Monday, March 19, 2007
Crankset and Chainrings for DT VII IH w/ SA8
I have done some experimenting for the Downtube VIII H w/ Sturmey Archer 8 speed hub relative to the choice of cranks.
See 28 Feb 2007 & 7 Feb 2007 for details on the parameters I was considering.
I decide to go with the Bulletproof BMX cranks than can be used as a single or double chainring setup for the folder config (pic). This crankset has a 110mm BCD with a wide range of chainring choices (min 34) and with a 107 mm Shimano sealed bottom bracket the chainline is nearly perfect.
The 34/23 combo with the SA hub provides the lower gears that I prefer for touring European city and rural landscape with my carry-on pack mounted on rack. [I will get to that some day.]
I would still choose the Touring triple with the 110/74 mm BCD if I were looking to have some range and what to use the more common 113mm square Bottom Bracket. For example, if you were looking to make the Cruz bike conversion with SA on the FWD, switch to a derailleur on front or back, or ... . A simple switch of chainrings which typically cost less than $25 is an easy upgrade plus there are many excellent used parts especially if you stick with the square BB.
by WrencherWOAC
See 28 Feb 2007 & 7 Feb 2007 for details on the parameters I was considering.
I decide to go with the Bulletproof BMX cranks than can be used as a single or double chainring setup for the folder config (pic). This crankset has a 110mm BCD with a wide range of chainring choices (min 34) and with a 107 mm Shimano sealed bottom bracket the chainline is nearly perfect.
The 34/23 combo with the SA hub provides the lower gears that I prefer for touring European city and rural landscape with my carry-on pack mounted on rack. [I will get to that some day.]
I would still choose the Touring triple with the 110/74 mm BCD if I were looking to have some range and what to use the more common 113mm square Bottom Bracket. For example, if you were looking to make the Cruz bike conversion with SA on the FWD, switch to a derailleur on front or back, or ... . A simple switch of chainrings which typically cost less than $25 is an easy upgrade plus there are many excellent used parts especially if you stick with the square BB.
by WrencherWOAC
Labels: BB, chainrings, cranks
Wednesday, February 28, 2007
What one 110mm BCD installation looks like
I recently junked out a MTB with a Shimano Exage triple crankset (110 BCD) with a 74mm - 5 bolt circle granny. The middle and large chainrings were total junk so I pitched them--actually I turned them it wall decorations, the land fills are no place for art.
The 26 tooth granny was in good shape, it is a bit smaller than I would prefer, none-the-less it is an easy ride and provides a basis for comparison. Hard pedaling is not a something to be forced put up with when checking out a bicycle for comfort and actual use.
I took this pic to show what it looks like with the chain on the chainring aligned with the DT SA8 hub. It is mounted in the inside position for chainline allignment. The 110mm BCD bolt pattern holes can be seen behind the granny(middle & large chainrings removed, i.e. "art"). If you buy new you can keep the larger chainrings or trade them. [click the pic for greater detail]
When I get around to it, I will probably replace the 26 tooth with a 32 but right now I am going for a MTB snow ride. The DT will be waiting to accompany me when I travel off to parts unknown.
(Speed)* is Not the Answer
Ride a Bicycle
wW
* War, Oil, well you fill in the blank
Labels: chainrings, cranks, junk
Wednesday, February 07, 2007
VIIIH Chainrings: Turning the outside in
After giving a bit of theory in post 960, I will give you some actual applications. When I got the new VIII H SA8 out of the box, I noticed the bottom bracket was so tight it would hardly turn. I pulled off the cranks to adjust that and while I had them off I figured I might as well experiment a bit.
I dug around in my junk parts cache and found an old Shamano STX triple crankset from a mid 90s Kona MTB. The STX is kind of an odd ball deal that has the middle chainring and the granny gear attached to the small 5 bolt circle. [Most triples have the middle and big chainring on the larger bolt circle and the granny on a smaller bolt circle.] Since I was just chasing gear and chainline combos, I just turned the granny (24) and the middle (32) around and put it on (the granny is just a weird looking washer in this configuration. (I guess I could cover the teeth with reflective tape :-)
Next I shortened the chain and used a connector link (good hint, pine cone) to put it back together. It is sweet, but let’s just say you aren’t going to see this in LBS. I am not so sure I want to go with it either, but it promises to give the gear heads whiplash and it does ride like a dream. No, this is not trick photography.
It gives me, and you, an idea of the kind possibilities there are when you are looking for used parts. I would still recommend going with the 110 (preferred) or 130 BCD with the 74 (see post 960) granny for chainline position, gear tooth flexibility and wider availability, but who knows what you might have to do in a pinch?
The Sheldon Brown "Touring Triple Crank" section lists two Sugino Cranksets for less than a $100 twhich I'm guess would be about for the DownTube VIII. It also provides a benchmark when shopping for used parts.
I also disassembled the stock crank/chainwheel and in can be seen in the picture below.
Click on the pic to see full size.
According to Sheldon Brown, it is possible to get a 130 BCD 5 bolt 38 tooth chainring ($20)that would get closer to a reasonable gear but merely replacing the chainring would not solve the chainline issue,o'wtf,how about washers and longer bolts ???.
Like I said before, "Change the cranks. You will be glad you did." And don't forget the new sealed bottom bracket!
by WrencherWOAC
I dug around in my junk parts cache and found an old Shamano STX triple crankset from a mid 90s Kona MTB. The STX is kind of an odd ball deal that has the middle chainring and the granny gear attached to the small 5 bolt circle. [Most triples have the middle and big chainring on the larger bolt circle and the granny on a smaller bolt circle.] Since I was just chasing gear and chainline combos, I just turned the granny (24) and the middle (32) around and put it on (the granny is just a weird looking washer in this configuration. (I guess I could cover the teeth with reflective tape :-)
Next I shortened the chain and used a connector link (good hint, pine cone) to put it back together. It is sweet, but let’s just say you aren’t going to see this in LBS. I am not so sure I want to go with it either, but it promises to give the gear heads whiplash and it does ride like a dream. No, this is not trick photography.
It gives me, and you, an idea of the kind possibilities there are when you are looking for used parts. I would still recommend going with the 110 (preferred) or 130 BCD with the 74 (see post 960) granny for chainline position, gear tooth flexibility and wider availability, but who knows what you might have to do in a pinch?
The Sheldon Brown "Touring Triple Crank" section lists two Sugino Cranksets for less than a $100 twhich I'm guess would be about for the DownTube VIII. It also provides a benchmark when shopping for used parts.
I also disassembled the stock crank/chainwheel and in can be seen in the picture below.
Click on the pic to see full size.
According to Sheldon Brown, it is possible to get a 130 BCD 5 bolt 38 tooth chainring ($20)that would get closer to a reasonable gear but merely replacing the chainring would not solve the chainline issue,o'wtf,how about washers and longer bolts ???.
Like I said before, "Change the cranks. You will be glad you did." And don't forget the new sealed bottom bracket!
by WrencherWOAC
Labels: chainrings, cranks, VIII H SA8
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
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.
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.
Labels: chainrings, cranks, VIII H SA8
