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interpreting frame geometry numbers

Old 10-22-15, 04:36 PM
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I ran the numbers assuming the same tires on all three bikes. Adusting my nuimbers to standardize the results to what Gunnar gives, I see:

Gunnar's trail - 61
Both Trek' and VO's 55

So, using the same tires for comparison, the Gunnar will be the most stable and the other two very close to each other. The VO will have a little more wheel flop. The bikes aren't very different. Here, wheelbase is a lot more different than the front end geometries. I"d look at that and balance between the wheels if I were l,inintingmy choices to these three bikes. (But at 4" difference in wheelbases, you are kinda comparing comparing a compact with a luxury car and saying they are not that different; they have the same number of turns of the steering wheel, lock-to-lock.)

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Old 10-22-15, 04:53 PM
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For sure the Univega, gorgeous. Hopefully it has a 130+ DO and a wheelbase 1060+. 2 years ago a guy rode the same bike SS around middle California. The OP bikes are uselessly short. That frame is IGH ready also, anytime. I would get a PHIL BB for slick power.

73o angles is rather goofy for a tour bike anyway.

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Old 10-22-15, 05:17 PM
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Originally Posted by 79pmooney
(But at 4" difference in wheelbases, you are kinda comparing comparing a compact with a luxury car and saying they are not that different; they have the same number of turns of the steering wheel, lock-to-lock.)
1011 - 986 = 25mm or approximately one inch.
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Old 10-22-15, 06:27 PM
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I'm not clear on what aspect of the bike's handling you are unhappy with. Usually people are referring to front-end geometry when talking about a bike's "handling" but a lot of other factors influence it as well.

Your bike's shorter wheelbase might be contributing to an overall twitchy feeling or it might be the lower trail number (or both). If you want to make the front-end handling less responsive, a fork with more offset would take you in the opposite direction and make your front-end even twitchier. A new fork with less offset would take you in the right direction but it would also shorten the wheelbase (subtracting from stability) and increase the chance of toe overlap.

My preference is for a bike with wider tires, a 71.5 degree head angle and a fork with rake in the low 40mm range. This gives over 70mm of trail and feels very stable when riding. I was riding a bike with 60mm of trail and did not like it at all. Very twitchy feeling.

Here is the calculator I use to figure out the impact of head angle, fork offset/rake, wheel size and tire size. My experience is that a few mm of trail doesn't make a difference.

Bicycle Trail Calculator | yojimg.net
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Old 10-22-15, 07:59 PM
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Bob, Here are a couple of web pages to at least raise more questions.

Understanding Bicycle Frame Geometry | CyclingAbout
Bicycle Trail Calculator | yojimg.net

Brad
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Old 10-22-15, 11:58 PM
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Originally Posted by engineerbob
1011 - 986 = 25mm or approximately one inch.
I mis-copied the Gunnar's wheelbase as 1086, not 1006, a three inch mistake.

Ben
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Old 10-23-15, 03:43 AM
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I am pretty sceptical of frame charts. Same of the ones I have looked at just aren't possible, like they may have the same wheel base for every size of frame. I wouldn't trust anything I hadn't measured myself. There seems to be pressure on some of the numbers, people come with expectations, and the charts reflect what clients will want, even if they are impossible.

A lot of the bikes seem to have 45mm offset forks, and 73 degree head tubes, or claim they do. Inability to ride hands free may have to do with twist in the frame.
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Old 10-23-15, 04:19 AM
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Originally Posted by engineerbob
It has 27" wheels. I just ordered a set of 27 x 1 1/4 tires.
Some have converted older 27 inch frames to 700c wheels by putting on longer reach sidepull brakes. Not sure if that is practical modification with cantilevers, but if it was me I would try to figure out if that is a viable option. The wider variety of 700c tires is a pretty strong justification. Used wheel sets with 126mm dropout spacing can be had for a pretty good price too, last winter I got a very nice set of wheels at a swap meet for a very good price.
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Old 10-23-15, 04:21 AM
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Originally Posted by engineerbob
Thanks. It appears to have been recently greased. The grease was much fresher than that which I found in the hubs.

