I think making the chainstay longer will actually put more of your weight on the front wheel.

---

"Are you sure we're going fast enough?" - Emil Zatopek

I have a 90's vintage KHS bike that has a curved seat post and correspondingly short chainstays. It is really fast and handles very well with a forward seat position and aero bars. I think the short chainstays really help the handling. Inspired by the retro thread, I'm planning on bringing it out of retirement for some races this year.

Thanks. I thought about it, and I can see how this might be so. My thought is that with a longer wheelbase, it will be a more stable ride (if you have ridden a tandem, you know what I mean), and move the rear wheel back farther so that it is on the end of a longer lever arm, thus having the weight of the disk wheel that much farther out to balance the front end. Thank you for your input. Any is appreciated.

Stephen J

---

I believe my local reality has been violated.
____________________________________________
Happiness = Results / (Expectations)^2

No probs - it's counter intuitive but I think correct. If stability is a key consideration I'd recommend searching for an old article Dan did about building a custom bike where he went into a lot of detail about the interaction of front center, rake and trail and all tha good stuff. It was on the old slowtwitch (the yellow one) but I'm not sure if it got migrated. If it's worth it to you and you can't find it it may be worth PMing him

---

"Are you sure we're going fast enough?" - Emil Zatopek

Hi,

Thanks again for the input. I was thinking about it some more; this time with some paper....Im not sure that the physics agrees with this. It would only change the weight distribution if you cross a critical geometry boundry which should occur at the pivot points (the wheel axles). When you go onto the aerobars, you do cross this threshold on many bikes (some more than others, and road bikes with shorter top tubes, more than tri bikes). The only other times when this would change would be when either ascending or descending which could again shift your weight relative to these 'pivot points'. Having a longer chainstay would not cause a higher distribution of weight to the front wheel in any of these scenarios; but again, this is armchair physics; and I did not pull out any equations or proofs. Really what I am concerned about is if lengthening the chainstays increases significantly the chance of getting a harmonic oscillation at a working speed of the bicycle, and if it will affect the structural integrity of the frame. My gut feeling would be 'no' for both of these (more so for the latter), but as a molecular biologist, I just dont have the experience to say definitively one way or the other. Thanks for the suggestion of a PM to Dan. I may just do that. Thanks again for your input, and if you think Im totally off my rocker with this reasoning, I would be very interested to hear why. Thanks!

Stephen J

---

I believe my local reality has been violated.
____________________________________________
Happiness = Results / (Expectations)^2

Most tri frames have a shorter chain stay than a road bike. All you are suggesting doing is going with a chainstay length that is similar to most road bikes. No problem there.

But if you are going to have a custom frame made, you owe it to yourself to understand all of the interplays between all the various measurements and angles...or at least be 100% sure your frame maker does and listen to his advice.

Longer wheelbase will mean more stable.

If you unshroud the rear wheel you add drag.

Stability may best be found through head tube angle and fork rake rather than chain stay length?

what jyeager said. if you're starting with the idea of, say, 38.5cm worth of chainstay (a typical tri bike length) and you want to go to 39.5cm, you aren't going to have any structural problems. your road race bike probably has 41.5cm.

keep this on mind, you wheelbase whore.

the wheelbase is closely related to two constituent quantities: chainstay and front/center. tri bikes already have a lot of front center. so, if you add too much chainstay, you end up with a LOT of wheelbase.

for me, (i'm 6'2") i find that about 105cm of wheelbase is the limit. more than that, i feel like i'm riding a longboard. why does this matter?

there are three basic properties that keep a bike upright: trail; precession; and your ability to adjust your weight along the coronal plane. while "trail" (the "castor effect") gets a lot of the credit for keeping the bike upright, probably the constant rebalancing through the shifting of your weight is the most important input.

but that rebalancing is not executed by moving your body over the bike, rather by moving the bike under your body. to that end, the longer you make the wheelbase (and the longer you make the trail) the more laborious it is to move the bike under you. so, i think there's an upper limit to this.

so be careful not to make your custom bike too long.

one last thing about chainstay lengths: these days this is usually not an elective spec. the more hip a company is, the more likely it's going to offer a faired rear wheel, and this chainstay length must be fixed in order to do that.

if you want to play around with some numbers, go to our custom bike geometry calculator.

