Effects of Reducing Crown Height

[greenmark]

Hi all - quick question on the effect of crown height. Specifically two options for a fork on a bike frame:

Option 1: Fork with 400mm crown height. 45degree rake
Option 2: Fork with 380mm crown height, 45degree rake.

Background is that I built a bike back in 2015 with 400mm crown height, based on the frame manufacturer's recommendations at the time.
But I've always been dissatisfied with the handling, I fired a new request to the manufacturer and they now say they recommend 370mm, based on their readings of the 2015 frame spec.

So just asking what I might expect with a 2cm lowering of the crown height. I would not want to change the relative riding position too much (eg handlebar stack).

 

[si_c]

You'll have a lower stack height and the effective head tube and seat tube angles will change and become steeper. This will make the handling feel a bit quicker. As well as this the handlebars will need raising relative to their previous position and the seat tube and saddle clamp will likely need adjusting as the saddle will end up pointing a tad downwards.

 

[Milkfloat]

I switched a fork on a 90s MTB from front suspension to a rigid fork (not suspension corrected). The crown height reduced by about 50mm, a few spacers on top and a different stem got the position reasonably close. I suffered no ill effects whatsoever, it was a slightly different feel, but hard to say exactly what.

 

[Cycleops]

I’d say you’ll notice almost no difference. Changing the stem length will have more effect on the handling.

 

[greenmark]

You'll have a lower stack height and the effective head tube and seat tube angles will change and become steeper. This will make the handling feel a bit quicker. As well as this the handlebars will need raising relative to their previous position and the seat tube and saddle clamp will likely need adjusting as the saddle will end up pointing a tad downwards.

That's what I'm thinking. Based on the bike's current stack and reach, I calculate that a 2cm drop in stack would be the equivalent of increasing head angles by about 1.5 degrees. Not sure how noticeable that would be. Also I know full well that how noticeable depends a lot on how sensitive I am to changes.

Currently the handling just feels wrong - sluggish with a fair bit of under-steer. I'll play about with the stem length and drop first.

 

[Pale Rider]

I recall an old Open University programme on bike frame design.

The presenter had a test bike with adjustable geometry.

By changing the front end a relatively small amount he made the bike lean out on corners.

In other words, it was impossible to ride in anything other than a dead straight line.

 

[figbat]

It won’t be much, but don’t forget the bottom bracket height will reduce, along with the crank-to-floor clearance.

 

[greenmark]

I recall an old Open University programme on bike frame design.

The presenter had a test bike with adjustable geometry.

By changing the front end a relatively small amount he made the bike lean out on corners.

In other words, it was impossible to ride in anything other than a dead straight line.

Well, this is what gets me. The published geometry for the head angle is 72 degrees. If that is what they expect for a bike with crown height of 370mm, then with a fork of 400mm then the actual head angle would be only 70 degrees. That's pretty low. .

I've tried measuring the actual angle many times with a spirit level and can't get anything consistent. I think it's time I just stuck in on some form of vice and get it all properly measured.

 

[CXRAndy]

My Kinesis Tripster V2 has an head angle of 70° . It has a lovely planted feel. The steering doesn't feel slow or numb, but very reassuring

 

[nickyboy]

Such an important matter they named a band after it

 

[davidphilips]

Agree with cycleops but am sure there will be many who notice great changes, what is it about the handling that you dont like? Know bike set up can be fun a bit like a spiders web pull on one bit and every thing else moves.

 

[greenmark]

Agree with cycleops but am sure there will be many who notice great changes, what is it about the handling that you dont like? Know bike set up can be fun a bit like a spiders web pull on one bit and every thing else moves.

I can't flick it around potholes. I just have to ride over them.

 

[Cycleops]

A pic of the bike might help @greenmark

 

[greenmark]

A pic of the bike might help @greenmark

 

[Ajax Bay]

Option 1: Fork with 400mm crown height. 45degree rake
Option 2: Fork with 380mm crown height, 45degree rake.

Apologies if I fail to understand, but I can't see 45 degrees anywhere. Are you saying both forks have an offset of 45mm? 'Rake' is the head tube angle (from the vertical eg 73 degrees).
The behaviour of a bike's steering is dependent on rake and offset.

See attached graph of offset v head tube / steerer angle (rake) from Tony Oliver's book 'Touring Bikes'.

View: https://www.amazon.co.uk/Touring-Bikes-Practical-Tony-Oliver/dp/1852233397
Jobst Brandt on 'Rake': https://yarchive.net/bike/rake.html