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Author Topic: On the subject of steering geometry  (Read 11157 times)

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Offline Narcoleptic Insomniac

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On the subject of steering geometry
« on: October 03, 2015, 08:20:41 AM »
Say we have two bikes. Bike A has a 74* headtube and a 32mm offset fork. Bike B has a 75* headtube and a 25mm offset fork. Everything else is the same.

Considering that reducing fork offset increases trail and more trail means slower steering, will the steering response of bikes A and B feel roughly the same?

Offline BilboBaggins

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Re: On the subject of steering geometry
« Reply #1 on: October 03, 2015, 09:35:02 AM »
It was always my understanding (what good is that? Lolol) that bike A would have a more relaxed feel. I ran a set of primo strands on a 75.5 ht and it was twitchy as shit but fun.

Offline montymitch

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Re: On the subject of steering geometry
« Reply #2 on: October 03, 2015, 09:56:08 AM »
I think that reducing fork offset will reduce trail, not increase it. A 25mm offset fork will have faster steering than a 32mm.

Offline Narcoleptic Insomniac

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Re: On the subject of steering geometry
« Reply #3 on: October 03, 2015, 10:03:12 AM »
I think that reducing fork offset will reduce trail, not increase it. A 25mm offset fork will have faster steering than a 32mm.

Looking at the bike from the side, if you reduce fork offset the axle and point of contact between wheel and ground will move backwards thus increasing trail.
« Last Edit: October 03, 2015, 10:08:03 AM by Narcoleptic Insomniac »

Offline montymitch

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Re: On the subject of steering geometry
« Reply #4 on: October 03, 2015, 10:40:25 AM »
Ahh. Well, less offset still equals faster steering. I need more coffee.

Offline G

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Re: On the subject of steering geometry
« Reply #5 on: October 03, 2015, 05:23:11 PM »
Everything else CANT be the same.

74 and a 32mm trail will create a 15mm or so longer wheelbase, the front wheel will be further out front. If you use a shorter toptube frame to compensate, then the bars will be furtherback, if you move the bars forward to compensate then the steering axis is behind... etc etc..

In terms of TRAIL yes, they will be close and feel similar, but when you "feel" the steering, you feel the stability due to trail AND the stability due to your weight pushing down, and the more relaxed angle and longer rake will definitely feel more stable because of it... more here:-

http://www.gsportbmx.com/2004/05/steering-geometry/

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Offline LeonLikesToRock

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Re: On the subject of steering geometry
« Reply #6 on: October 03, 2015, 06:24:26 PM »
I had a frame 75.5 head tube angle. I had 27mm forks and it felt rubbish to me, so I swapped them out to an old set of forks with 35mm rake. When everything was dead straight it felt a little bit different to before but as soon as you turned it felt twitchy as fuck again. It did not really feel like a slacker head tube angle at all.
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Offline Prodigal Son

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Re: On the subject of steering geometry
« Reply #7 on: October 04, 2015, 01:18:13 PM »
I was having an argument the other night after the skatepark turned out their lights were the dude was saying the rake on forks was different because the legs were welded at varying angles from the steer tube. I thought it was preposterous not being 180 on all forks. Affirmation of this would make me feel better.

Offline Narcoleptic Insomniac

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Re: On the subject of steering geometry
« Reply #8 on: October 04, 2015, 04:47:59 PM »
Nonsense.   

Offline BonerhasBent

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Re: On the subject of steering geometry
« Reply #9 on: October 06, 2015, 05:33:15 PM »
From what I understand(which isn't much), a bike with low trail, will handle well at low speeds, but will be unstable at high speed. I believe a lot of this has to do with the type of bike and the riding you are doing, along with the tire size. A little off subject but most Randonneur bikes use low-trail. This is because the bike is loaded with a front bag. When the weight from the bag is put directly over the axle, or just behind it, it make the handling of the bike far better than a high trail or mid trail bike.

G's write up is spot on. The variable are so great, manufacturing tolerances, fork offset, tire size, stem length all play a factor.

I have no idea what a BMX bike will use for trail. I've never actually looked into it.

Offline @ss4oLe

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Re: On the subject of steering geometry
« Reply #10 on: October 08, 2015, 06:15:06 PM »
bmx + trails = rules!

Offline condrbkr

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Re: On the subject of steering geometry
« Reply #11 on: October 09, 2015, 12:55:03 PM »


25mm offset, 75.5 headtube, super chicagoed bars. I'd want to ride that bike.

Offline Narcoleptic Insomniac

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Re: On the subject of steering geometry
« Reply #12 on: October 09, 2015, 05:57:49 PM »
I've found Chicago bars fuck up the handling of any bike.

Offline Prodigal Son

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Re: On the subject of steering geometry
« Reply #13 on: October 09, 2015, 11:31:15 PM »
When your backend is 13" and tt is short, it's going to feel fucked up anyways.

Offline condrbkr

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Re: On the subject of steering geometry
« Reply #14 on: October 10, 2015, 01:29:08 AM »
I'm gonna try giving that amount Chicago a go tomorrow. It's definitely a weird feeling but it seems more responsive for whatever I don't understand. I generally jib so maybe it'll work out for the better. I also run 20.5 so let's see what something a bit longer would feel like.

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Re: On the subject of steering geometry
« Reply #14 on: October 10, 2015, 01:29:08 AM »

 

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