Tuesday, February 06, 2007


Investigation of Downtube VIIIH -----------------------------SA8 Gearing

I purchased a DownTube VIII H with the 8 speed SA hub and front suspension. I have seen some discussion on the bike forums (DownTube) about the high gearing on the bicycles equipped with the SA8 hub. The SA8 is direct drive (1:1) in the lowest gear (1) all the rest, i.e. 2-8 are overdrive. The stock crankset has a 48 tooth chainring (lower quality 130mm BCD removable) so the gearing is way to high for all but the most aggressive hammer heads–who typically don’t ride folders.

The Sheldon Brown Internal Gear Calculator (http://sheldonbrown.com/gears/internal.html) makes it a snap to compute the ratios. For illustration I use 20 inch nominal wheel size, gear units of inches, and the stock 23 rear tooth size. I used 30, 36, 48 for the chainring sizes.for insructive purposes.

This computes some numbers that are basically meaningless to most people. The good thing about the inch gear unit is that it makes comparison between bicycles of different wheel size. In other words, a 60 inch gear will require the same pedaling effort regardless of the wheel size of the bicycle. When you go to the LBS (local bike store) for a "test ride" to get the feel of what a 60inch gear is like, tell them you want to ride a bike with a 60 inch gear.

I find that an even more useful number is to divide the inch gear by 5 to get the speed in miles per hour (mph) for a normal rider (about 68rpm) ok, ok if you spin at a bit over 80rpm like you should, divide by 4. Mph is a much more common number for the American cyclist. If you are metric and think in kph like most of the world divide by 3 and 2.5 (gotcha).

Okay to put it to the test hit the compute button and

gear 30x23 36x23 48x23

8 79.6 95.5 127.3
7 62.1 74.5 99.3
6 54.8 65.7 87.7
5 48.5 58.2 77.6
4 42.8 51.3 68.5
3 37.8 45.4 60.5
2 33.4 40.1 53.4
1 26.1 31.3 41.7

This table was taken from the SB calculator. Thankx.

Well average rider, you do the math, how often do you ride 127/5 = 25 mph, hoe about 80/5=16 mph with a wind aided spinning 80/4 = 20 mph. Now how about climbing the local nut buster hill at 41.7/5 = 8 mph (for about 2 min or 1/4 mile for your average rider), now 26/5 = 5.2 is about all you can do on a steep hill. Put 25 pounds of baggage on that trip to your favorite hideaway and you will probably still have to walk the steep ones with the 30 or 36, but it will be a bunch better and a whole lot less often than with that 48.

You really have to look at the middle range gears 4 or 5 to investigate your average riding speed–most people can ride 12-13 mph in calm or low wind conditions without looking like they just came out of gym after trying to impress the coach on how tough they are (you know what I mean). That would be gear 3 with 48 tooth chainring ( stock) and gear 5 with the 36 tooth chainring. If you only ride downhill or with the wind stay with the stock, otherwise, change the crankset.

I see “pine cone” (p37- nice job) changed to a 39/53x tooth and that’s probably okay (a bit high in my opinion), but he didn’t say anything about chainline.

I have found that the SA sprocket requires the front chainring to be very close to the bottom bracket to get a straight chainline that looks and performs like it should, i.e. chainring and rear sprocket are in the same plane and the chain runs straight reduce friction and wear.

The LBS will know what I’m talking about and your will find it in the discussions on the Sheldon Brown single speed sections. Although the SA hub provides 8 “speeds” its chain configuration is like a one speed so the chain should be straight. It is not straight with the stock 48 tooth crankset ??? btw, the Sheldon Brown site is superb.

How do you get what you want and need?

Tell your LBS you need a standard road triple with 110mm or 130mm BCD and a granny BCD of 74mm. This is very common and believe me there lots of them around, new and used. The best part is you don't need the middle and large chainrings which are usually the ones that are worn out.

While you are at it tell them to replace the bottom bracket with a sealed unit (the best $40 you will ever spend).

The 74 mm granny bolt circle is the key. Its interior position provides the correct chainline and chainrings are available in any size from 24 to 36 tooth, check the 74mm BCD page from Harris (http://sheldonbrown.com/harris/chainrings/74.html) to see what I mean.

Hey you will be glad you did if you really plan on riding this honey of a folding bike. Make the changes, get what you want and enjoy the ride. After the gears get a quality seat and start getting the pleasure you deserve. Look you will have a custom for $5-600 which is almost too good to be true.

Change the cranks. You will be glad you did.

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I'm not sure if I understand what you're saying.

Which chainring are we supposed to actually use? The 110mm BCD or the 74mm one?

Also, are we supposed to put the other chainring inside or outside of the actual chainring being used?

The second chainring is just for spacing right?
anon.??? who is that?
You may do whatever you like,

I recommend the 74mm (granny) chainring of your choice (24-36tooth). See Harris Cycle

The pic is a of a crank set that has neither a 110 or a 74 so I improvised with the junk (58 BCD with granny and middle chainring on this bolt pattern) that I had and mounted the 32 tooth chainring in the inside (granny position) just to illustrate how used/give away parts could be used.

A typical "110 or 130 triple 5 bolt pattern" with a 74 granny would not require the spacer. Simply mount the 74mm chainring on the inside (granny) bolt position. The other chainwheels can be left off to save weight. The inside position is to assure the chainline is correct and the tooth choice is to get proper gearing.

Stay tuned I may post a pic of a standard installation. In the meantime, ride slowly and enjoy the sights.
But don't you need the 130mm BCD chainring to bolt it to the crank?

If I want to use just the 74mm BCD chainring then how do I bolt it to the crank?
But don't you need the 130mm BCD chainring to bolt it to the crank?

If I want to use just the 74mm BCD chainring then how do I bolt it to the crank?
Only if the crankset is a "double" with a granny attachment (not a recommended option). A true "triple" has 2 bolt circles.

I see it is a bit confusing how the self referential crank talkers sound out their blather. For all the wrong reasons size matters in the crank world too ;) so only the big numbers are used to designate the crankset size, even on a "triple," i.e. 3 chainring thingy! The triple actually has 2 bolt circles a big one (such as 110 or 130) and a smaller one that is on the side closest to the bottom bracket, that is the 74mm. Just bolt the 74mm chainring on the 74 bolt circle and wola: there you have it.

Look on the 28 Feb 07 post for a pic n roll with it.
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