interpreting frame geometry numbers
 

I’m posting this in the Touring forum because I’m looking for a bike with geometry suitable for touring even though it will never carry more than twenty pounds.

Background: I’ve been riding the same Trek 1200 for 26 years. However, since moving to the mountains, I find that the Trek’s handling is a bit more responsive (read “twitchy”) than I would prefer when descending. Anything north of 35 mph seems like a bad idea. I run 28mm Gatorskins at 85 psi.

Following the recommendations of other forum members, I have looked into a couple of other frame sets. Relevant numbers are as follows.

[table="width: 500"]
[tr]
[td][/td]
[td]Trek 1200[/td]
[td]Gunnar Sport[/td]
[td]VO Pass Hunter[/td]
[/tr]
[tr]
[td]head angle[/td]
[td]73.5[/td]
[td]72.5[/td]
[td]73[/td]
[/tr]
[tr]
[td]trail[/td]
[td]53[/td]
[td]61[/td]
[td]56[/td]
[/tr]
[tr]
[td]fork offset[/td]
[td]45[/td]
[td]45[/td]
[td]48[/td]
[/tr]
[tr]
[td]wheelbase[/td]
[td]986[/td]
[td]1006[/td]
[td]1011[/td]
[/tr]
[/table]

Would I be correct in concluding that all three bikes will have a similar feel? How much difference does one degree of head tube angle yield?

Thanks.

Bob

do you understand what trail is?

IME, anything less than 71 or greater than 74 for headtube or seattube (assuming about 52cm frame) would ring alarm bells for me. as for rake and trail, etc. i wouldn't give it a second thought. my admittedly hasty and less than thorough experiment, which i have related previously on BF (attaching it 180 degrees from it's intended orientation), demonstrates that just about anything will work.

My 1991ish Trek 1200 was "bonded" aluminum with aluminum lugs, is yours similar? When in doubt and all other angles similar, the longer wheelbase will feel more stable, at least that makes intuitive sense to me and has been my subjective experience.

only a 8mm range 61-53.

If I were to draw an imaginary line through the head tube and extend it to the ground, "trail" would be the distance between the end of that line and the tire's point of contact with the ground. Trail tends to maintain a wheel's direction of travel (not a good explanation) and can best be seen in the behavior of a grocery cart's front wheels.

If a fork had no offset and the head angle was 90 degrees, I would expect trail to be zero and the bike to be a hand-full.

Please correct me if I am mistaken.

Most of what you feel as quick or slow steering is influenced by trail, not offset.
Below 55 is very quick, hence why your Trek feels twitchy. The Gunnar and VO are both on the the more stable side. Splitting the difference between the wheel base and the trail, I'd say they're going to feel similar.

My bike has a trail of 57.5, HA of 73 and wheelbase of 1005. It is very stable at speed, but still not a dog in the corners.

That's exactly what mine has.

Then slow down! Seems like you should be asking about brake pads instead.

Look at sports tourer with front disc brakes. You could probably fit a Surly or Salsa disc fork onto the Trek 1200, add an Avid BB7 + cable/housing + LBS install for $300.

I know if had been riding a 89 Trek 1200 it would have been pretty worn out and technologically obsoleted by 1994.

I'll toss a very strange personal experience into the scenario. I have been riding a Specialized Allez Comp for the past 3.5 years and have over 60,000 miles on it. This spring I bought a Specialized Secteur. I loved the idea that I could ride the thing without having both hands on the handlebars. It was a 2010 model year that never got sold so I ended up getting for $550...another MAJOR reason why I ended up buying it. I went on the bike trip this summer and had some nice long stretches riding without hands on the handlebars at all, sometimes just to try to stay awake. I got back home and had to drop the Secteur off at the bike shop, it's still there waiting to see what Specialized is going to do about the rack insert problem, may end up with a new bike out of it. I had to go back to the Allez Comp. The problem was simply I could never ride the Allez Comp without both hands on the handlebars If I wasn't doing at 18-20 mph I would head straight for the ditch everytime I tried to take both hands off the handlebars. Wildly, now I can ride the Allez Comp without the hands on the handlebars even down at 11 mph and keep it going straight without even trying.

The bike never changed, but somehow or another the rider did. I can't explain what happened any other way.

Just something to think about.

I do not think I have ever been over 35 mph on a bike. Have been over 25 several times each week, but when I start getting close to or above 30 mph I start to apply brakes.

Have been over 85 mph on motorcycles, but that does not count here.

