Effects of Reducing Crown Height

[greenmark]

The spec sheet for my existing fork says the Rake = 45. I guess that should be in mm, and not degrees.
It is the same figure quoted for the forks that are recommended and the ones I am considering. All their specs quote "Rake = 45".

The fork recommended to me by the manufacturer in 2015, Rake = 45mm, crown height = 397mm
http://www.topsbikes.com/products/Lynskey-Endurance-Road-Carbon-Disc-Fork-2016.html

The fork that I'm using now, Rake = 45, crown height = 398mm
https://www.kinesisbikes.co.uk/Catalogue/Forks/Adventure/ATR-DISC-FORK

Fork that the manufacturer now recommends, based on their reading of the 2015 frame specs. Rake = 45mm, crown height = 368mm
https://lynskeyperformance.com/lynskey-pro-road-carbon-fork-5-wide/

Fork I am now considering, as it has mudguard mounts, Rake = 45, crown height = 380mm
https://www.kinesisbikes.co.uk/Catalogue/Forks/Road/TRACER-15-DISC-TA

Frame
https://www.chainreactioncycles.com...e-disc-titanium-road-frame-2015/rp-prod131500

 

[Ajax Bay]

Option 1: Fork with 400mm crown height. 45mm offset
Option 2: Fork with 380mm crown height, 45mm offset
So just asking what I might expect with a 2cm lowering of the crown height.

Fork that the manufacturer now recommends, based on their reading of the 2015 frame specs. Rake = 45mm, crown height = 368mm
https://lynskeyperformance.com/lynskey-pro-road-carbon-fork-5-wide/
Fork I am now considering, as it has mudguard mounts, Rake = 45, crown height = 380mm
https://www.kinesisbikes.co.uk/Catalogue/Forks/Road/TRACER-15-DISC-TA

No discernible difference in handling (Edit: see my next post: reducing the fork length effectively increases the head tube angle) - handling depends not on crown height but (see graph image) on [edit] top head tube angle (rake) and offset (often wrongly termed 'rake' just to confuse the unwary - "rake" has to be an angle, doesn't it - you thought so); but maybe someone will come along and say different. Assume the 378mm crown height will give you the clearance you need.
I have a very similar Lynskey 'Endurance' carbon disc fork (like this, I think) and its offset (shown on the steerer tube) is 45mm. My measured distance from centre of dropouts (ie wheel mid point) to crown is 374mm (no idea of detailed spec). I have not built up the frame (Lynskey) yet but will be running 'normal' mudguards - it has the necessary threaded holes on each dropout.
A 622 wheel with 32s will have a 'top of the tyre' radius of 350mm, so 378mm seems plenty of room for a mudguard (disc brakes) - you don't have to 'drop' the mudguard as you would for the calipers on a rim-braked wheel.
Have read your CRC review - thank you for the effort.

 

[greenmark]

No discernible difference in handling - handling depends not on crown height but (see graph image) on top tube angle (rake) and offset (often wrongly termed 'rake' just to confuse the unwary - "rake" has to be an angle, doesn't it - you thought so); but maybe someone will come along and say different. Assume the 378mm crown height will give you the clearance you need.
I have a very similar Lynskey 'Endurance' carbon disc fork (like this, I think) and its offset (shown on the steerer tube) is 45mm. My measured distance from centre of dropouts (ie wheel mid point) to crown is 374mm (no idea of detailed spec). I have not built up the frame (Lynskey) yet but will be running 'normal' mudguards - it has the necessary threaded holes on each dropout.
A 622 wheel with 32s will have a 'top of the tyre' radius of 350mm, so 378mm seems plenty of room for a mudguard (disc brakes) - you don't have to 'drop' the mudguard as you would for the calipers on a rim-braked wheel.
Have read your CRC review - thank you for the effort.

I'm guessing that you mean head tube angle, not top tube angle. But reducing the crown height by 3cm steepens the angles by about 1.5 Degrees.
I spent a good hour measuring everything on my bike. What I have is my head angle is currently 71.5 degrees, what the original spec sheet is that it would be 72.5 degrees. My BB drop is 6.2cm, the original spec is that is should be 7.0cm. So currently the crown height a little too high.

