter · #14 ·
Jayem said:
The terrain would have to be pretty crazy for you to "not be able to hold a line" on a "flexy" bike. Sure, the flex and suspension binding DOES come into this, but if you're talking about steve pete or any other very experienced rider (such as myself), they can usually do it on virtually any bike. As an example, I had a marzocchi shiver SC, which was an EXTREMELY flexy front fork, but I took the bike down wicked rock gardens and through all sorts of other stuff. Did it cause me to take lines that I didn't want to? Sure. Did I crash because of it? Not often if at all. Did I go through rock gardens with much more confidence and precision when I had a stiffer bike (front and rear)? Yes. The benefits are there, but it doesn't mean that steve pete couldn't go through a rock garden. Now, it may mean that said rider can't pick the line they want and that may cause them to be slower and not win, but that's a little different situation. As an example of a more direct situation, I found that it was much harder to choose the line I wanted on off-camber stuff when I had a bike with a flexy rear end. If you've ever tried to ride many off-camber lines with relatively flexy and rigid bikes, you'd notice a huge difference with the rigid one, as you have to fight the flexy one a lot more to keep it from going downhill in the direction it wants, and sometimes you can't help it.

Only in the most extreme situations would you see a big difference in terms of crashing or not crashing IMO, but that doesn't mean that there aren't big differences besides that. The overall precision, ability of the suspension to absorb vertically, and other factors do make a decent difference. It's kind of like looking back at the stuff that was ridden on 6 and 7" "downhill" bikes back in the day. Has the terrain gotten that much crazier? No, I can point out plenty of examples from back then. Have the riders been able to ride the crazy terrain with much more confidence and control? Yep. That's kind of the difference. It's not like it'd be impossible one way or the other.
very very well put man! thanks a bunch!
 

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Jayem said:
More laterall stiffness is always good
I dont think you can say that as an absolute. If tires (a part of your suspension) had 100% lateral stiffness they would loose an increadible amount of traction.
Absolutes are never good. (think about that statement)
 
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