Geometry quiz!

[tv_vt]

OK, test has one question. Here goes:

Two bikes, both carbon, 59cm size, same relatively short chainstay length, head tube length, and 43mm fork rake.

Difference is that Bike 1 has the following geometry for the front end - 74* head angle, trail of 5.2, and front center of 58.6cm. Bike 2, though, has a 73* head angle, trail of 5.8cm, and front center of 60.2cm.

The question is: how would bike 1 ride differently than bike 2?

Ready? Go.

(Thx for any info!)

T

[mandasol]

That's interesting. Not that I have a clue how to answer that, but it would seem to me that handling would depend on the dimensions of the person riding? What's perfectly balanced for one person could be crap to another... right, wrong, maybe?

[martinrjensen]

Well, when you are riding the first bike, it's going forward and the other one is not. When you ride the second bike, that one is going forward and the first one is not.
Why don't you tell us (first) how they feel different, since I'm assuming you have them? Any comments here are going to be just theoretical, you that you know right?OK, test has one question. Here goes:

Two bikes, both carbon, 59cm size, same relatively short chainstay length, head tube length, and 43mm fork rake.

Difference is that Bike 1 has the following geometry for the front end - 74* head angle, trail of 5.2, and front center of 58.6cm. Bike 2, though, has a 73* head angle, trail of 5.8cm, and front center of 60.2cm.

The question is: how would bike 1 ride differently than bike 2?

Ready? Go.

(Thx for any info!)

T

[Ozz]

lemme guess....you are looking at two similarly sized bikes with the geometry listed above....they probably are not available for test ride so you would like someone who know this stuff to let you know what to expect the ride to be like for each...

I have no idea as to the answer....good luck.

[JonB]

Its been awhile since I've considered this stuff, but I believe the smaller trail number, steeper head angle and shorter front-center of bike 1 should make it quite a bit twitchier. Possibly good for racing/crits, but don't try and sit up in a crosswind to take off your jacket.

Bike 2 should be more stable at speed due to more trail and more stretched out with that longer front-center.

[bzbvh5]

Its been awhile since I've considered this stuff, but I believe the smaller trail number, steeper head angle and shorter front-center of bike 1 should make it quite a bit twitchier. Possibly good for racing/crits, but don't try and sit up in a crosswind to take off your jacket.

Bike 2 should be more stable at speed due to more trail and more stretched out with that longer front-center.

I agree with JonB.

More on the fun side. I thought you were going to ask, why do the sum of the squares of 2 sides of an isosceles triangle equal the square of the third side? Now that's geometry?

[torquer]

More on the fun side.
Here's a good one from the Times earlier this week:
http://www.nytimes.com/2009/10/20/science/20tier.html?_r=1&scp=1&sq=geometry%20puzzle&st=cse
http://graphics8.nytimes.com/images/2009/10/20/science/1020-sci-TIERNEY.190.jpg
Toothpick Giraffe Puzzle. Five toothpicks form the giraffe shown above. Change the position of just one pick and leave the giraffe in exactly the same form as before. The re-formed animal may alter its orientation or be mirror reversed but must have its pattern unchanged.

[tv_vt]

Well, Ozz is pretty close. I do own one of the two bikes and am intrigued by the other and am just wondering how they might differ in ride, handling, steering response, etc. Seems like there are a few a frame gurus on the forum, so thought I'd throw it out there.

Thanks for any thoughts.

The toothpick Q I'll take a pass on!

T

[Tobias]

The question is: how would bike 1 ride differently than bike 2?
T, ride wise I'd bet other factors have a greater impact. Simply being carbon doesn't mean they are the same unless they are otherwise identical.

Handling wise what others have stated makes sense to me. The 73 degree HTA, longer front center, and greater trail would be my choice. Everything else being equal that bike seems a little more comfortable and easier to ride long distances.

Having said that, if you want a real difference you have to go after real differences. I don't find these enough for me to get excited about. At least not enough to replace a bike unless I was going to do it anyway.

[EDS]

Here's a good one from the Times earlier this week:
http://www.nytimes.com/2009/10/20/science/20tier.html?_r=1&scp=1&sq=geometry%20puzzle&st=cse
http://graphics8.nytimes.com/images/2009/10/20/science/1020-sci-TIERNEY.190.jpg
Toothpick Giraffe Puzzle. Five toothpicks form the giraffe shown above. Change the position of just one pick and leave the giraffe in exactly the same form as before. The re-formed animal may alter its orientation or be mirror reversed but must have its pattern unchanged.

That was an easy one.

[Dave]

The smaller trail and shorter front-center both make for a quicker steering bike.

[Peter P.]

Bike 2 has either a steeper seat angle or a longer top tube, leading to a longer front center. Personally, I'd ignore front center dimensions because they're the result of other dimensions on the bike.

I tend to look at the fit of the bike-seat angle and top tube length-separately from head tube angle and fork rake. While each combined influence the end result ride of the bike, I don't think it's critical enough to sweat it.

I presume you're content with the seat angle and top tube lengths of frames A and B, that's why you didn't list those specs.

Most builders steepen the head angle and lessen the fork rake on taller frames to keep the wheelbase within their design limits. I'm no framebuilder (I only play one on TV) but I can't see why keeping the wheelbase of taller frames close to that of smaller ones is so important. It seems to me that you can construct a fine, tall racing frame regardless of the head angle/fork rake combination. Just choose the handling you want and be done with it.

