Chainstay philosophy?

[MadRocketSci]

Originally Posted by RPS

Chainstays mostly have to resist chain tension in compression (more so on the drive side) and lateral bending due to chain load being off-center...

It seems that controlling pedaling input and rider weight are just part of the cs/ss/dt's functions. Additional considerations are when there is no rider input but the bike is banked in a sharp turn. Here there are lots of torques and everything needs to resist twisting. Much harder than handling pure compression.

The OS downtubes are there not just to stiffen up the bottom bracket, but to keep the angular deflections down between front and back wheel, along with the chainstays of course.

Just my guesses. Haven't touched a finite element package since school.

[MadRocketSci]

Originally Posted by fierte_poser

When you ride the bike, if you hit a sharp edge of pavement or what not, you are essentially applying an impulse or step input to the spring+mass system. The response of the spring+mass is going to either be underdamped, overdamped, or critically damped depending on the parameters of the spring+mass.

Spring-mass systems have no damping and hence will be completely underdamped, like a tuning fork vibrating forever in an ideal sense. Damping in the system comes from the tires, air in the tires, and the rider's body, which is why they tell you not to lock your elbows. Adding materials like elastomers and/or composites will increase your damping, thus reducing the amplitude and settling time to impulse inputs. in theory, at least.

[jl123]

Is anyone else on here having trouble reading
this thread on their browser? (I'm using Firefox)
My computer is displaying the threads rather
haphazardly, that is out of order todays posts
before yesterday posts etc?
Would like to hear if anyone else is having
this trouble. JL

[zeroking17]

Originally Posted by jl123

Is anyone else on here having trouble reading
this thread on their browser? (I'm using Firefox)
My computer is displaying the threads rather
haphazardly, that is out of order todays posts
before yesterday posts etc?
Would like to hear if anyone else is having
this trouble. JL

It appears to be in sequence on my machine. I'm using Firefox 2.0.

[RPS]

Originally Posted by fierte_poser

OK... well, I'm way out of my area of expertise here, so I'm not sure what you mean by stating that the damping factor is very very low. Further, I have no idea why that would be a good thing.

The point I was trying to make is that I believe you can analyze the rear triangle of the bike as a sping + mass. The spring constant, to first order, is going to be the seat stay. In the case of a straight stay, the constant is so high that no appreciable deflection occurs at the rear axle during typical riding. In the case of a terraplane or dks stay, the constant is intentionally lowered to the point where rear axle deflection occurs during normal riding.

Having said that (assuming its a valid comparison), once you deflect the rear axle upwards, you have stored energy into the spring (stay), and you need to dissipate that energy (damp the system), otherwise the system will resonate.

When you ride the bike, if you hit a sharp edge of pavement or what not, you are essentially applying an impulse or step input to the spring+mass system. The response of the spring+mass is going to either be underdamped, overdamped, or critically damped depending on the parameters of the spring+mass.

In the DKS seat stay, was the elastomer providing the damping for the system or was it providing some other function?

Just trying to understand...

Kent

Kent, I’m not sure I can help, but will gladly express my opinion on the subject.

I was referring to the fact that the natural damping of real engineering materials is quite small. For some steels, it's in the range of 0.0006, which means that for all practical purposes any energy that is stored by a steel spring will be returned. The natural damping of aluminum, for example, is even lower. IMO real material “damping” is insignificant, and should not be confused with what most people report as a smooth or damped ride.

The reason I stated that IMHO low damping is a good thing is a little complicated to express without art (I tried that a couple of weeks ago and was shot down). Anyway, before I even try to describe my reasoning, I want to iterate that in my opinion, road and MTB suspensions are two different animals and shouldn't be compared as one.

On a road bike we are dealing mostly with forced vibration as is the case on an MTB; but IMO most of the interest is in reducing high-frequency low-amplitude buzz associated with rough surfaces; and not that associated with jumping over rocks, logs, etc… which we seldom do on road bikes. Having said that, damping is mostly useful when the forcing frequency is around the range of the suspension's natural frequency (i.e. – damping can limit oscillation amplitude when the system resonates) – but much below and above that it doesn’t buy much if anything. In fact more damping than optimum for a given condition is often counterproductive.

Logically, we can predict that any bike with a low-enough suspension spring constant to do any good (like Terraplane, DKS and others) will have a lower natural frequency that the forcing frequency of a few 100 cycles per second that an accomplished rider will often experience at or above 20 MPH. Because of this we can conclude that no or very little damping will be beneficial. It will allow the wheels to oscillate up and down over the road's imperfections without giving the rider enough time to move up and down himself -- hence what we feel as smoothness or a smoother ride.

Obviously, the lower the spring constant of the suspension, the easier it is for the wheels to ride up and down over imperfections on the road without having enough time to affect the rider to the same degree. Also, lower spring constant lessens shock forces when the bike strikes objects like rocks or pot holes.

If you try to run the math on curved seatstays acting as springs, don't forget that it gets complicated because the springs (i.e. -- seatstays) do not behave as linear or progressive springs as is the case in most vehicle suspensions. The more they bend the lower the spring constant. Where it gets complicated is in the rebound phase because the spring constant increases and you can experience a catapulting effect.