Rake & steering sensitivity

For discussions about bikes and equipment.
ukdodger
Posts: 2992
Joined: 18 Aug 2007, 5:32pm
Location: Sunny Surrey

Rake & steering sensitivity

Post by ukdodger »

What exactly causes steering that is too sensitive. My son asks me to make a frame for him and in sorting out his size I asked him to try my bike. He complains that the steering is too sensitive (it isnt to me). I chose the rake from Talbots book which puts it in the 'Neutral' zone (about 2.25inches). However.. I checked my shopper bike which frame I did not make and although it has the same amount of rake it is less sensitive. Sooo the only difference I can find between the two sets of forks is the shopper is slightly shorter but the rake starts further up the blade. If I try to copy that bend it will mean finding or making another former to bend it around. Alternatively I could change the amount of rake. But which way for sure? Fork blades arent cheap and you only get one shot.
User avatar
Vantage
Posts: 3050
Joined: 24 Jan 2012, 1:44pm
Location: somewhere in Bolton
Contact:

Re: Rake & steering sensitivity

Post by Vantage »

Never built a frame, nor would I want to, but afaik, headtube angle and wheelbase also come into it.
Bill


“Ride as much or as little, or as long or as short as you feel. But ride.” ~ Eddy Merckx
It's a rich man whos children run to him when his pockets are empty.
LollyKat
Posts: 3250
Joined: 28 May 2011, 11:25pm
Location: Scotland

Re: Rake & steering sensitivity

Post by LollyKat »

The tyre size has an effect - presumably the shopper's ones are bigger and the wheel diameter may be bigger?

There is a long post on the Thorn forum about trail and handling here. There are several interesting-looking links at the end of it.

Or maybe your son is simply not used to your bike's handling - what does he normally ride?
ukdodger
Posts: 2992
Joined: 18 Aug 2007, 5:32pm
Location: Sunny Surrey

Re: Rake & steering sensitivity

Post by ukdodger »

LollyKat wrote:The tyre size has an effect - presumably the shopper's ones are bigger and the wheel diameter may be bigger?

There is a long post on the Thorn forum about trail and handling here. There are several interesting-looking links at the end of it.

Or maybe your son is simply not used to your bike's handling - what does he normally ride?


He doesnt Lolly. But he's decided to take part in the London to Brighton event. Both bikes are the same wheel and tyre size but the wheelbase on the shopper is about two inches shorter and frame about two inches smaller (it's actually too small for me but is still comfortable). Thanks for the link. I've looked at several of those but have to admit
to ending up confused.

Head tube angle is also the same - 72deg.
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: Rake & steering sensitivity

Post by 531colin »

The one word (one number?) answer is trail. http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics (scroll down!
Trail derives from head angle, rolling diameter of the wheel, and fork offset.
The shape of the bend in a bent fork doesn't matter, its how far the axle is offset from the steering axis....straight blades rake from the crown.
Conventionally, the more trail, the more stable.
Look at Wiki's diagram, you can see that tyre drag will try to straighten the steering (like a caster)....tyre drag is the force, trail is the lever.
It follows that with identical geometry, a "draggier" front tyre, will feel more stable....so will the same tyre with more weight on it.
Wheelbase has a tiny effect.
So, if the fork offset and rolling diameter is identical on your 2 bikes, and they steer differently, either the head angle is actually different (one degree makes a difference) or the tyre drag/weight distribution is different.
I'm very used to 54mm fork offset with 71 deg head and 700c tyres ranging from 28 to 35mm, with drop bars...it suits me.
With flat bars, it feels "too light" for me, but I expect you could get used to it.

I also think a shallow head angle is more "forgiving" than a steep head with the same trail.....but I can't prove that!
ukdodger
Posts: 2992
Joined: 18 Aug 2007, 5:32pm
Location: Sunny Surrey

Re: Rake & steering sensitivity

Post by ukdodger »

531colin wrote:The one word (one number?) answer is trail. http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics (scroll down!
Trail derives from head angle, rolling diameter of the wheel, and fork offset.
The shape of the bend in a bent fork doesn't matter, its how far the axle is offset from the steering axis....straight blades rake from the crown.
Conventionally, the more trail, the more stable.
Look at Wiki's diagram, you can see that tyre drag will try to straighten the steering (like a caster)....tyre drag is the force, trail is the lever.
It follows that with identical geometry, a "draggier" front tyre, will feel more stable....so will the same tyre with more weight on it.
Wheelbase has a tiny effect.
So, if the fork offset and rolling diameter is identical on your 2 bikes, and they steer differently, either the head angle is actually different (one degree makes a difference) or the tyre drag/weight distribution is different.
I'm very used to 54mm fork offset with 71 deg head and 700c tyres ranging from 28 to 35mm, with drop bars...it suits me.
With flat bars, it feels "too light" for me, but I expect you could get used to it.

I also think a shallow head angle is more "forgiving" than a steep head with the same trail.....but I can't prove that!