---

Dan Empfield
aka Slowman

Under normal circumstances I would agree with all the points you raise, with the additional point of saying that the short chainstays are an artifact of tucking the rear wheel. One caviate is that we are talking about a Titanflex which has a fixed size for the fuselage. I am already getting the longest front-center that I can with this bicycle. In addition, there are limitations of what kinds of fork you can get which are aero, and 1" diameter, and have the correct rake. If you make corrections for this by changing the head tube angle to match a 40mm rake then you are shortening the front-center measurement, and putting more weight forward (albeit by just a smidge). My issues are not with the front of the bike; I already have all the parts settled for this; I am asking about the back end which is also limited by the fuselage. I am also doing something a little unique with the rear brake which requires a little bit more clearance than normal, but this clearance can compensated by fabricating a 'reshroud' of rear wheel...my real concern is not stability per se; but durability issues that arise from longer chainstays. With this said, I have done the looking around at geometries, and bicycle part lengths, and most touring bikes have chainstays that are longer than 44cm; but I would imagine that they are also beefier, but I dont have volumes of personal experience with this area. Anyway, I do appreciate your input, and as I have said previously, any input adds to the data set from which I can draw. Thanks!

Stephen J

---

I believe my local reality has been violated.
____________________________________________
Happiness = Results / (Expectations)^2

Aha! titanflex. That makes things much more clear.

They still have a triangulated rear end (ie. chainstays + seat stays....or what passes for them). I can't imagine adding 1 cm having any durability issues. The upper tubes ('seat stays') are going to be under compression.

Given everything else you've said...what overall wheelbase would you have with your 1cm increase in the stays? If that number is in the ballpark of a road bike I expect you will be happy.

"most touring bikes have chainstays that are longer than 44cm"

can you point me to these?

also, don't worry about what a change in the offset will do to the front/center. its affects are minimal compared to the affects on trail. adding/subtracting 3mm of trail will have a huge affect on handling, less affect on weight displacement. worry about getting the trail right first.

honestly, i'm concerned about what it is you're doing. you seem to be really under the gun time-wise, and, you've got some very non-standard ideas about this bike's geometry. i smell problems.

---

Dan Empfield
aka Slowman

For stephen:
http://www.cannondale.com/.../1323-9TR1-Touring-1

point taken. i note that the middle of the 5 sizes has a wheelbase of 106cm, and, that's a very long wheelbase, even with a pretty average "tri" front/center of 61cm.

one thing about this touring bike: it seems to be set up for a style of touring opposite of what i'm used to. i think most folks would prefer the cargo's weight to be on the front of the bike rather than the year. touring bikes set up this way have a lot in common with tri, because of that weight on the front. when i tour, it's with an old blackburn lowrider rack and panniers. this weights the front, and, long front/centers, shallow head angles, lots of offset, make that all work nicely with a front-weighted bike.

---

Dan Empfield
aka Slowman

Just a quick compilation that I found with a quick google:

http://home.comcast.net/.../bike-geometries.xls

You know, I always get myself into trouble when I use words like 'most'....I am usually more careful than that. I should have written that many do, and if you look for recommendations on geometry, many suggestions are for longer chainstays.

In terms of the measurements, they are the same as the V700-ST, which matches up very nicely with my current configuration which I sprung for a fit last year with Ian Buchanan (FitWerx)...which I was both happy and disappointed with...happy that I had arrived over the years at darn near the optimal position he came to, but disappointed that I did not get some instant speed.

