Phamilton

Phamilton

rhm

Ah, don't let the bent forks get you down!

Assuming a fork is bent by a head on crash, the result is to reduce fork offset ("rake") and so increase trail. The same crash is likely to bend the frame, increasing the head tube angle.

All these things can be measured. If you have a frame you like, disregard the likelihood that it may be bent, and measure it as is.

But that's a hypothetical. I'd recommend trying a few frames that are guaranteed NOT bent, see how you like them.

63rickert

It's my experience that pretty much every vintage steel frame is bent. They are fairly large and lightweight tubular structures that are used very hard. By the time they are thirty or forty years old they aren't that straight. If you do something crazy like riding them instead of looking at them they will get progressively less and less straight. Every time I have a frame aligned it rides a great deal better than it did before. Aligned as in on a frame table by a framebuilder. Or you can straighten them to 'good enough', as in good enough that the gears shift, the chain doesn't fly off, and it doesn't pull too hard to the side. Or you can ride them as is and then discuss the fine points of frame geometry.

It's just like tuning a piano. You keep doing it. A good pianist can play an out of tune piano and make great music. For anyone who uses the piano hard the instrument is noticeably further from tuning at the end of the performance than at the beginning. So you call the piano tuner back. Or the instrument slides downhill to junk.

old's'cool

Well, normal riding over any length of time or miles will not change the shape of a good steel (or any other material) frame. Only extraordinarily heavy impacts or other abnormal stresses that exceed the designed yield or fracture strength of the frame will do that.

MNwj48

I know, I am not sure if this thread is even discussing the original posts.

In Minnesota we have pothole season. If you hit a pothole hard enough to notice something right away, then it would have gotten fixed, usually. The lesser hits accumulate, go unnoticed. What happens is more of a slow realization that something is less than ideal. They could ride the bike for years that way. When they have another bike to compare is when they realize that it could or should be better. Most people start adjusting the adjustable things, searching for a solution. The fork will not appear bent, and even trying to measure it may not show it is bent. But, this is one thing that would make it harder to turn.

Another case is when a smaller wheel is put the front (for the weird look I suppose, it wasn't me that tried it). It really messed up the steering response and was nearly un-ride-able. Nearly the same effect but due to tilting the frame forward.

Putting more of the riders weight on the handlebars:
I would expect this to make the bike more difficult to turn. For one it increases the static restoring force which will make the steering slightly more heavy in the hands. If you just shifted forward in the seat, that heaviness would make it hard to judge the steering input. With more weight forward, the front tire now has to turn that weight too. My expectation would be that the front tire would be more likely to skid, or hop, during the more extreme events. My expectation somewhat depends on the fact they throw a great deal of sand about in the winter here, which a rider may encounter is small or large amounts.

The danger of spilling over the handlebars should be considered too. About 4 weeks ago I came around a slight bend and met 4 riders, one of who was alongside. He tried to stop suddenly to avoid hitting me. He managed to stop the bike in about a 10 feet. He came over the handlebars and did a face plant into the asphalt too. The rest of us stopped in about the same distance without such an incident. I suspect that his weight was a bit too far forward to safely handle the unexpected.

For the general reader,
because you never know how many bikes are out there with subtle bends in the their front forks from hitting things like potholes ::
Anyone who has ever hit a pothole hard, or gone down into one of the slots of a gutter grate, may want to check their steering balance, even if the front fort "looks" ok.
But, It is too hard to measure directly without serious equipment.

Much easier to find a curved line, ride down it and try to stay on the line.
Next, you borrow a friends bike and try the same thing, just to make sure it is not you that is the problem. (For the general public, those who do not have 2 bikes)
The friends bike logically should be harder to steer through this test because it is not as familiar as your own.
But, If the friends bike turns out to be easier to steer this way. then you could end up staring at your bike for long periods of time.
When you cannot stay on the line, especially if it is easy at one speed, but more difficult at slower or faster speeds, you should suspect the fork, (IMHO of course).
This is, of course, unless you have been adjusting the frame into unusual configurations, in which case you would probably want to return it to a normal configuration and
test it again, before bending anything.
Also, the general public should not be bending anything themselves unless they already know how to bend it in tiny amounts in a precise controlled manner (which generally they don't),
and know how to measure how much they have bent it.

noglider 

I've been thinking about this thread a lot, though I'm not sure I have any new insights. I own lots of bikes, and they all feel different, though I believe I have the fit adjustments of most of them dialed in well enough to work well for me.