So says Talbot and it sounds right. Imagine a 90deg HT/TT angle and no rake at all.. Would be almost impossible to steer. But there's my problem I have another homemade frame with less rake (marginally less than 2inches) and the same 72deg angle but it too is less sensitive (though not much). The tyres and wheels are the same only the wheelbase and height are each an inch shorter. That said if I increase the rake on my son's frame on the face of it I'll be making it more not less sensitive. I''m not saying it isnt right just that I must be missing something.
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: Rake & steering sensitivity

Post by 531colin »

See if I got this right......
"my bike".....2 1/4" offset, 72 deg.
"shopper".....2 1/4" offset, 72 deg....but less sensitive
"another home-made" ...< 2" offset, 72 deg, less sensitive..

the last one makes sense....everything else being equal, less offset = more trail = more stable

How do you measure head angle?
ukdodger
Posts: 2992
Joined: 18 Aug 2007, 5:32pm
Location: Sunny Surrey

Re: Rake & steering sensitivity

Post by ukdodger »

531colin wrote:See if I got this right......
"my bike".....2 1/4" offset, 72 deg.
"shopper".....2 1/4" offset, 72 deg....but less sensitive
"another home-made" ...< 2" offset, 72 deg, less sensitive..

the last one makes sense....everything else being equal, less offset = more trail = more stable

How do you measure head angle?


The lugs are 72. Hang on. I've assumed trail and rake are the same when they're opposite. Duh :oops:

Talbot's graph claims that 'quick' steering equates to oversteering and is the result of less Rake which I've taken as meaning more sensitive. Think that's cleared it up.

http://img834.imageshack.us/img834/6898/0isd.jpg
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: Rake & steering sensitivity

Post by 531colin »

Talbot seems to say less offset = quicker steering.
In fact, more offset = less trail = quicker steering.
Brucey
Posts: 44517
Joined: 4 Jan 2012, 6:25pm

Re: Rake & steering sensitivity

Post by Brucey »

in relation to the latter point I agree with Colin.

In relation to the original query, whilst tyres etc all make a difference, it could well be that more weight on the front end (in part because of the shorter wheelbase) makes for slower steering, even at the same trail value.

Time to get the bathroom scales out and measure wheel loadings.

Also; note that the MOI of the steering assembly has a role as well. Bikes with more MOI in the steering and/or that carry a front load need less trail to feel like they have slow enough steering, I think. To test the MOI hypothesis, just add weights to the handlebars of a bike. The difference is usually remarkable.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
ukdodger
Posts: 2992
Joined: 18 Aug 2007, 5:32pm
Location: Sunny Surrey

Re: Rake & steering sensitivity

Post by ukdodger »

Brucey wrote:in relation to the latter point I agree with Colin.

In relation to the original query, whilst tyres etc all make a difference, it could well be that more weight on the front end (in part because of the shorter wheelbase) makes for slower steering, even at the same trail value.

Time to get the bathroom scales out and measure wheel loadings.

Also; note that the MOI of the steering assembly has a role as well. Bikes with more MOI in the steering and/or that carry a front load need less trail to feel like they have slow enough steering, I think. To test the MOI hypothesis, just add weights to the handlebars of a bike. The difference is usually remarkable.

cheers


MOI??
Brucey
Posts: 44517
Joined: 4 Jan 2012, 6:25pm

Re: Rake & steering sensitivity

Post by Brucey »

sorry- jargon infestation alert!

Moment Of Inertia

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
NATURAL ANKLING
Posts: 13780
Joined: 24 Oct 2012, 10:43pm
Location: English Riviera

Re: Rake & steering sensitivity

Post by NATURAL ANKLING »

Hi,
As said tyres will make a difference, I always thought my tourer was not that stable on handling (531colin queried the stats) but on putting the original tyres back on which were taller 10 mm (adds more trail) and wider heavier too 37 opposed to 25 mm wide, transformed the bike.

Edited -
http://cyclingtips.com.au/2011/02/the-g ... -handling/
In this article it appears the offset is identical across frame sizes which with an increasing head angle for larger frames means less trail :?: Is that correct :?:
geometryChart.jpg

img006_aa.jpg

I will stick my neck out and say that Offset is self centering value, more is more stable :?:
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
Brucey
Posts: 44517
Joined: 4 Jan 2012, 6:25pm

Re: Rake & steering sensitivity

Post by Brucey »

NATURAL ANKLING wrote: http://cyclingtips.com.au/2011/02/the-geometry-of-bike-handling/
In this article it appears the offset is identical across frame sizes which with an increasing head angle for larger frames means less trail :?: Is that correct :?:...


yes that is what is happening there in that sample table. Less trail in a larger frame. But arguably it is because a) they are trying to keep the wheelbase short even in larger sizes, because it is a 'racing bike' or something, b) they can't buy the 'same fork' with different offsets and c) they are hoping that the reduced trail is mitigated by the increased weight that a taller rider (using a longer stem etc) will be putting over the front wheel. It doesn't always work.... I'm sure Colin will explain more.

I will stick my neck out and say that Offset is self centering value, more is more stable :?:


A commonly held misconception....but in fact; more offset = less trail, less self centring in the steering.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: Rake & steering sensitivity

Post by 531colin »

More offset = less trail = quicker steering.......a shorter lever for tyre drag to centre the steering, if you like.

I reckon changing 25mm tyres to 37mm alters the steering feel due to tyre drag, not the small increase in trail.

In your tables, steeper head angle on larger frames with the same offset will give less trail and less stability.....yes, that's what it means, yes, lots of manufacturers do it, and no, I don't think its right for a bike meant for anything even vaguely like touring......If you search the race bike geometry tables of the really big manufacturers you can find them sourcing carbon forks with different offset for different head angles...I think Spec. and Trek..?

And, just for fun, I don't think any of the articles we have all quoted are completely right.....Wiki is wonderful, trotting out the fallacy that touring bikes have lively steering, then giving a list of trail values that says the opposite!....but their diagram of "trail" is right, at least!

Oh , BTW, shimmy is a problem for tall riders on tall frames.....I think they need fat tubes, not quicker steering!
Post Reply