I calculate that dropping the crown height by 2cm with the new fork will match the original spec. But, it will only be a 1 degree difference and I really think that I probably won't notice it. At least not worth spending £200 to find out.

Thanks though for that chart. It really is helpful.

 

[Ajax Bay]

Yes, sorry: head tube angle.
Relja (Bike Gremlin) tries to tie this down (also note rake v offset text) and his diagrams are good:
https://bike.bikegremlin.com/832/bicycle-frame-geometry/
I (too) have done the maths with approximations (thank you for providing the catalyst) and agree ('normal' bike and wheel) that each cm reduction of fork blade length reduces the head tube angle by half a degree. The spec sheet assumes a 'normal' 380mm (say) fork length - why the manufacturer suggested a 400, who knows - maybe they had one to 'get rid of' - you can see on Tony Oliver's graph it's on the margin so your feeling that it's not right is soundly based.
A shorter fork will give you better steering (my assertion (above) that the difference would not be discernible is wrong). Only you know whether that'd be worth it to you - but I would like to avoid potholes myself, and the replaced fork will have resale value.
There's a reason why fork lengths for road bikes 622 wheels are generally around 380. If greater clearance is required for higher(wider) tyres, the frame is designed for it, to maintain the desired head tube angle.
You might appreciate the Geometry Geek site.
HTH

 

[greenmark]

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.
View attachment 553591
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

AB, there is one thing about that diagram from Oliver's book that doesn't make sense. According to the chart, the steeper the head angle then the slower the steering. The shallower then it says the quicker it is.
That's against everything I learnt and also against the BikeGremlin link that you sent. There's a typo in that chart, right?

 

[Ajax Bay]

No: the chart shows both limits of 'steering' stability.
(If you want ) You need to define 'steering', 'slower' and 'quicker' in terms of the two main inputs to steering: lean, and bar input, and consider 'steering' at low speeds and higher (ie normal riding) speeds and what might be meant by 'sensitivity'. Apologies if this sounds pedantic and also if I've got some of this wrong.
Head tube angle Head tube angle (measured in bicycles from the horizontal) dictates how much effort is required to turn the front wheel (bar input to steering). As the angle increases (steeper), the front wheel becomes easier to turn and, thus, the manoeuvrability of the bike generally increases with the angle of the head tube. But at riding speed it's not the bar input which is dominant.
Trail Trail is the distance between where the tyre touches the ground and the point where the steering axis (an imaginary line) meets the ground. The contact patch of a bicycle wheel invariably (almost) trails behind its steering axis and will tend to align with the direction of the bike (like the wheels of a shopping trolley). This castor effect becomes increasingly important at higher speeds by helping the front wheel remain centred without any input from the rider (‘stable’). The amount of trail depends upon three factors:
1) Inversely proportional to the sine of the head tube angle (ie steeper head tube = smaller trail),
2) Inversely proportional to the amount of fork offset,
3) Proportional to wheel size.
http://www.kreuzotter.de/english/elenk.htm
Trail assists steering: as you lean the bike to the left or right, the steering axis moves the contact point in the opposite direction, and thereby turns the wheel (a small amount) in that direction as it pivots on the point of contact with the road. Trail also assists the bike in holding a straight line.

 

[Ajax Bay]

https://www.sheldonbrown.com/rinard/forklengths.htm
Damon Rinard says: "If all else remains the same, a steeper head angle is quicker handling, while a shallower head angle is slower handling."

 

[greenmark]

OK an update - for anyone still following or anyone in the future with the same question and eventually finds this thread.

I changed the fork and reduced the crown race by 2cm, which led to a steeper head angle.

The handling seems to feel better. I'm less worried about under-steer. I'm not sure how much of that feeling is real and how much is psychological bias trying to confirm that there was a change.

But what I didn't expect was a better ability to keep on the drops and be faster on the flats. I also found it much easier to get out of the saddle and jump on the pedals for quick climbs or sprints. All round a more fun experience.

I did not change the setback of my saddle on the seatpost. Although I reduced my crown height by 2cm I increased my headset spacers by only 0.5cm. So in effect I opened up my hip angle a little, brought myself forward a bit while also flattening my back. I'm guessing some of the extra twichy steering from the head angle is offset by pushing my centre of gravity forward and down a bit.