Bob
If your bottom bracket is cup and cone, check the grease in that too.
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Old 10-23-15, 09:44 AM
  #35  
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Don't know if it's just me, but I find talk of what quantifies "twitchy handling" to be very much determined by the individual and what they personally find twitchy or stable. Have seen first hand when switching bikes with friends that some people are just not comfortable with a given bikes handling characteristics and can find it nervous, where it perfectly fine for someone else.
Perception comes into it I'd say, just as being comfortable at speed, as shown in some of the answers here concerning what different individuals feel as a comfort level at a given speed.
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Old 10-23-15, 12:57 PM
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Originally Posted by bikenh
I have been riding a Specialized Allez Comp for the past 3.5 years and have over 60,000 miles on it.
Impressive- this is an average of 47 miles per day every single day for 3.5 years.
An average of 3.5-4 hours of riding every day for over 1200 days- that's seriously getting after it.
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Old 10-23-15, 01:12 PM
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Originally Posted by djb
Perception comes into it I'd say, just as being comfortable at speed, as shown in some of the answers here concerning what different individuals feel as a comfort level at a given speed.
That's right, but you can have norms, the industry designs around some concept of what the public wants, even if they have different ideas about that. I also think different riders, as you say, will experience things differently possibly because of different weight, or objective factors as well as subjective factors. I think there are objective as well as subjective issues, or one couldn't design anything.
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Old 10-23-15, 01:27 PM
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Originally Posted by mstateglfr
Impressive- this is an average of 47 miles per day every single day for 3.5 years.
An average of 3.5-4 hours of riding every day for over 1200 days- that's seriously getting after it.
Give up driving, stop owning a car and live 30 miles roundtrip from the nearest decent grocery store and you'll quickly start to ride the miles. Add in 17,309 miles of open road bike tripping/touring in the past 4 years and it adds up quite quickly. By the end of the bike trip this summer I had a 365 day total of 23,459 miles. It's back down to just under 22,500 miles right now. Hoping to have 22,000 miles for this year alone.

As for speed. I can routinely hit 40 around my house and do it all the time. On the multi-speed to fixed gear convert I love hitting 40. I always know when I hit 40 mph, it feels different. Yeah, I'm riding 52x17 so I can still climb the hills but I can also descend down them without having to put on the brakes. I've been over 50 once and almost hit 50 this summer down in CT on the bike trip. I was shocked to find something faster on a state/federal highway than what I have here in NH. I have one spot 6-7 miles from my house that used to be the fastest descent in the NE 1/4 of the United States at 48.7 mph. That changed this summer. The biggest shock this summer was hitting 35.6 mph just east of Tallahassee, FL. I figured if I were to get down around Claremont, FL I might be able to get going that fast but anywhere else in FL, not a chance. Boy, was I wrong.
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Old 10-23-15, 04:07 PM
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Originally Posted by MassiveD
That's right, but you can have norms, the industry designs around some concept of what the public wants, even if they have different ideas about that. I also think different riders, as you say, will experience things differently possibly because of different weight, or objective factors as well as subjective factors. I think there are objective as well as subjective issues, or one couldn't design anything.
absolutely, it's like gearing, having a given gear inches number gives a black and white number to a low gear for example. What I find interesting is that of my bikes that I ride during the season, a 26in mtn bike, a 97 or 98 Rockhopper comp, and a cross/touringish 700 dropbar bike, the mtn bike steers very very quickly, something I like with city commuting and avoiding potholes etc, it steers really really fast and precise. Compared to the Tricross it is much quicker steering, and you really notice it over about 50kph, the rockhopper still steers very quickly at speed--but the interesting thing is that it is still stable and for me, not worrying or giving any indication of concern at whatever speed I can get it to.
I compare this to a friend of mine who has a Trek 520, and he finds the Tricross way to twitchy for him.
So its interesting to me that even with a much faster steering bike like the Rockhopper, it still remains stable and confident at 70kph, or about 45mph , thats about the fastest Ive gotten it to, so it does seem that numbers can indicate that a bike will steer quicker than another, but it may not clearly how the bike feels when it does get going fast, or with a load on it and going fast.
This is one of the reasons why I am so fond of this old Rockhopper, it works well unloaded and it takes a good rear end load really really well also, stays very composed and like I said, while still fast steering, it doesnt display any concerning handling issues.

