Baldone

I'll take it.

Any chance to get the frame while the fork was off? I guess that means pulling the cranks, I haven't done that yet but probably will.

620g seems reasonable.

I like the non suspended option on these, I think being light weight is important for a folding bike, one you may often need to haul around on your back, make airline shipping weight restrictions with other items etc...

SesameCrunch

No, there would have been a lot more to take off before I get a clean frame to weigh.

The NS is a lot lighter. I don't know what kind of riding you do, but if you do fast sporty rides, the problem with the NS is that the wheelbase is about 3" shorter than the non-suspended bike, so it's twitchier when going fast or downhills. It is good if you're going to use it with a camper or plane or boat. Or if you have to carry it a lot.

For just commuting or riding around town, I personally would go with the IX, or even better, the VIIIH. Comfortable and better looking, I think. The additional 5 lbs won't be a big deal in casual riding.

SesameCrunch

Yan:

Look at what I did to your nice beautiful bike design .

It's a kit from Cruzbike.com. Initially, I had put it on my mountain bike, but it turned out at 37 lbs and felt every ounce of it when I was riding. So, I decided to convert this IX, put the NS fork on it and added the Cruzbike kit.

It's pretty light, I would guess 27, 28 lbs. Blast to ride. I'm still tweaking the riding position, but I am so glad I did it. This will be the bike I ride in my double century this year!

I know, sacrilege to mess with a nice bike, isn't it?


It doesn't fold quite right because the seat pan is obstructed by the hinge. I'll have to work on that...

bookishboy

That looks like a blast to ride!

Do you have any plans for being able to still fold it?

Can that kit be made to work with under-seat-steering?

I'd like to see an in-depth writeup on how it performs, when you've had more pedal-time with it.

SesameCrunch

Yeah, it's pretty light and nimble. The small wheels aren't as uncomfortable as I was afraid of.

I will fix the fold eventually, it will require a custom bracket, but not that difficult.

I don't know anything about how USS linkages work, so I can't answer your question. Sorry.

I will indeed give a full writeup when I get a few long rides under my belt. I want to be able to climb hills with this bike, so that's my main concern.

BigMacFU

Now that's the coolest DT hack there is!

downtube

Very cool mod....I love it. Please share your thoughts on the ride.

Thanks,
Yan

WrencherWOAC

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

The Sheldon Brown Internal Gear Calculator (https://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.

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.
In other words your can ride a regular sized bike and get the feel of what a 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 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

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 tooth and that’s probably okay 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 (chainring and rear sprocket are in the same plane and the chain runs straight like it should to reduce friction and wear) that looks and performs like it should. Your 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 ??? How do you get what you want?

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. 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 is the key. It provides the correct chainline and it is available in any size from 24 to 36 tooth, check the 74mm BCD page from Harris (https://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.

WrencherWOAC

More like an addendum

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. 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 it isn’t 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 wide availability.

bookishboy

Thanks for the reply, Yan.

WrencherWOAC

Don't get me wrong about this DT bike forum. It is great but for me it is inconvenient, ntm bassackwards & unhandy linking to specific posts, so I have decided to start putting my full length posts on my blog MavandJen_Europade. I will make announcements of posts here, and I invite others to comment. btw, with permission, I would like to list some of the people on this forum that I find authoritative and useful.

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.


For more on DownTube VIIIH SA8 visit [URL="https://mavandjen.blogspot.com/"]MavandJen_Europade[/URL

WrencherWOAC

Yan

[QUOTE=Dahon's cheapest model in 2007 is a Speed 7 for $379. All our 20" bikes are priced between $299 and $399 and are much higher grade than the Speed 7. Our retail prices have stayed steady for over a year....I doubt we will ever approach the $600+ average selling price for a Dahon. Hence I think we are a value priced folder and sales are confirming my theory ( sales are up 300% over last year )
.[/QUOTE]


There seems to be a bunch of misunderstanding about what is important for making choices about buying a DownTube product....

How about coming up with a scoring sheet that considers preferences, etc. to guide the potential buyer? I mean not everybody is shopping by price or, or ... alone.

I believe it would be an excellent marketing tool for online/mail order buyers, and it would help the buyer self-select and ultimately be more satisfied. There is also nothing wrong with a Q: like "how much do you expect to spend after receiving to stock bicycle? Perhaps with a what the improvement does and approximate cost.

What do you think?

People who are trying to reduce weight on the DT or any bicycle for that matter should consider this.

When an ounce is not an ounce

ahorner1946

Wrencher WOAC: I enjoyed your post here and also on your Blog regarding the gearing of the 8H. However, there is still one piece of information I have yet to see! What is the BCD of the stock DT 8H Chainring, 130mm or 110mm?

Thanks,
Al

Polaris43

110 mm

WrencherWOAC

For some reason this sentence was left out of the post--it is in the first paragraph:

Tuesday, February 06, 2007

Investigation of Downtube VIIIH -----------------------------SA8 Gearing
48 tooth chai
"The stock crankset has a chainnring (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."

