Yet Another Cycling Forum

General Category => The Knowledge => Topic started by: quixoticgeek on December 05, 2019, 11:29:38 pm

Title: Brake steer
Post by: quixoticgeek on December 05, 2019, 11:29:38 pm

I notice when riding the work bike that when I apply the front brake, the bike pull slightly to the left. The bike is a 2 wheeled cargo bike (long, big box at the front), with hydraulic disk brakes.

Is this brake steer, or is this something else caused by the bike being slightly out of balance?

J
Title: Re: Brake steer
Post by: Brucey on December 05, 2019, 11:50:06 pm
it could be the fork flexing due to the braking loads, but it could also be a badly aligned frameset.

cheers
Title: Re: Brake steer
Post by: quixoticgeek on December 05, 2019, 11:53:52 pm
it could be the fork flexing due to the braking loads, but it could also be a badly aligned frameset.

I've noticed it on 2 different units of the same make/model.

J
Title: Re: Brake steer
Post by: grams on December 06, 2019, 12:28:29 am
Is this the kind with the steering connected by a linkage on the right hand side?

If so, applying the front brake slows down the front wheel before it slows down the rider at the back, compressing the frame. The steering linkage stays the same length, so pushing the front wheel to the left.
Title: Re: Brake steer
Post by: quixoticgeek on December 06, 2019, 12:33:48 am
Is this the kind with the steering connected by a linkage on the right hand side?

If so, applying the front brake slows down the front wheel before it slows down the rider at the back, compressing the frame. The steering linkage stays the same length, so pushing the front wheel to the left.

BINGO! That makes perfect logical sense.

(https://pbs.twimg.com/media/EK4Co-UW4AEK5Km?format=jpg&name=medium)

J
Title: Re: Brake steer
Post by: Jakob W on December 06, 2019, 09:35:29 am
How noticeable is the effect? I guess Bullits are stiffer than my Bakfiets, but even at 200+ kg AUW, I've not noticed any steering weirdness under braking. (I suppose the rollerbrakes are going to impose lower braking loads, and what steering effects there may be are probably lost in the general torsional flex when heavily laden...)
Title: Re: Brake steer
Post by: Brucey on December 06, 2019, 09:38:32 am
if there is the slightest free play in the headset then the fork will deflect significantly when the brake is applied and this will cause the steering to move.  This will happen at relatively low braking loads, so ought to be easy to distinguish from a lack of bending stiffness in the fork or the frame.

It has been my observation that the steering linkage is often mounted part way down the fork in bikes of this type, and the head tube is very short, which means that any free play in the headset is exaggerated  in terms of movement in the steering linkage.

cheers
Title: Re: Brake steer
Post by: bludger on December 06, 2019, 10:24:06 am
BINGO! That makes perfect logical sense.
 (image)

 :o H . E . F . T . Y   :o
Title: Re: Brake steer
Post by: quixoticgeek on December 06, 2019, 11:19:13 am
How noticeable is the effect? I guess Bullits are stiffer than my Bakfiets, but even at 200+ kg AUW, I've not noticed any steering weirdness under braking. (I suppose the rollerbrakes are going to impose lower braking loads, and what steering effects there may be are probably lost in the general torsional flex when heavily laden...)

Pretty noticeable.  Enough to scare the crap out of me when it first happened. (I had 2 fall due to spd unclipping errors during my 1st shift).

if there is the slightest free play in the headset then the fork will deflect significantly when the brake is applied and this will cause the steering to move.  This will happen at relatively low braking loads, so ought to be easy to distinguish from a lack of bending stiffness in the fork or the frame.

It has been my observation that the steering linkage is often mounted part way down the fork in bikes of this type, and the head tube is very short, which means that any free play in the headset is exaggerated  in terms of movement in the steering linkage.

I'll check the headset when I'm next at the office. I've seen at least one other bike there which had a loose headset, so it may effect more of them.

:o H . E . F . T . Y   :o

I'm trying a modified Emily Chappell TCR training plan. It's only 30-40k per shift (plus the 20k round trip commute), but with the weight of the bikes, and the winds, it's a bit like an interval training session. Also a great bike handling test, esp as you can't see the front wheel position, so you have to be read the road more than you might otherwise.

Getting back on my race bike at the end of the shift to cycle home is quite a shock...

J
Title: Re: Brake steer
Post by: Jakob W on December 06, 2019, 12:23:27 pm

 :o H . E . F . T . Y   :o

I keep meaning to get some patches made up with a drawing of a bakfiets and (in the appropriate hell's angels/heavy metal typefaces) text saying something like 'heavy metal - cargo biker'...
Title: Re: Brake steer
Post by: quixoticgeek on December 06, 2019, 12:24:48 pm

 :o H . E . F . T . Y   :o

I keep meaning to get some patches made up with a drawing of a bakfiets and (in the appropriate hell's angels/heavy metal typefaces) text saying something like 'heavy metal - cargo biker'...

Put me down for one!

J
Title: Re: Brake steer
Post by: rogerzilla on December 08, 2019, 11:49:07 am
I noticed a lot of fork twist with a disc brake and a P2 rigid fork.