But I went to Philly this weekend, and my friend loaned me one of his bikes for two 17-mile rides. (We rode from a suburb into the city for the Philly Bike Expo and then back in the evening.) I had trouble discerning what was unique to the design of the bike and separate it from the fit. The fit was very unusual, with the handlebars a lot higher than I'm used to. It took a long time to feel comfortable on it because of the fit, not because of the bike's design.

Then after we got back, he had me try two more of his bikes. One of them also had the handlebars high, but for some reason, it felt terrific. The other one had the handlebars much too low, and not surprisingly, the bike felt awful, but the awfulness was clearly from fit and not the bike.

LDBenjamin

Is anyone familiar with frame sink? This is the effect of rake, trail, head tube angle, and wheelbase, where as the front wheel is turned, the front of the bike lowers a tiny bit. It's significant because it requires effort to turn back to center and raise the bike back up against gravity. Supposedly this affects handling.

Chombi1 

I always thought that "caster" built into the front fork design should cancel that and that is why we can also ride our bikes without hands on the handlebar.....

Chr0m0ly 

Dfrost 

pedilato

Hi iab,

Can you please let me know how you did the above drawings?

Thanks a lot

Road Fan

I applaud your care and precision! One question about the saddle setback: you measured saddle tip to a plumb line aimed at the BB center and the TIPS are the same distance behind. But the distance from saddle tip to where your weight is supported might not be the same if the saddles are not the same model and perhaps the same year. I use the widest point of the saddle as the location for my sitbones. At least based on sticking my fingers under my butt when pedaling comfortably, I think ... that's a decent indication. One may ague the point, but its quite clear if you look at them, a broken-in Brooks Swallow, a brand-new Brooks Pro (or B17), a Selle Anatomica which has stopped stretching, and a Specialized Toupe will have different distances from saddle front tip to the center of rider pressure. In part it's geometric distortion due to extensive loading with rider weight. The Swallow and the Selle AnAtomica are commonly seen with elongated noses and tension screws nearly "all done," where Professionals, B17s, and Toupes change relatively little. I tend to measure from saddle rear tip forward or saddle center along the line connecting the sitbone center of contact.

Road Fan

63, where do you take frames to be aligned and get good results, in your area? I can't think of too many Michigan shops I would trust to do it correctly and whom are still in the game. I had a Trek aligned once by Ron Boi, and the improvement was magical. We used to have Mike Nobilette here in Ann Arbor, but that was >25 years ago. My local good shop went out of business less than a year ago. but they also were not 100%. I don't really have a problem sending or carting frames to Chitown to have a good alignment done.

SpeedofLite 

I just happened to notice resurrection of this thread and thought this would be the perfect time and thread to post this Bicycling! 1980 article co-written by a frame builder and a mechanical engineer.

pedilato

Thank you speedoflite, you gave us a nice work for the weekend

pedilato

I am don't think that its really fit in this post but I am sure will be quite few people interested about this Bicycle Geometry 101

Thanks and respect to JohnDThompson!!!

63rickert

Suggestion for Michigan would be Matt Assenmacher in Flint. A friend just broke his tandem steerer while riding to Toronto with his daughter and Assenmacher had him back on road the following morning. On the other hand the website for that shop says they are getting away from framework so you should check to see if the service is really available if you don't have a great story and a great angle. I mean, most people don't succeed in keeping a tandem up when the steerer breaks or manage to break down one mile from a framebuilder.

Ron Boi has been a friend for fifty years. He is barely in business. He did a fork for me just a couple weeks ago. Each time I am less sure what will happen. Talk to him, he loves to talk. If he takes the job it will be good.

Andy Muzi and Tim Bouche at Yellow Jersey in Arlington, Wisconsin do most of my frame work. Have only known Andy 46 years.

You're right, it's magical. Bikes are fun when they are close enough. When truly straight they ride like magic.

iab

I think it was about a 135mm lens on a DSLR on a tripod. Lens center at about 50cm off the ground. Camera far enough back so a single bike fills the viewfinder.

Bikes are vertical, BBs are set at a particular spot. Picture taken of each bike.

Pictures brought into Illustrator, outlines created of each bike. I measured the diameter of each rim and scaled the Illustrator drawings to full size. I cross referenced the seat tube, top tube and wheelbase dimensions to check for accuracy. Everything was within 3mm. Then it is just a simple job of playing with combinations of the outline drawings.