The only non-standard is the chainstay length...which I was going to push out 1cm. I did re-read your articles regarding fit and geometry, and I think that there is a small mistake regarding the physics of weight distribution...extending the wheel out rearwards will not change the distribution of weight between the front and back wheels, given a static system. Now, when you introduce acceleration into the picture and you get some vectors going, then moving the rear wheel out will only help, as opposed to hurt the balance of the ride, and you have a longer 'fulcrum' to counter weight you while decelerating (negative acceration...not sure if this is the right term, as I have not has physics since college) or descending. If you move your entire body forward or behind the pivot points (in the case of a bicycle, I was making the assumption...however poorly... ) that these pivot points (for drawing force vectors) are vertical intersections to the ground drawn from the wheel axles. The still can not see other downsides besides a more stable ride and possible decrease in strength (due to there being more leverage on the chainstays, and therefore a greater force on smaller area of the bottom of the chainstay connection to the bb). Actually now that I think more about it, wouldnt a shorter chainstay cause a higher force on the bb/chainstay interface, as you dont have as much length to bleed off some energy in bending of the chainstay...which of course translates into reduced drivetrain efficiency. On the upside, I will get a better chainline, a more stable ride, and placement of the brake where I want it.

Then again, my reasoning may be total crap, and I may be totally off my rocker; which is why Im looking for someone to tell me something that makes sense. I totally respect your opinion, and both my wife and I have owned superforms in the past (one was I was able to pick up super cheap in the 'back room', and another I rescued from a dumpster...poor thing). This will be the first frame that has cost me more than $300, and it is sort of a big deal to me; otherwise I would never post this stuff on an open forum. Thank you again for your input! Knowledge is power.

Stephen J

---

I believe my local reality has been violated.
____________________________________________
Happiness = Results / (Expectations)^2

let me make it simple. stick to geometries that are proven. if you bake a cake and you just start throwing stuff into the mixing bowl that you've never tried, and that no good cook countenances, it's a crap shoot whether it's going to taste any good. now, imagine that's a $3000 cake. just how much experimenting are you willing to do? as long as it's your $3000, i'll be an interested spectator of the process.

---

Dan Empfield
aka Slowman

That is an outstanding point. Of course with this point, I have personal experience with butting up against accecpted dogma, getting beat up professionally; only to be proven correct years later by having my work stolen from me by significant figures in my field. I have a really bad habit of being lucky, and right at the wrong time. I think that as long as it looks nice, Ill be happy with it. I guess Ill do some more thinking on it. Thanks!!!!

Stephen J

---

I believe my local reality has been violated.
____________________________________________
Happiness = Results / (Expectations)^2

Weight distribution is about the relationship of your support points to your centre of mass. Support points being the axles. COM can be calculated from a measurement of WD (scales under the wheels). For instance my COM in my road position is 34.1mm ahead of the BB. Having a location for the COM allows us to then work out the WD on other frames (because the COM is dominated by body position). If you calculate the moments for FC and RC (with COM as the pivot) you will discover that shorter chainstays reduce the front WD.

Here is some waffle with a chart with calculated WD comparisons.
http://forum.slowtwitch.com/...post=2677656#2677656

"scales under the wheels"

i think one scale is enough. and easier. i *think* a bike suspended by the rear axle, in the trainer, ought to suffice. a scale under the front wheel, bike level, assume the position, read the scale. the remainder of the weight (you standing on the scale holding your bike, minus the weight under the front wheel of the bike) is the weight on the rear wheel. calculate. rinse. repeat.

smarter minds than mine must be employed to determine whether this is enough to calculate weight displacement.

---

Dan Empfield
aka Slowman

One scale is sufficient, I just said scales to indicate "some method of weight measurement." I do it with a block the same height as the scale and swap from front to rear. Then measure total weight to verify that I haven't done something horribly wrong (the numbers never quite match up - just have to make sure they're fairly close). Calculating COM is easy from there.