If you like fast handling bikes without a lot of that "on rails" feeling at high speed, get the 74 degree-er. This is a bike for crit lovers and people with quick reflexes.

The 73/43 combination is more calm and middle of the road. It will feel more stable at higher speeds say, over 30mph. If you don't have the reflexes for the 74 above, the 73 will be the bike you enjoy, especially on longer rides where you sorta just want to point the bike down the road and it just holds a line on its own.

Either bike will be a competent racer, it's just a matter of which flavor of steering you prefer.

[RPS]

Most builders steepen the head angle and lessen the fork rake on taller frames to keep the wheelbase within their design limits. That’s one possibility, but it may also be that that combination yields better handling and that it can’t be scaled down to smaller sizes because the front center would be too short to be practical. I suspect you are right, but can we know for sure without trying it?
I'm no framebuilder (I only play one on TV) but I can't see why keeping the wheelbase of taller frames close to that of smaller ones is so important. It seems to me that you can construct a fine, tall racing frame regardless of the head angle/fork rake combination. Just choose the handling you want and be done with it. I think the idea of proportionality (when applied to larger sizes) sounds easier in theory than it is to implement in practice. That’s not to say it can’t be done, but IMO the end result would look very goofy if based on scaling up the size while using the same size wheels. When scaling down for smaller riders simply using 650C or 26-inch wheels and tires can decrease size by up to 10 percent while keeping things in proportion, but when going up in size the same doesn’t apply because there are no viable wheel and tire sizes in much larger diameters.

Scaling up 10 percent (like comparing typical 5’-10” versus large 6’-5” riders) would probably be most visible in the gap created between the rear wheel and the seat tube due to the very long 45 ~ 46 CM chainstay length that would result from pure dimensional scaling. It may work OK but would look very “old fashioned”, like something from the 1960s. And there is also the issue of longer cranks to make everything proportional. You can specify custom cranks that are 10 percent longer (about 190s), but that would force the bottom bracket to be higher for adequate ground clearance. And since the tire diameter would be about the same, the amount of BBKT drop would need to be less, also making the bike look “different” and out of proportion.

I’m just trying to say that scaling up or down without being able to scale the wheel/tire size would look odd IMHO. I personally don’t find very large bikes elegant any more than very small ones built around 700C wheels but they are probably a good compromise. To me the more common “average sizes” look best.

[Tobias]

If you like fast handling bikes without a lot of that "on rails" feeling at high speed, get the 74 degree-er. This is a bike for crit lovers and people with quick reflexes.
Not sure I accpet the premise that quick steering should go with quick reflexes. The very opposite may be true.

As to geometries specific to tall riders, simple physics dictates that taller riders have a slowing effect on bike steering. For that reason alone (and there may be others), they may not need bikes with proportionally longer wheelbases and similar steering that may be too slow for their size. What works best for a short rider doesn't have to work best for a tall rider because the dynamics are not the same. Just food for thought. ;)

Good subject. Hope this isn't too much of a thread drift.

[salem]

I agree, to a point, with the common consensus of a steeper head angle being a quicker handling bike (for a given fork rake), but I throw a wrench in the works with this one: I can corner faster and harder with a slacker head angle and the corresponding longer front center.

Why? Well, largely my background. I have a long history of riding and racing mountain bike, including some downhill racing. Look at mountain geometry and you'll find slack head angles paired with very little rake to give loads of trail, and therefore a slow handling, stable bike, and yet look at mountain bike trails, and you'll find lower speeds and sharp 90 degree corners on single track, so why the slow stable bike?

Answer: different technique. Steep angles work well with cornering that originates from pushing down and out on the inside handlebar. This is a more intuitive way to make a bike turn and feels most comfortable to most people. With a stable (slack, slow handling) bike, however, the rider can be more aggressive with inputs because the bike won't lose stability under him or her so quickly. The more aggressive corner technique is to fall into the corner with your body's weight, leading from the hips.

Most people are very uncomfortable with this second technique because it truly involves momentarily falling and then bringing the bike back underneath you. In teaching riding clinics, I have to actually trick people into putting themselves into this cornering posture because their sense of balance and equilibrium won't allow themselves to make the initial fall. The end conclusion is, however, a slow, stable bike, allows be to use a more destabilizing corner technique, which allows me to corner faster.

So what does that mean for the original question? The answer depends very much on what the rider wants the bike to do, along the lines of what others have said. For me, bike two would be the choice without hesitation.

[Peter P.]

Salem, your explanation describes what I never could express about WHY I prefer slacker head angles on my road bikes. I much more enjoy my bike that has a 72.5/47mm setup (25mm tires) than my ubiquitous 73/45 with 23mm tires. I'm even tempted to try 72/50-53 with my next road bike.

Thanks for putting in words exactly what I feel when I dive into turns at high speed on both bikes.

[mister]

the higher trail bike is going to be more sensitive to your body if you lean. theres a reason lots of the rando guys like a really low trail geometry. i've read comments about a bike that's real easy to ride and i know what they mean...but they don't accomplish the low trail with a steep head angle and you also gotta keep wheel flop in mind.

i have a bike that has 74* head angle and 45mm rake, that equates to 50mm trail and the handling is interesting. did notice it wanted to try to wobble if i tensed up a bit but it was still manageable and it does have a certain stability to it that my 58mm trail bike doesn't have.