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Q for you geo geeks

Im trying to figure out what is better for me as far as trail is concerned. My last bike had a trail of 60mm, with a HT angle of 73.5. The bike handled great, it was quick and responsive but I found it to jack-knife with very little warning. The jack-knife was not often but enough to notice it and since Im in the process of designing a new fork I was wordering on which way I should go with my trail to help elimanate it or at least give me better warning. The jack-knife happens when im riding at slower speeds to

Any thoughts?

no one?

60 trail with a ~30 rake and ~73 or 74 degree head tube is pretty standard for a polo bike. Don't know what to tell ya about the tendency for your wheel to flip but I wouldnt believe your trail number to be the issue.

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fixcraft.net

Tires can also contribute to jack knifing- a front tire with a "pointy" profile will encourage it & a more squared off profile will discourage it. might be a cheaper solution to try before you put in for a new fork.

hey thanks guys. I'll definitely keep that in mind. Regardless, Im getting a new fork, plus a new frame to go with it cause my polo bike "shiva" got stole like 5 months ago. Its time for a new one and I remembered about that problem I had with it. Im going to change the geo a little. Plus, Im going from 700c to 26, so Im sure that will make it feel different too.

I was gonna come in here talking about how bmx cranks suck because they throw off your Q factor killing power and contributing to pedal strike. Then I realized you meant Q as in question.

Anyways...

You just have to know every bike has limits. I find I only ever jack knife in real competitive game where I'm pushing my bike to very limit of its handling or perhaps on a unfamiliar surface. (I've only ever played maybe 10-15 games on asphalt and it always shows.)

Also Jackknifing rarely happens with warning. Sometimes the wheel will "skip along" and you'll either regain control or step one foot off anti climatically but most often it happens with little warning.

Lastly 26s will help.

- Sincerely
Olsen Aviles

I switched from chopped down mini bmx bars to a flat bar that was only 1 to 1 and half inches wider and almost eliminated oversteer.

I was running a 70mm stem and went up to a 90mm- no more jackknife. I think the added stem length increases your control while leaving the geometry nice and nimble.

Longer lever = increased torque, same idea as a longer handlebar length.

> try to set your seat back or get a layback post. Moving your center of gravity back a bit puts less weight on the front wheel which will decrease the tendency to flop.

> keep your speed up. Increasing the rotational inertia of the front wheel will tend to counter the wheel flop effect.

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry#Wheel_flop

I agree on the longer lever - longer stem or wider bars - but I wonder about the seat thing.

I had quite the issue with jackknifing on my old pompino polo bike. Trying to correct it I kept pushing the seat back until it was suggested to me to do the opposite - push it forward. I did this, putting my body over the cranks more. Additionally I put on a longer stem (up from 50mm to a 75mm I think) and the jackknife issue faded considerably. Perhaps a particular bike frame vs rider frame combination should be considered. Where the riders weight is distributed must have an effect on how the whole bike/rider geometry works. I'm built like a weasel - long in the body with legs short relative to my torso.

Now I'm riding a Scrambler and have kept my hips and knees quite forward and over the cranks and have a longish stem/bar combination keeping me upright and forward but not quite in a "sit up and beg" position. I rarely jackknife unless pushing the bike too hard at too low a speed, or when I'm just being stupid.

Thats a good point on the short stem. I had a a really short stem, dont know the size but it was short. that and using 700c, I can see how that would make me jack-knife in those moment.

I'd love to see this thread go into, why a certain trail should be used pro's/con's.

The equation is easy:

A bike jackknifes when the trail gets 0 or less. Trail gets less with more turning angle.

Just an example:
74°, 26'', 60mm trail -> zero trail @ about 70°

74°, 26'', 80mm trail -> zero trail @ about 84°

I built forks for my bike with about 55, 65 and 78mm trail. I prefer 75-80mm trail.
I´ve also ridden bikes with similar geometry with 100mm trail (straight fork). That was way to much for my taste.

The max-power fork has 2 dropouts, giving you somewhat around 60mm trail and the other around 84mm trail on the maxpower bike. Most people I saw chose the 84mm dropout.

ok so your saying to use a trail of 75-80(personal preference of coarse) to get a comfortable polo ride. Im totally down with trying that, but when you run that high of a trail combined with that HA(74) you have to extend the TT. I bring that up only because Im short. So when does it become a disadvantage, as far as frame size is concerned. my last frame had a ct of of 530mm, in order to do what your saying I have to extend it to 550mm min. to even clear my feet from toe overlap. That and combine with trying a longer stem this time(100mm) it lengthens my cockpit by over a 100mm. Thats a pretty big change. Also, with the trend that most of these polo bikes are going as far as short wheel base(reach) were does it become to much. Or is that why its starting to settle at around 60mm.
Im going to try the higher trail this time and see how it goes. I just hope that the extra 100mm in the cockpit doesnt feel like to much. the wheebase is basically the same, so we will see.

10 cm of cockpit is probably gonna be noticeable.

there's nothing in the wheel flop equation about stem length. if you push out the top tube to increase steerer angle without getting a lot of pedal strike then you can run a shorter stem to keep the same cockpit. this is how most "polo geometry" bikes are being designed, anyway.

the other way for short dudes to get better trail geo without pedal strike is to go to 26 inch wheels.

Yes, I need a "long" HT, because I´m a tall guy (191cm = little more than 6 feet 3)
No toeoverlap, wheelbase 940mm, 26inch wheels. I think 70 or 80mm stem.

  • pitponyII_19.jpg

problem solved.

When the rotation maxes out at the end of the groove, what stops the fork from continuing? It looks like it's clamped to the steering tube spacers which could still rotate.

Combination of choice: Smash + Bang

Looks to me like it's clamped directly to the steer tube. That would certainly make more sense.

How did you calibrate that?

- Sincerely
Olsen Aviles

Build a new frame.

Legalize Hand Throws - 2014

This looks terrifying.

shotgun your bike!

No jackknife ever with this, adjustable A-C, and looks burly as hell ;)

I've never used one of these "centering" springs that used to be on some old-timey townies, but this thread reminded me of them. Besides the huge dork factor, I wonder if these could actually be useful for peeps with jackknifing issues? They're cheap enough at $10 a pop. I imagine one good crash would destroy it...

Then there's the Hopey damper :)