It would be interesting for me to try to find the specs of these bikes, I really dont have a good understanding of trail and all that, and certainly havent retained the numbers for my bikes, even if I had seen them at one point. Would be interesting to compare to other bikes I have had and ridden, and to other bikes mentioned here.
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Old 10-24-15, 04:39 AM
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Originally Posted by MassiveD
I am pretty sceptical of frame charts. Same of the ones I have looked at just aren't possible, like they may have the same wheel base for every size of frame. I wouldn't trust anything I hadn't measured myself. There seems to be pressure on some of the numbers, people come with expectations, and the charts reflect what clients will want, even if they are impossible.

.
Ditto, unless there are other factors than trail affecting handling. I've had a 56cm 700c LHT, presently have a 26" wheel LHT and a 56cm CrossCheck. According to the trail calculator they all have nearly the same trail but the 700c LHT is out in left field wrt wheel flop and lack of responsiveness at low speeds compared to the other two.
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Old 10-24-15, 07:33 AM
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Originally Posted by LeeG
Ditto, unless there are other factors than trail affecting handling. I've had a 56cm 700c LHT, presently have a 26" wheel LHT and a 56cm CrossCheck. According to the trail calculator they all have nearly the same trail but the 700c LHT is out in left field wrt wheel flop and lack of responsiveness at low speeds compared to the other two.
Everything, not just the trail measurement affects handling. I looked up the figures for four of my bikes* and the touring bike has the least trail of the four. As expected, the touring bike has the slackest HT angle and the longest wheelbase and is the most stable in a straight line.

I can write up the numbers if anyone wishes.

Brad

* All four are Cannondales, three are roadies.
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Old 10-25-15, 08:49 AM
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Originally Posted by corwin1968
If you want to make the front-end handling less responsive, a fork with more offset would take you in the opposite direction and make your front-end even twitchier.
Yeah, you're right.

Wheelbase, head angle and trail seem to be the critical dimensions. And, admittedly, it's subjective. My 32-year-old Univega is looking better all the time.

Bob
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Old 10-29-15, 04:02 PM
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I finally completed the clean/lube/adjust everything process, and put the Univega on the road. I would say that this bike has the handling that I have wanted and for much less money than I might have spent. Unfortunately, I don't think that I can accurately measure the frame, certainly not the head angle.

Bob
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Old 10-30-15, 11:20 AM
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Hardware- Builders supply have angle Finders . You're the "engineer" You should already know this or how to plot an angle..
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Old 10-30-15, 11:51 AM
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Originally Posted by engineerbob
I finally completed the clean/lube/adjust everything process, and put the Univega on the road. I would say that this bike has the handling that I have wanted and for much less money than I might have spent. Unfortunately, I don't think that I can accurately measure the frame, certainly not the head angle.

Bob
Sounds like problem solved! It's one of my biking dreams to find a cool, high quality older steel bike that has that perfect ride for me. I'm sure plenty of modern bikes can meet my ride quality criteria but they have no soul. I want to find something older, with a little class, that I can customize to be my own. Your Univega sounds like it fits the bill for you.
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Old 10-30-15, 01:56 PM
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Originally Posted by corwin1968
I want to find something older, with a little class, that I can customize to be my own.
My closest friend that I used to tour with had over a dozen high end bikes but he loved his old damn Giant Iguana steel mountain bike for touring as he claimed it was the most comfortable bike that he owned for spending long hours in the saddle.

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