Center of mass will not relate to what the scales will read if you have two scales. It is the same as if you have a weight sitting on a table, and four scales under each leg. No matter where on that table you place the weight, you will get the same reading on all four scales. At first I thought that I had it totally wrong, thinking about it like a car which has a heavier front end than a back end (an example brought up by a Ph.D. in physics; not that it makes him an expert, but does lend a little credence), but this example does not hold, as the pivot point is the front and rear wheels, and the engine hangs over the front wheels, creating more force over the 'tipping point' or fulcrum. Another example which was offered was lifting a table with the weight more at one end than the other; but as long as the table is kept level all four scales will read the same. It is only when one person does not lift his/her share that the weight 'increases' for that person. Therefore, as long as the amount of weight crossing the pivot points/fulcrums does not change, the amount of weight on each wheel will not change, until you get to the point you make the chainstays so short that they are underneath the rider; at which point they will get a greater contribution on the rear wheel (of the rider weight). Now, this changes when you start to place vectors on things such as acceleration. This can cause the amount of force under the wheels to shift; as will going up or downhill. The negatively impacted part will be the front end though, as shortening the chainstays will only help while going uphill or accelerating to a higher speed; it is the downhills and braking that you would be concerned about more weight being shifted to the front end; which will not be impacted by lengthening the chainstays.

Just a gut check, is the reason you think a 44cm chainstay would be bad due to history and tradition; or is there really something obvious and concrete that I am just missing...which has happened before, and Im sure will happen again. Thanks!!!

Stephen J

---

I believe my local reality has been violated.
____________________________________________
Happiness = Results / (Expectations)^2

id really suggest you call dave greenfield at elite bicycles.

Ive spoken to him before. Really nice guy from what I remember...also very tall. Problem is that he does not make a beam bike.

Stephen J

---

I believe my local reality has been violated.
____________________________________________
Happiness = Results / (Expectations)^2

Yea, I think that would work. I also think that if you then extend the point that you are holding the rear of the bicycle in the rear back 1cm that the weight on the front vs. the rear will not change. In theory it should be sufficient to tell; although I can not claim that I have any smarter mind.

Stephen J

---

I believe my local reality has been violated.
____________________________________________
Happiness = Results / (Expectations)^2

If you think you will get the same reading on both scales (or under the legs of an unevenly weighted table) you really should do the experiment.

Your weight is oriented to the back (note that bikes have a seat tube *angle*) so the car illustration is comparable.

If you build with long CS (but not correspondingly long FC) you will have a more frontward WD on the flat. The amount of change onto the front wheel you get going downhill will be the same regardless of CS length but if you start at 46% and add 4 you are still a lot higher than starting at 42%.

My former SS roadie was a 64cm Reynolds 531 (long point lugs with Zeus tips) 27" wheeled touring geo frame. Long wheel base and long CS - good for big tyres and mudguards. Not good for sprinting or tight handling. I've also ridden (with the same position) a road bike with a longish RC and very short FC (toe overlap was a new experience given I'm 6'4") - much more forward and not the handling I'd want for a long steady state ride.

In the case of a tri bike my main objection is around aerobar stability. I've had several bikes where I set my position based on the point at which I felt handling/stability in the aerobars degraded by going further forward. Obviously if you don't accept that longer RC means more forward WD this isn't a problem for you.

I also don't ascribe to the more is better approach to WB and stability - you still want to turn the bike and you don't want the back end to feel like a sponge (which I'd expect to be a risk for Beam bikes anyway - note that I've not ridden one to find out). And don't worry - I'm not wedded to tradition - much of what is done in bike design is quite irritating to me because of the encumberance of history and unimaginative contemporary thinking.

"I also don't ascribe to the more is better approach to WB and stability"

two "stable" paradigms, and i've ridden both:

1. really big trail: the way you steer is, you correct for veering offline by veering even MORE offline, to get your CG between the tire contact patch and the line you wish to re-acquire. when a bike has a lot of trail (more than 64mm, let us say), it's very hard to keep getting your CG between your tire contact patch and the line you endeavor to ride. you're trying to ride a straight line, but you end up riding large amplitude drunken-sailor waves down the road.

2. long WB: the big problem here is when you're out of the saddle. the turns you try to execute (esp with a long CS) are only executed at the front wheel. imagine trying to slalom ski if your skis were twice as long from the boot back to the tail. not a nice bike to ride. and, for me, the break point was 106cm.

a bike that is "too" stable is probably worse than a bike that is nervous. i can sort of get used to the nervous bike, but it's hard getting used to the dog. of course, best is a bike appropriately built.

---

Dan Empfield
aka Slowman