If you are building a bike, or upgrading a fork on a bike, it is important not to upset the geometry too much. If you do upset it, it can lead to sudden unpredictable wipeouts etc and possible resultant injuries.
In order to achieve the best end result, you really need to know the axle to crown measurements for the given fork you are putting on, to make sure it equates with the design intention for your frame. If you are dealing with a 140/150 travel fork then you can have a difference of 25 mm between the various different options available, which can effect the handling quite a bit.

SO…….I was thinking that it would be good if the cycling press adopted a default position of measuring the axle to crown lengths on the products they test and feed them out to the public in their reviews.

It’d also be good if the manufacturers quoted that measurement on their product descriptions.

well to start with, you can get A-C measurements for most forks from the manufacturers websites normally.

alot of frames come with an indication of the fork length or amount of travel that should be used.

Plus I don’t really think that +/- 10mm makes any bike an unridable death trap, might change its ride characteristics a tad but it’s not quite that big a deal in real life….

Might you be overestimating the scale of this issue?

billyboy – Member
…it can lead to sudden unpredictable wipeouts etc and possible resultant injuries.

1 degree = approx 17mm, or 20-25mm sagged.

Might you be overestimating the scale of this issue?

+1 cynic-al.

A-C measurements are a good idea, but

billyboy – Member
…it can lead to sudden unpredictable wipeouts etc and possible resultant injuries.

is a bit of a stretch.

I think most people understand that putting longer forks on can upset things a bit, but a few people would be interested in knowing that not all same-travel forks have the same A-C.

The “Cycle Press” are just a bunch of enthusiasts with advertising revenue to maintain, why would it fall to them to publish all of this apparently essential data?

If you want some proprietary data for fork dimensions the manufacturers supply it already, just go and look for it. You already have a interweb connection.

If you want a fork travel/A-C dimension recommendation for a given frame, again the manufacturer tends to supply this, or you can request it.

Why do you need a table in the back of MBR/MBUK to help you assemble the stuff you get from CRC.

Supposing you get to the point where all of this information leads you to pick a frame and a couple of fork options and you discover for instance that the unsagged HA for the bike would be 66.0 degrees with fork ‘A’ and 66.5 with fork ‘B’ what are you actually going to do with this knowledge?
Will it inform your buying decision at all? will your interpretation of this data affect your decision or will the price or the colour sway you more?

I generally assemble my own bikes, I have enough common sense to go and researh the appropriate parts for on my own, if you need your hand holding to that extent, maybe its better if you just buy an off the peg bike… Or just get a U-turn fork.

If it’s that big a deal, how do you cope with the change in handling as the fork goes from one end of travel to another – that’s a full 150mm for the type of fork you’re talking about?

A mate once lent me an old ti hardtail with a 100mm fork on it. No problem I thought, nice forgiving ride, but then it lifted at the first hint of a climb and wiped out on the first descent, with no provocation, leaving my knee skinless. After a bit of investigation we found out it had been designed to have an old rigid fork and was thus about 4 to 5cms higher at the front than it should have been. Bit of an extreme example…but it hurt and it was on a nothing corner

ever since then I’ve made sure I get the right fork for whatever frame….

I’m convinced it does make a difference

In my experience the a to c measurement is not normally provided….you have to ring up to get it……

surprised that an old Ti hardtail was designed sooo slack that an additional 2deg left it unable to corner.

Totally agree with the OP, my bike’s been all over the shop since I changed the fork.

I’ve used my intense with forks ranging from 115mm-200mm and I’m not dead…. Like has been said, gives different characteristics. I use 115 on climbs, 160 for everything else (uturn lyrik) then for my alps trip I had a 200mm boxxer on it. A to Cs all over the place, but still fine!

Should I remember to lock mine out so they stay constant then?

but then it lifted at the first hint of a climb and wiped out on the first descent, with no provocation, leaving my knee skinless.

I would suggest a certain amount of user error, and possibly a degree of running-out-of-talent. Bit of a poor show to blame other people for this.

indeed, I think you might be overstating it a little…

What about bikes that have been designed around 100 – 140mm forks, I mean that’s a big difference, and yes, it alters the handling, but it won’t make it an instant death trap, and travel adjustable forks allow big variations in legnth, even up to 60mm on some forks.

