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The Street => The Bike Shop => Topic started by: Narcoleptic Insomniac on October 03, 2015, 08:20:41 AM

Title: On the subject of steering geometry
Post by: Narcoleptic Insomniac 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?
Title: Re: On the subject of steering geometry
Post by: BilboBaggins 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.
Title: Re: On the subject of steering geometry
Post by: montymitch 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.
Title: Re: On the subject of steering geometry
Post by: Narcoleptic Insomniac 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.
(http://photos1.blogger.com/blogger/2220/1860/1600/Trail.jpg)
Title: Re: On the subject of steering geometry
Post by: montymitch on October 03, 2015, 10:40:25 AM
Ahh. Well, less offset still equals faster steering. I need more coffee.
Title: Re: On the subject of steering geometry
Post by: G 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/

:)
G.
Title: Re: On the subject of steering geometry
Post by: LeonLikesToRock 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.
Title: Re: On the subject of steering geometry
Post by: Prodigal Son 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.
Title: Re: On the subject of steering geometry
Post by: Narcoleptic Insomniac on October 04, 2015, 04:47:59 PM
Nonsense.   
Title: Re: On the subject of steering geometry
Post by: BonerhasBent 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.
Title: Re: On the subject of steering geometry
Post by: @ss4oLe on October 08, 2015, 06:15:06 PM
bmx + trails = rules!
Title: Re: On the subject of steering geometry
Post by: condrbkr on October 09, 2015, 12:55:03 PM
(http://s3-eu-west-1.amazonaws.com/digbmx/articles/_landSingle1200/Ben-Lewis-Bike-Check-DIG-2015-1_1.jpg)

25mm offset, 75.5 headtube, super chicagoed bars. I'd want to ride that bike.
Title: Re: On the subject of steering geometry
Post by: Narcoleptic Insomniac on October 09, 2015, 05:57:49 PM
I've found Chicago bars fuck up the handling of any bike.
Title: Re: On the subject of steering geometry
Post by: Prodigal Son 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.
Title: Re: On the subject of steering geometry
Post by: condrbkr 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.
Title: Re: On the subject of steering geometry
Post by: tim_sch on October 10, 2015, 05:54:23 AM
I can't ride bikes without chicago bars
Title: Re: On the subject of steering geometry
Post by: Kinchy on October 10, 2015, 10:04:31 AM
I always ran my bars at the same angle as my forks
Title: Re: On the subject of steering geometry
Post by: Hank Chinaski on October 12, 2015, 02:08:33 AM
I always ran my bars at the same angle as my forks

Anything else feels horrible
Title: Re: On the subject of steering geometry
Post by: JFax on October 14, 2015, 02:22:27 AM
I always ride 1 or 2 degrees of chicago. That Chicago in the pic is just silly.
Title: Re: On the subject of steering geometry
Post by: GabeR on October 14, 2015, 11:40:16 AM
I always ran my bars at the same angle as my forks

Anything else feels horrible

Yup, just ever so slightly forward of being parallel with the forks is where it's at for me.
Title: Re: On the subject of steering geometry
Post by: ediotism on October 15, 2015, 01:43:28 AM
I always ride 1 or 2 degrees of chicago. That Chicago in the pic is just silly.

it's probably just "silly" because of how odd it looks relative to the toptube (ane the bike frame). bear in mind that modern frames are a lot more slanted that they were years ago.

but you notice that against the background, it's directly vertical, which makes it only a tiny bit more forward then most riders. i just took my bike to check the bars, which are almost vertical. i've always wanted it a tiny bit more forward for flatland-style stuff but it's at the perfect angle for street/ramps, bearing in mind that i run 18mm offset forks.

if you look at bikes from the Etnies Forward era or even a bit later, most bars are quite vertical. back then, backends were longer and TTs were shorter in comparison, and a rider'd look really cramped on a bike if the bars are pulled back past the fork angle