CrimsonEclipse

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HillRider

Your are correct that "high normal" means the derailluer, under control of only its spring, goes to the smallest cog and "low normal" means the opposite. High normal is by far the most common and, so far, universal on road bikes.

Shimano has several low normal MTB derailleurs. Sheldon Brown had a pretty good explanation as to why low normal had some advantages for MTB use, most importantly that it's easier on the chain under high load conditions.

well biked

Shimano has changed their terminology from low normal and high normal to low normal and top normal. Top normal means the spring pulls towards the smallest cog (means the same thing as high normal). I liked the low normal/high normal terminology better.

I've used a low normal (aka "rapid rise") rear derailleur for mountain biking, it does have a real advantage in emergency downshift situations, which are common in mountain biking, and that's Shimano's reason for offering it-

Wordbiker

Just say NO to Rapid-rise/low-normal.

IMO, Shimano introduced it in a slow year when they needed something "new" to sell, much like Biopace. It solves a problem that doesn't exist and introduces a new one: Shops have to stock two of each rear derailleur type for each component level, increasing stocking costs and driving up the price for the consumer.

operator

Ok, having never really mountain biked, can you give us an example of a situation where you might need to emergency downshift?

well biked

Coming around a blind corner in a high gear, and facing an immediate steep and technical climb.

Crossing a rocky creekbed, getting slowed down by the rocks more than you planned, and needing a low gear fast so you can climb up the steep bank on the other side.

It's smoother and faster to get to a lower gear with a low normal derailleur.

Wordbiker

You've just dropped off a huge cliff after climbing to the top in a low gear...then, uhhh...you need to execute a shift to a high gear, to uhhh...keep speed when you land?

No, it is much more common to need an emergency shift to a lower gear, and almost always under chain torque that having cable to pull the derailleur over is much more effective than the relatively weak derailleur spring pulling it over.

I was concerned for a while that Shimano was so in love with Rapid-rise that they'd no longer offer High-normal components. Fortunately enough other mountain bikers felt the same way and the outcry forced Shimano to offer Rapid-rise as an "option". Did I mention they still stand firm on Biopace being the best system for every rider?

EDIT: Had to clarify the post due to it being a response to an erroneous assumption that Rapid-rise assists in emergency shifts to a lower gear...it doesn't. Well...that and I've had a couple beers.

well biked

Shimano first introduced their Rapid Rise derailleurs in XTR only in the late '90's I believe. They had serious problems, my brother in law had one and we worked almost constantly to keep it shifting properly. When Shimano reintroduced Rapid Rise in '02 or '03, they came out and more or less admitted the old ones sucked. The newer versions are much better, for the most part they will at least hold their adjustment like a derailleur should, at least the one I've used seems to be a-ok. I agree that it's nice they still offer high normal on their mtb derailleurs. But low normal does have its merits-

Wordbiker

Shall we agree to disagree then? I think the "merits" are far outweighed by the disadvantages of parts availability and the creation of a "new" standard for no real reason.

well biked

Sounds good to me. Honestly, I'm not passionate about Rapid Rise or anything, but I have used it enough to realize it does have some benefits.

BigBlueToe

When I built two bikes in the last year I went with high normal on both of them, including the mountain bike. I didn't want to change from what I was used to unless I understood that there was a good reason to do so.

One question I'd like answered for my own curiousity. On low normal bikes does it mean that the action of the shifter is contrary to the old way? In other words, if I'm used to pulling on my Rapid Fire lever to go to lower gears in back on high normal, would pulling on the lever make it go to higher gears on low normal?

The fear of this was one reason I stayed with the tried and true. I'd be especially leary if I had one type on my road bike and one type on my mountain bike.

well biked

Yes, the rear shifter works oppositely with a low normal derailleur than than it does with a high normal model. I've only used low normal with trigger shifters. If you need to downshift quickly through several gears, in situations like I described above, using the index finger trigger instead of the thumb shifter to click through the downshift is advantageous in my opinion.

As a side note, this also means that the front shifter and rear shifter work the same in regard to upshifting/downshifting-

HillRider

Sheldon Brown's explanation of the benefits of low normal rear derailleurs said they were easier on the drivetrain.

High normal rear derailleurs let you make a downshift by brute force on the shifter whether the chain wants to move or not. Low normal rd's moderate the side force on the chain and let the cog's ramps and release teeth complete the shift under less stress.

Wordbiker

Yes, and I'm sure Shimano says the same thing.

AFAIK, Sheldon never raced MTBs, and in that particular discipline, forcing a downshift is IMO more important than saving wear on the drivetrain in the long term. When a bike is encrusted in mud, it's the low gears to get going again you're concerned about, not whether you'll replace a cassette in August or September.

Funny that Shimano never developed Rapid-Rise for their race-level road groups.

cyccommute 

Anyone who has tried to dump a chain off of a middle chainring at the bottom of a steep climb can tell you all about how well those derailer springs work at downshifting under load. Some of Shimano's thinking was that they wanted to have both shifters moving in the same direction...not that anyone who rides even a little can't figure out the shifters on a bike.

Back in the late 80's, Suntour made a front derailer that was high normal. That was a much better way of going then Rapid Fail.

rodar y rodar

Now there`s a scarey thought that I can imagine happening- just like the near extinction of canti posts on mtb frames and mid to high level eight-speed shifters. Why give the customers option when the manufacturers know better what we need to be riding? Yikes, glad that one didn`t come to pass!

Svr

It was 1996. The concept was introduced with M950 XTR. It seemed stupid to me then, and still seems stupid to me now.