Axle to Crown Confusion

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Manc33
Posts: 1838
Joined: 25 Apr 2015, 9:37pm

Axle to Crown Confusion

Post by Manc33 »

I measured the axle to crown on my old 26" suspension fork and it's 48cm when there's 5KG of weight on the handlebars.

I was looking around for a rigid fork and the 26" version of this aluminium rigid fork I was looking at, only has an axle to crown measurement of 40cm. That's a whopping 8cm shorter than my 26" suspension fork! I know there's maybe 3cm sag when you sit on the bike, but there certainly isn't 8cm of sag - so why is the rigid 26" that much shorter?

Have I just got a really long 26"? :lol:

Even the 29" version of that same rigid fork says the axle to crown is 47cm - which is still 1cm less than my old 26" suspension fork when it's got 5KG of weight on the handlebars.

These are what I was looking at for anyone interested:

Code: Select all

https://www.ebay.co.uk/itm/144015584636
What if I get the 29" and put that on? Not that there's any left in stock. :roll:

I mean if my current axle to crown is 48cm anyway, putting a rigid fork on that's 47cm isn't going to raise the handlebars or lengthen the wheelbase, right?

It would look real stupid with a 26" wheel, the mud clearance would be a vast expanse :lol: although this wouldn't be the first time I've run something like this and had a wheel way too small in a fork way too big for it. It doesn't look silly on the suspension fork because it's got 120mm of travel.
When two cyclists get married, they should throw anodized cable crimps instead of confetti.
colin54
Posts: 1949
Joined: 24 Sep 2013, 4:34pm

Re: Axle to Crown Confusion

Post by colin54 »

Thorn make this suspension replacement fork,The Mt Tura with an axle crown length of 420mm & 48mm offset.
Described as 80/100mm suspension corrected. To correct a suspension fork that rides at about 430mm in neutral conditions.
Made with Reynolds blades, £109. Rear facing brake mounts from the picture I notice, that's a Thorn thing for some reason.
https://www.sjscycles.co.uk/forks/26-th ... att-black/
Nu-Fogey.
Manc33
Posts: 1838
Joined: 25 Apr 2015, 9:37pm

Re: Axle to Crown Confusion

Post by Manc33 »

Cheers colin54 but I need at least 470mm axle to crown. I'm trying to fit something the size of Marble Arch under the down tube. Scrapping the suspension fork will take away the problem of the suspension bottoming out. Then it will be the opposite to a hardtail, with suspension at the rear but not at the front. :lol:

I think it's going to be one of these in a 29", about £70 when VAT is added...

https://www.aliexpress.com/item/1005003203705560.html

This bit with the "friendly reminder" made me laugh...
https://ae01.alicdn.com/kf/H29943e50c8b ... ec37fB.jpg
When two cyclists get married, they should throw anodized cable crimps instead of confetti.
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andrew_s
Posts: 5431
Joined: 7 Jan 2007, 9:29pm
Location: Gloucestershire

Re: Axle to Crown Confusion

Post by andrew_s »

colin54 wrote: 21 Sep 2021, 7:23amRear facing brake mounts from the picture I notice, that's a Thorn thing for some reason.
Rear facing mounts allows the use of Suntour self-energising rear brakes on the front.
https://www.sjscycles.co.uk/brakes/0-su ... akes-rear/
colin54
Posts: 1949
Joined: 24 Sep 2013, 4:34pm

Re: Axle to Crown Confusion

Post by colin54 »

andrew_s wrote: 21 Sep 2021, 7:49am Rear facing mounts allows the use of Suntour self-energising rear brakes on the front.
https://www.sjscycles.co.uk/brakes/0-su ... akes-rear/
Thanks Andrew.
Nu-Fogey.
Manc33
Posts: 1838
Joined: 25 Apr 2015, 9:37pm

Re: Axle to Crown Confusion

Post by Manc33 »

When I am sat on my bike (with 26" suspension fork) while the fork is sagging, it has an axle to crown of 460mm. This is still more than any of the "corrected" rigid forks I have seen. One was 400mm which is 6cm under. Others like the SJS one linked to above is 420mm which is still 4cm under. They are all shorter then they should be. :?
When two cyclists get married, they should throw anodized cable crimps instead of confetti.
Mike Sales
Posts: 6277
Joined: 7 Mar 2009, 3:31pm

Re: Axle to Crown Confusion

Post by Mike Sales »

I may well be completely adrift, because I have never used suspension.
Does not the suspension fork have to absorb a momentary higher load by telescoping, than the stationary which stays the same length, when coping with impacts?
That is, there has to be a clearance greater than on a rigid fork in order to handle the movement needed when coming down hard.
Manc33
Posts: 1838
Joined: 25 Apr 2015, 9:37pm

Re: Axle to Crown Confusion

Post by Manc33 »

Mike Sales wrote: 21 Sep 2021, 7:14pm I may well be completely adrift, because I have never used suspension.
Does not the suspension fork have to absorb a momentary higher load by telescoping, than the stationary which stays the same length, when coping with impacts?
That is, there has to be a clearance greater than on a rigid fork in order to handle the movement needed when coming down hard.
I guess. When a suspension fork "bottoms out" maybe that's an axle to crown closer to the rigid forks, or, the rigid forks are using a middle ground figure which seems to be the case, in other words half the travel of a suspension fork.

My full axle to crown is 480mm on my 26" suspension fork that has 120mm of travel, so if it was bottomed out that would become an axle to crown of 360mm - half way between those two figures is 420mm - exactly the same as the SJS fork linked to above and 2cm more than the other rigid one I saw that, I guess wasn't suspension corrected.

I ride on the road though and never bottom out the travel, in fact I doubt it uses up half the travel that often. Only reason I have one is because of all the potholes everywhere, but with 1.75" tyres on now, I'm considering a rigid fork. It's not the front where suspension is needed on the road with potholes, it's the rear. I just like being comfy and couldn't care less how heavy my bike is, I'll just put lower gears on it. 8) The rear triangle does wiggle about a little though, which is a bit annoying.
When two cyclists get married, they should throw anodized cable crimps instead of confetti.
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