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Old 09-01-13, 07:39 PM   #1
treadtread
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Stability at speed

What makes a bike stable at speed? Long wheelbase and tires are the only thing I can think of - but most road bikes I see don't really have a long wheelbase. I have the following bike:
http://www.specialized.com/us/en/bik...irrus#geometry

and yesterday, I hit 41.27 mph downhill - and it was freaking scary. The bike was distinctly skittish - I don't think I'll try that again. I also lost a light - fell off with the vibration. And I'm running 700x32 tires (stock Nimbus) - thinner tires would make it worse.

So .. is there something inherently wrong with the bike geometry to not encourage higher speeds? Or is it the wider handlebar, or not-so-great tires? Or is it just that I need to HTFU? Not that I intend to repeat the experiment - just curious
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Old 09-01-13, 07:51 PM   #2
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Drop bars are gonna work a lot better for high speed descents. Gets the weight over the front end. Those bars also have a lot of leverage which makes things twitchier.
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Old 09-01-13, 07:56 PM   #3
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"trail" is the most important aspect of stability.

http://davesbikeblog.squarespace.com...g-bicycle.html
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Old 09-01-13, 08:17 PM   #4
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There a lot of things that can make the bike, "seem" skittish. Are your wheels true. Is your head set too tight or too loose. Are you gripping the bars in a death grip? That bike should track fine, Try again with slicks and relax a bit to see if that doesn't help.
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Old 09-01-13, 08:33 PM   #5
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A long WB recumbent is great for this. I've hit 59 mph on a down hill.
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Old 09-01-13, 08:54 PM   #6
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Probably confidence. These guys aren't using long-wheelbase bikes:

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Old 09-01-13, 09:21 PM   #7
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I never thought of weight distribution in the context of drop bars, which is just duh I suppose. But that isn't going to change very soon.
Looking at the bike stats and the article about trail, geometry shouldn't be a significant issue (applying my very limited knowledge of geometry!). Wheels are true, headset - dunno. How do I tell?


I'll pay attention to whether I'm gripping too tight - I usually don't but maybe did it inadvertantly at that speed. I'll also remove the rack and bag (which was lightly loaded) - that should help with weight distribution. And next time I don't barrel down that hill at 40 mph, maybe I'll do better
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Old 09-01-13, 09:47 PM   #8
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Quote:
Originally Posted by Mobile 155 View Post
There a lot of things that can make the bike, "seem" skittish. Are your wheels true. Is your head set too tight or too loose. Are you gripping the bars in a death grip? That bike should track fine, Try again with slicks and relax a bit to see if that doesn't help.
Ditto to all of the above. I've seen 55 mph (and accelerating) on a supposedly skittish short-wheelbase recumbent, so I think consciously relaxing your grip and any muscles not engaged in pedaling helps enormously.

This is Sam Whittingham at close to 83mph on the flat. Notice how smooth he is:


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Old 09-02-13, 07:45 AM   #9
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There are so many varyables on each an every bike concerning geometry etc that you really have to ride a specific bike to find out what they are going to do at speed.

Probably one of the best "feeling" bikes I have owned was a Univega touring bike. Above 40 mph it seemed to have a life of its own. The gyro effect of the wheels seemed to make it super stable.
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Old 09-02-13, 08:54 AM   #10
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Aside from the good advice above (stay loose), please keep in mind that there is absolutely no reason to go faster than you are comfortable going. Falling off a bike going 40 mph is no fun.
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Old 09-02-13, 11:26 AM   #11
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But It's a flying 200M speed. in the record books..> Jeff<. The Hour record is much lower speed ..



OP, you should Buy and read this Book [print or e version are sold]

http://mitpress.mit.edu/books/bicycling-science



Add Wiki Quote.. Bio on Sam :

The 200 m flying start (single rider, World Human Powered Speed Challenge, Battle Mountain, NV):
133.284 km/h (82.819 mph) on 2009-09-18.

First unpaced cyclist ever to break the deci-mach mark (1/10 the speed of sound,
World Human Powered Speed Challenge, Battle Mountain, NV): 132.50 km/h (82.33 mph)
on 2008-09-18. To date, still the only person to have accomplished this milestone.

The 1000 m flying start (single rider): 128.40 km/h (79.79 mph) on 2001-10-06.

The 1 mile flying start (single rider): 126.55 km/h (78.64 mph) on 2001-10-06.

The hour record: 90.724 km (56.373 mi) on 2009-07-17.[1]

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Old 09-02-13, 11:36 AM   #12
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But It's a flying 200M speed. in the record books.. Jeff. The Hour record is much lower speed ..

OP, you should Buy and read this Book [print or e version are sold]

http://mitpress.mit.edu/books/bicycling-science
You can stuff the snarky comments Bob .. I never implied or hinted at records. This probably doesn't apply to saints like you, but sometimes normal people do things just because. Curiosity is a human trait.

I'll read the link you suggested, thanks.
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Old 09-02-13, 12:44 PM   #13
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Have a nice day try to keep the speed within your comfort zone .

Now you know what happens past your limit.
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Old 09-02-13, 01:56 PM   #14
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Have a nice day try to keep the speed within your comfort zone .

Now you know what happens past your limit.

You've got style Bob, mock the guy trying to learn and understand. Learning and improving are clearly foreign concepts.
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Old 09-02-13, 10:09 PM   #15
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Quote:
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But It's a flying 200M speed. in the record books..> Jeff<. The Hour record is much lower speed ..



