aham23

^^^^ hey, let us all leave the Psimet alone. he is, or should be, busy building some custom wheels. later.

ericcox

^^I guess he never said "tip of everyday."

Psimet2001 

8. Pull ratios - the meat and potatoes of compatibility - or "the source of 90% of the 'can I run x with y' threads"


#28 - Pull Ratios

The way that rear derails and shifters work together and are either compatible or "incompatible" ios because of their pull ratios. Basically put the ratio describes the amount of lateral (side-to-side) movement you get from the derail for any given movement of cable (pull).

You pull the cable...and the derail moves. Simple right?

The difference in ratios come about from design of the parallelogram of the derail as well as the mechanical limitations or desgin of the shifters. The shifters and derails of a particular system are designed to work together.

Usually the shifters and derails from one manufacturer will work interchangably. This is not always the case. In particular I am thinking of how Shimano Dura Ace used to...and will again with 2009...have a different throw on the derail than the Ultegra, 105 or down levels had on 8 spd.

Now...you can change pull ratios. Sometimes it can come from mounting the cable through the anchor in a different position. Sometimes it can be done by using a device that will basically use 2 differnt sized "pulleys" to change the ratio (shiftmate comes to mind).

With this in mind it starts to become apparant that you can almost get just about any shifter to work with just about any derail...if you really want to and don't mind having to find ways to try to affect the ratios needed.

My explanation is probably pretty poor. It can be hard to explain this one. This is why many times it is easier to just tell someone, "Campy with Campy. 10 with 10, 9 with 9" and so on.

In actuality it can sometimes be fairly easy to make a Srimangolo drivetrain work well together, but maybe it would be better to just need to ask..."why?"

But in general any rear derail will work across any "speed" version in a manufacturer's line. The shifter just needs to match the number of cogs on the cassette and the pull needs to actuate a throw on the rear derail that will match the spacing on the cassette.

Campy and Shimano both use ratios. Most say they are fairly close, but shiftmates work well to go between the two systems.

SRAM uses "exact actuation" or a 1:1 ratio. This means for every mm of cable pull you get a mm of throw or lateral movement on the derail. There are many who believe this makes for a more robust system over the Shimano system which has a low amount of pull to high amount of throw on the ratio...meaning small cable adjustments or stretches make big changes.

In response it appears that Shimano has changed the ratios for 2009 mechanical DA to make it more robust - need less adjustment.

....that's the theory anyway. All of this is moot with eDA

Oh...and I have been busy blogging instead of tipping....sorry.

KiddSisko

You should never admit this. Besides, we're not necessarily known for our perfect reading comprehension either. Our confusion is part of the ongoing fun of bike ownership.

StephenH

Handy hint: If your eyebrows are bushy enough, sweat will drip over your glasses instead of onto your glasses. I haven't tested this one extensively yet, though.

johnileo61

Now THAT'S funny!!!

CyLowe97

Holy thread bump, Batman. Now you're going to force Psimet to post some sort of tip of the day.

Luckily for all of you he got a chance to work on my wretched rig the other night, so I'm sure he'll be full of hints and advice after he gets through the shock and denial of even knowing me.

So what's that again about brake pads and toe in, Psimet?

Psimet2001 

*sigh* ......errrr..*psi*

ColorChange

So when does Dr. Psi have late night shop hours? I have a brb relatede creak and need a Psi Super Tune on my shifting.

I'll bring another sixer.

Psimet2001 

I think you guys are messing up your bikes so you can come over and make me drink beer and gain weight so that I get slower.

merlinextraligh

I think you're about 300 daily tips in arrears now.

CyLowe97

Way to crack the code, Sherlock!

Nachoman

Now it's double that. What happened to our daily tips?

shelleyspins

Don't post them all at once - it might take me a few hundred days to process the previous tips (sometimes I'm not just slow on the road ; ) Good stuff, thanks for bumping this thread!

Psimet2001 

*sigh*.....oh how much has changed....

Let me come up with at least 1 new one now....

Tip#WTF: When buying a new frame/bike be sure to make arrangements to buy a replacement derail hanger. If you ride a lot you may eventually need one. if you race it is almost assured that you will need one. It usually happens when you are no where near a location that would actually have one and it is usually a few model years after yours was made. This is especially true of fly by night frame importers.

Wheels Manufacturing makes most replacement hangers available but I still advise all new buyers to buy one at time of purchase.

Top#WTF2: Treat your hook-up well. If they are a good hook-up then odds are they go through a lot for you without you realizing it and probably lose money on you than you realize. I do this for many people but I call them "friends" and they have always been grateful, but I can see why many shops and owners change their tone quickly after a few bad experiences.

tuxbailey

What if your hanger is welded on... SOL?

Aero Sapien

Why does one put a higher psi in their rear and a lower in the front? I usually fill them up at the same psi

alpha_bravo

The rear wheel bears more of you weight while riding than the front, and thus needs a higher PSI to support. You don't NEED to have less air pressure in the front, but it makes for a much more comfortable and better handling ride.

Psimet2001 

Yes. Unless it's steel.

radeane

I've found this to be a useful thread throughout the years and now I have question for those at the far low end of the spectrum. I bought my nephew a Fuji Ace with 650 wheels. He's about 100 lbs. I'll weigh him when I see him this weekend.

Does the formula still apply? I'm coming with 86 and 78 on 23 tires. Is this ok or are those tire pressures too low for road wheels.

Velo Vol 

More like 1100 now.

ColinL

I have found out to my displeasure that these weight-based formulas can lead you to pinch flats. if you do a good job of avoiding sharp bumps and deep potholes, no problem.

however my wife rides right over a lot of stuff. she pinch flatted twice (months apart) when I set the pressure according to her riding weight. instead I use sidewall minimum up front and 10psi above that in the rear. I would expect similar from your nephew-- however, for training rides, it can't hurt* to see if he has any problems at the mathematically-derived pressure.

also beware that tire volume and carcass rigidity are not equal. therefore one tire that suggests 100 psi minimum and another only 85 minimum should convey some kind of useable information to you, even if both tires are the same rated size.


*technically it could hurt in the form of a pinch flat, or less likely, a bent rim.

Psimet2001 

Sorry. I started getting paid for my tips

zigmeister

Wow, nothing like a 2yr bump.

How come you don't include the weight of the bike with water bottles half-filled or something to give a realistic weight of what the tires will have to support?

My tire pressure jumps nearly 7psi by doing this, which, the formula works out to just a few lbs less than I really run anyway.

What about clinchers V tubulars?

Sorry, not sure if this was all covered in the thread, didn't feel like going throw 150 posts to find that already.

Thx

Psimet2001 

It was all covered. The formula isn't something that I personally derived off of hours of empirical testing.....

It is a derivation of pressures and ratings that the late great Sheldon Brown posted. They are starting places and have worked for the vast majority of riders trying them. In general and after 20 years of doing this I have found that the vast majority (near all) of riders ride at pressures that are simply way too high.

Argue all you want but it won't change that truth. Funny how in the last year or so everyone has started raving about lower pressures and big tires and wide rims.... guess this thread was before its time.