@engineerbob I too would suggest you consider a fork swap if you are otherwise happy with your Trek 1200. That said, I see nothing wrong with a 1980s or even 1970s or earlier bike. Mine range from 1976 to 1994 vintage. I also see no reason for a disc brake, nor for any LBS labor on this project.

The plot on the ground is correct, there are Varying Opinions on what The Ideal trail Is ..

there is one group favoring shorter trail when there is a Load to be carried On the front wheel

You can call this the Porteur group , fans of that French Newspaper Publisher to Seller's Kiosk delivery Bikes .

If I understand you, I could possibly get the handling characteristics I want from a fork with more offset? Interesting.

Regarding an '80s or '70s bike, I'm going through a 1983 Univega that I recently bought off of C/L. If it works out as I hope it might, then the search is over and I will have saved a boat load of money.

http://i605.photobucket.com/albums/t...psxmogdntm.jpg

Bob

Either of those two steel frames will be much more comfortable than your 1200 as to absorbing noise and reducing the harshness of an irregular road . Aluminum frame design has come a very long way since those early days of Trek's foray into glued frames. You really deserve some reward for hanging with that bike for so long, and I say that because of the countless bikes I've owned in my lifetime, that frame had to have been one of the worst and what gave aluminum such a bad reputation (along with the Cannondales and their over sized down tubes) back in the late 80's and early 90's. Bonding frames never caught on and it wasn't a matter of the lugs coming loose or failing as much as it was the poor ride characteristics. Shortly thereafter, Trek went to TIG welding like everyone else.

Hence the 28mm tires at 85 psi.

Hi Bob;

Yes, you can get the handling characteristics you want with a fork swap - the tricky thing is to find the right fork...

Univega - NICE. Get some Koolstop Salmon brake pads (they do make a huge difference). Looks like it has room for much larger than 28mm tires if you desired. And it has a Suntour RD :) The old Suntour (before bankruptcy, etc) had top notch engineering; too bad they had no understanding of business.

That Univega - if you are new to steel frames, grease the seatpost. It can reduce the amount of rainwater that gets into the frame and reduce the chance of the steel frame bonding to the seatpost from dissimilar metal corrosion.

It has 27" wheels. I just ordered a set of 27 x 1 1/4 tires.

ETRTO 32-630 :)

The Kenda has some 37 or 38 630s if you want wider. There are also 28-630 (27 x 1 1/8) and 25-630 (27 x 1) if you want narrower. On pavement, I like have narrower in the front.

Seat post greasing - definitely, doesn't matter what kind of metals are involved - I use boat trailer wheel bearing grease because it is inexpensive and highly water resistant.

The biking book "Just Ride" gives some numbers which I have personally embraced, but it's dependent on whether you consider yourself a racer or an "unracer".

Grant Petersen has some very specific ideas about bike riding which are in line with my preferences. Here's what he has to say in a nutshell:

Page 159: Frame Arithmetic

Seat Tube angle: Most are 71-75 - He designs all this frames with a shallow seat tube of 71-72.5

Head Tube Angle: For road riding, he likes 71.5-73 degrees... For touring and mountain, 71-72 degrees.

Trail: Whatever feels good to you. He's a "normal trailer" but appreciates that riders who are convinced that low trail bikes handle better under certain circumstances.

BB Drop: He like low BB heights and therefor lots of drop.

Chain stay length: Extra length helps with panniers and adds stability on rough ground and adds comfort at high speeds, including touring bikes. Longer also has more suspension characteristics.

300 for a fork change, and related gear, is starting to climb into the saddle for new frame prices. I saw some good touring frames being sold off for 275 not long ago. Admitedly that just throws open the money pit since probably not much of your current gear is worth porting over, so you have a ways to go to get back up, but with sensible choices or a nearly free donor bike it might not be all that expensive.

Though there is mention here of 55 on downhills, that is relatively crazy, though nothing wrong with crazy on your own dime. 25 is probably where I start to ratchet back, but then the places where I would hit that are probably rough compared to a long run out in a mountain, on a tourist route.

One has 5 times the energy at 55 vs 25, and even at 35 twice the energy. So pretty much everything we know (not much) about the design envelope of everything from brakes to helmets is out the window.

THIRTY-FIVE, not fifty-five. Although I have passed 40mph on the Trek and on our tandem, that's a neighborhood I prefer to avoid.

Thanks. It appears to have been recently greased. The grease was much fresher than that which I found in the hubs.

Bob

I hit 35MPH every day, and often push to 40MPH. I topped out at 43 the other day. I have big hills here so I use them to my advantage. My bike is stable. It is a Surly LHT. Many bikes get more stable at speed. Larger tires help in some ways, acting as a gyroscope.