OP, you should Buy and read this Book [print or e version are sold]

http://mitpress.mit.edu/books/bicycling-science
Who said anything about the hour? The OP mentioned hitting 40 mph downhill... there's very few places where you can ride downhill for an hour (although I've ridden one).

I have a copy of the book, and I've had a couple informative email exchanges with author David Gordon Wilson. Also, I'll be packing up this Saturday and heading to Battle Mountain, Nevada for this year's WHPSC. It ought to be interesting with Graeme Obree's entry:
http://www.dailymail.co.uk/sciencete...ld-record.html
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Old 09-02-13, 10:20 PM   #16
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Quote:
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You've got style Bob, mock the guy trying to learn and understand. Learning and improving are clearly foreign concepts.
He's kind of like Krusty the Clown, smoking, drinking, trying to make people laugh.

Nevertheless, going fast on a bicycle can be tricky. It's a very dynamic system, and small adjustments can have large results. The upright posture of your hybrid bike may be putting to much weight on the rear wheel... next time try hunching over the handlebars, with your hands and lower arms pointing straight rearward. This can be tough (especially if you don't have upper body strength like me), and holding this position while not putting a death grip on the bars can be tough.
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Old 09-03-13, 05:59 AM   #17
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A long WB recumbent is great for this. I've hit 59 mph on a down hill.
As a lover of speed, I would love to see that...

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Old 09-03-13, 06:43 AM   #18
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Hi,

Not mentioned is the state of the road your doing 40+ mph downhill on.

Too high tyre pressures, especially on the front, is a further possibility.

rgds, sreten.
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Old 09-03-13, 07:45 AM   #19
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Quote:
Originally Posted by treadtread View Post
What makes a bike stable at speed? Long wheelbase and tires are the only thing I can think of - but most road bikes I see don't really have a long wheelbase. I have the following bike:
http://www.specialized.com/us/en/bik...irrus#geometry

and yesterday, I hit 41.27 mph downhill - and it was freaking scary. The bike was distinctly skittish - I don't think I'll try that again. I also lost a light - fell off with the vibration. And I'm running 700x32 tires (stock Nimbus) - thinner tires would make it worse.

So .. is there something inherently wrong with the bike geometry to not encourage higher speeds? Or is it the wider handlebar, or not-so-great tires? Or is it just that I need to HTFU? Not that I intend to repeat the experiment - just curious
From what I know, I'd agree with the fellow who mentioned trail. A small amount of trail can give you more responsive steering, but that also translates into every twitch of your body gets translated back to the steering. I don't think that drop bars will have an appreciable affect.

I've got an old Specialized Hard Rock with street slicks and a part of my regular ride that is about a 40 mph downhill. I just relax. It's not tires, wide handlebars. Have you double checked to make sure your rear wheel is aligned to the frame?

My recumbent did seem skittish until I learned to relax because the tension in my arms was over-driving how much effort was needed to keep straight. But downhill, in a corner, it went like it was on rails.
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Old 09-03-13, 08:26 AM   #20
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After reading comments here, and thinking back, I think the advice to relax a bit is most spot-on. I was leaning forward on the bar ends, and I probably leaned mainly on my hands, not utilizing the upper body as much. Tire pressure may be relevant, I would need to experiment with different pressures. And I do hear the advice about the dangers of speeding - I don't do this sort of thing regularly. Just wanted to understand what I should be doing right when going fast.
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Old 09-03-13, 09:31 AM   #21
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If you search through Dave Moulton's blog postings, there is an article in there somewhere on how to handle a descending bike that has too little trail, and is prone to high-speed shimmy. In a nutshell it is:

1. Keep all weight on pedals, either by putting all weight on the pedal at 6 o'clock position(preferred), or hold crank horizontal and equally weight both pedals.

2. Pinch top tube between knees.

3. Keep weight *off* handlebars.

4. Keep weight *off* saddle.

I used this method to hit 45mph, on a bike that had previously gone out-of-control repeatedly at 35 mph. The bike was a tall frame with too-steep headtube, thanks to UCI's idiotic front-to-center limitations(*). Ordering a custom-made fork with a smaller-than-standard rake was the permanent solution. With the new fork, I've hit 55mph with that bike w/o incident.

Note that high-speed shimmy is an *interaction* between the rider and the bike, as the rider's reactions try to stop the shimmy. Also, the worst situation I've experienced to *trigger* the shimmy, is to be riding in a strong crosswind from the left, and then be passed by a large vehicle which momentarily blocks the wind.

(*) http://velonews.competitor.com/2010/...-poorly_121162

Last edited by Shimagnolo; 09-03-13 at 11:58 AM.
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Old 09-03-13, 10:24 AM   #22
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There could well be something off on your bike so that the wheels are not properly aligned causing a bias to one side or the other. I have a Specialized Secteur that did not seem to track well and pulled to one side. After a lot of thought and measuring, I found the front dropouts were not perfectly horizontal when the bike was plumb. One dropout was glued into the fork just a tiny bit higher than the other. I worked out that if one dropout was one half mm higher than the other, the rim would be 3 to 4 mm out of plumb. I was able to easily fix this with a round file to remove just a bit of material in the dropout. The bike can now be ridden no hands but the best part is that it tracks much better and seems nearly on rails when cornering.

This may not be the problem and solution on your bike. You could visually check that the wheels are centered within the frame. Possibly dish on the rear wheel could be off and the front wheel also may not be centered due to spoke tension being off.
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