Also, while you’re worrying yourself over ~20mm of fork length have you given any consideration to the perils of running mismatched tyre sizes front and rear, that can change change the angles too…

Or manufacturer A that reckons head angle X is the one true way, where as manufacturer B is convinced it’s head angle Y, I mean surely certain brands of bike would be un-ridable, and that’s before you throw top tube lengths, stem lengths, wheelbase, BB height, chainstay length etc into the equation.

Or running a shock at either end to soft or too hard, or having a handlebar too low or too high, or your seat too far forward or back…dear god! the list of ways you can turn a bike into an instant human killing death trap or almost endless!

might it be more to do with the fact you rode something unfamiliar and weren’t used to it?

I’m running a mix of radial and crossplies on my bike. Will I die?

I’m running a mix of radial and crossplies on my bike. Will I die?

it’s a universal inevitability

billyboy for OP of the year!

I don’t know if you guys have engaged brain here……………

Here we are, we are all consumers, we are spending loads of dosh, and I’m just asking that we be given more information by the people who provide us with the kit we pay them a lot for………

And you are having a pop at that………….

Do you like being bloody mushrooms, fed shit and kept in the dark about reality?

This has got a nice label, don’t know shit about it, but I’ll buy it, even if it causes me injury……………..

The industry loves guys like you…………………… so does the bloody government………………

🙄 it won’t cause you injury. you’re overstating the issue. it may make your bike feel a bit funny then after about 15 minutes you’ll get used to it.

billyboy – Member
I don’t know if you guys have engaged brain here……………

erm Billy, I could go outside right now and adjust my fork (RS Boxxer) by around 50mm. It would change the handling but in no way would it become dangerous, especially as I am able to change technique to suit

I don’t know if you guys have engaged brain here……………

One of us seems to have not done.

That’s you, in case you where wondering. 🙄

Billyboy.

I will admit that “if” I were to throw a set of 200mm Boxxers on a frame designed for rigid non suspension corrected forks the handling and stability might become a bit of an issue.

Would it be fun to ride? Certainly entertaining.

Would it be a death trap? Errrrm……….

Would I need my head examined? Yes.

We’re all grown ups, have the use of the internet, and can make informed decisions. If you want an argument go down the pub and spill someone’s beer. 🙄

Wow, that’s one hell of a post/rant!

Oh and fork manufacturers do list axle to crown measurements, and even if they didn’t, I’m guessing you are familiar with the operation of a tape measure?

Think about it though, if being the tiniest bit out on the correct a2c is potentially life threatening why would companies sell adjustable travel forks? (before you claim the a2c doesn’t change, it does. By a lot).

While I’m at it, why is the On One 456 named so? Is it because 456 is the number they made? Nope. Is it the number of cups of tea consumed in the design process? Unlikely. Is it because it was designed to take 4, 5 or 6 inch travel forks? Bingo!

Seems to me you’ve been searching for a reason you had a bad days riding. This isn’t it.

/rant

I agree that AC measurements are handy

I have an old frame designed and wanted new forks. But of research showed that new 80mm forks are way longer than the ones when 80mm was state of the art. So I bought some Z2s of ebay.

Its not just head agle is it, its BB height as well. If you like a bike the way it is the you’d go for similar

If you don’t care about AC the you can of course ignore the information

I seem to remember Cy at Cotic specified forks for his bikes in terms of AC not travel as he wanted to limit the leverage applied to the head tube

When I phoned Orange they said they tweaked the geometry for each new generation of fork with a new AC

Ok……got a bit carried away there……..

All I’m saying is that it’d be nice to have that piece of info without always having to ring distributors or frame manufacturers to get it.

To give another minor example………
I love my Pace 303 and so treated myself to a 325.5 frame. The 325.5 is designed for a 140/150 fork, the 303 was for a 130. I have 130 and 140 forks so I measured them and found they had the same axle to crown measurement which made me wonder what specific axle to crown measurement they had designed the 325.5 frame for. So I rang and found out it had been designed for the newer generation of forks with longer axle to crown lengths than the older 140s I had. I bought one of the newer Revs and I’m very pleased with the bike build I ended up with.

I agree there are adjustable travel forks out there but I’d assert that if you start descending hard with the fork set at its shortest travel you are going to compromise the ride experience. Just the same as climbing steep stuff in the longer travel setting, it’s going to be more difficult…….etc. The wrong length fork will make a difference, so the more info you know, the better.