WrencherWOAC

The way to determine the BCD is to take off the chainring and measure center-to-center from one bolt arm to the next. See Sheldon Brown ... chainwheels(sic) for details. Take that measurement and multiply by 1.7

The chainrings that were on the DT I have (VIIIH SA8) have a measurement of 76.4 and 1.7 X 76.4 = 130. You may be looking at chainrings from another model. This is a "Prowheel Forged 2N 170" the 170 is the crank length is independent of the BCD.

Just for the record, a 110mm BCD would have a measurement of 64.7mm. It is an easy error to make (all the same digits--just mixed up ). The road double 110s are often called a "compact" that was popular 15 years ago, faded, now coming back. A double won't solve the chainline issue. Read my blog for more.

Caio

jur

I read that blog entry and it is contrary to my knowledge of kinematics. Please prove how rotating mass is more important than plain old mass.

There is no property of a spinning rigid body that will provide some mystery braking effect on horizontal translational movement. You can take an extra spinning bike wheel on your bike and the top speed will just be the same as with a stationary wheel.

There is no property of a spinning rigid body that will provide some mystery extra weight on vertical translational movement. You can take an extra spinning bike wheel on your bike and the amount of work you must do to maintain a certain vertical speed will just be the same as with a stationary wheel.

makeinu

The bike isn't sliding, it is rolling.

If you take two wheels, one with the mass concentrated near the center and one with the mass concentrated near the rim, the wheel with the mass concentrated near the center will be easier to spin. Picture a figure skater doing a spin; As the figure skater pulls in her extremities she spins faster. This is because the mass of her body is becoming more concentrated near the center, making it easier for inertia to spin the body. Therefore, the rotating parts of a bike will be easier to spin if they have a lower "rotating mass". Since the speed of the bike is proportional to the rotation, lowering the rotating mass will increase the speed for the same amount of effort.

However, I wouldn't compare reducing the rotating mass with lowering the weight of the bike, as the result of reducing the rotating mass should be more similar to making the drivetrain more efficient than reducing the weight of the bike. As you pointed out, the total weight of the bike and its affect on translational motion is an entirely seperate consideration. I also don't know why Wrencher brought it up, as it seems people were discussing lowering the weight for carrying purposes, which has nothing to do with rotating mass. Besides, the only way to reduce the rotating mass would be with lighter rims, spokes, and tires.

jur

Actually I am completely aware of the various effects. What I am ranting about is the sloppy imprecise language which confuses those with less knowledge. My point was, whether a wheel is spinning or not, it does NOT affect translational movement in any way whatsoever. You post is also imprecise, but not quite as bad. It is the acceleration which is affected, so a lighter wheel would be easier to accelerate. But the magnitude of that effect is quite overrated. The original post apparantly places wind resistance which affects translational motion on the same par as acceleration. And that is simply wrong.

makeinu

Well, the spinning wheel does affect translational motion because the bicycle (by its very purpose) couples the rotational motion of the wheel to the translational motion of the bike. I know you want to consider the physics of the translational motion by itself, but the translational motion does not exist by itself with a bicycle.

Also, I think it is a little misleading to say that only the acceleration is affected by the rotational mass. In an ideal physical sense this is true, but in an ideal physical sense all the energy put into the pedals will result in acceleration. However, the bike will slow to a halt if the rider stops pedaling. So it is necessary to "reaccelerate" the wheels to compensate for deceleration due to other factors. I'm pretty sure the smaller wheels will increase the efficiency of this process, even though the actual speed of the bike is not increasing, but I'm not that kind of engineer, so perhaps you can clarify.

jur

I dislike at this stage to hijack this thread any more . I suspect most readers are saying, enough already, go away!

barneybarney

cheers yan, thanks again.

mcgurme

Actually, with the standard crank that comes on the bike, you can put the chainring on the inside rather than the outside of the crank arm spider if you want to move the chainline. That would be the cheapest solution.

One one of my FS bikes, I actually put a 38T on the inside, a 52T on the outside (they are 130 BCD on the Prowheel crank), and it works pretty well (but this is on an FS with the external cog in the rear). I can shift down while riding just using my foot to put gentle pressure on the chain. Upshifting is harder, though occasionally it will auto-upshift because of chainline issues, when it is on the smallest cog in back and small ring in front. I was originally going to get a slightly longer BB spindle to prevent this, but I've decided it's not so bad - if I'm on small-small, it usually means I'm going downhill and higher gearing is good. I just wish it was more automatic

For my other bike, I put a shimano 105 double crank on. They're still on sale at Nashbar. Note you'll need a 115 or longer Octalink BB - the standard 109.5 doesn't work well.

bookishboy

It's still an interesting conversation, even if it is heading away from the thread subject. If you two started a new thread and carried on, I'd still read it.

Speedo

This topic (rotating vs. non-rotating mass) was discussed in the Swift thread. The science is described in this post:

https://www.bikeforums.net/showpost.p...&postcount=721

Speedo