Effect of changing Axle-To-Crown measurement on handling?
 

Is there any rough guide to this?

I am looking at replacing a rigid fork with a short travel suspension fork.

I know that the head tube angle will change (I think it is roughly 0.5 deg per 10mm A2C difference)

But will changing the A2C affect the handling, how big a change does it have to be for it to actually be noticeable?

Or are these unanswerable questions?

Check this link out:

http://www.sheldonbrown.com/rinard/forklengths.htm

That should answer your questions. In General:

If your fork is changed to a shorter one it will become easier to turn
If your fork is changed to a longer one it will "Track" better (Going in a straight line)

But it depends on a lot of variables.

Maybe you've already seen that link, so instead try to ride a bike with a different head tube angle and similar riding position, and see how much you notice the change.

it certainly changes, mathematically speaking, but can you really feel the difference? IMHO on a MYB unless the change is greater than 1" either up or down I doubt most riders would notice.

I'd say it's pretty much unanswerable in specifics, as people's sensitivity to this varies a lot.
There's noticing change, and there's being bothered by change, which are two entirely different things.
I've used a 100 mm travel fork on a bike with rigid geometry. It certainly made it sluggish in comparison, but 2 minutes into a ride, it was entirely a non-issue.
Eventually swapped it for a 80 mm travel unit. Made the difference less, but didn't have any significant impact on my ability on the bike.

Back in the days when retrofitting sus forks on rigid geometry frames was more common, there were regularly posted warnings about this placing undue strain on the head tube/down tube joint as well as the fork. Can't remember ever reading about this leading to anything though.

CC, The most difference is when the bike is unladen, but once you're mounted on the bike the fork will compress some (called sag) which makes the difference less. How much the fork compresses will depend on the fork's spring rate and your weight. Some forks have spring preload adjusters to adjust the sag.

Brad

This is an excellent distinction and should be posted as the answer to MANY questions posted here. I agree that many geometry and handling differences that may be noticable at first disappear in short order after a few minutes riding.

Thank you...

@Wallstreetjazz - Cheers for the link, I had missed that.

@dabac - A very intuitive answer, thank you.

@bradtx - I understand about the sag, I was figuring that in, but thank you for pointing it out.

I am little confused now though, maybe it is just me.

My current forks supposedly have an A2C of 400mm - I measured (not very accurately I might add) and came to a similar conclusion. This was a direct measurement between crown and axle and not parallel to the steerer tube as suggested on Sheldon's site. That said, the difference is negligible. Using trig and the rake of 45mm, I calculated that if the measurement of 400mm was direct, the parallel would be 397.46mm, and if the measurement was parallel, the direct would be 402.52mm. So whichever measurement is used, I doubt 2.5mm is going to be noticed at all.

The problem I then have is this. My front fork holds a 20" wheel, but is designed for 26" (I think) - yet all the 700C wheels shown on that link to Sheldon's site have a A2C between 25-45mm shorter, but are for larger (diameter) wheels. How is this possible?

I expect the fork designed for a 26" wheel may be longer than ones for 700c wheels to clear much larger cross section (and therefore height) tires and possibly fenders. Many 700c forks have very tight clearances if intended for road or racing use. For example a 700c wheel with 23 mm tires has a diameter of about 668 mm. A 26" (ISO 559) wheel with 2.125" (54 mm) tires will have a diameter of 667 mm so the diameters are the same and with knobs and mud clearance, the 26" fork has to be longer (and wider).

BTW, substituting a 20" wheel in a frame and fork intended for 26" wheels will have a dramatic effect on trail since the radius of the wheel is part of the trail calculation along with headtube angle and fork rake. It will also have a significant effect on the actual headtube angle unless you use a 20" rear wheel.

hadn't thought of that, and it makes sense, thank you.

The bike (2009 CD Hooligan 1) is manufactured with 20" wheels front and rear, its just the fork seems to be designed fro 26", probably to correct the frame geometry and keep the BB high.

http://4.bp.blogspot.com/_PrVauTpmio...eHooligan1.jpg

If a 26" wheel suspension corrected fork ? it certainly is longer..


I think you have a decent bike, just leave it as it is.

or contact a framebuilder and start all over again,
and specify all the changes the OTS bike has you wishing for.

I own a 20" wheel bike, a Bike Friday Pocket Llama, travel bike.
it has disc brakes too, and an IGH, + dynohub for lighting.

I take your point, but I like to play with things :D.

Yes, I am putting a lot of thought into a suspension fork right now, doesn't mean I will go with one though - I might just plump for the Big Apples instead. I am just exploring my options and trying to learn. In the end I want to cut down on vibration through the bar (its a boneshaker with the tyres at 95PSI), but I also want to maintain speed and possible soak up some kerbs as well.

This is a lot to ask from a 20" wheel bike. Smaller wheels are inherently more difficult to roll over obstacles than larger wheels. You would need a very long travel suspension fork to allow a 20" wheel to 'easily' roll over a curb. A short travel fork will give it a bit softer ride but it won't make it into a monster truck. Learn to bunny-hop if you want to get up on the curb.

Quite probably. Out of 3 bikes and 4 forks I had available built to the rigid 26" standard all end up thereabouts. The fork on the bike being labeled as a "hybrid", a tad shorter. The one off a Merida a tad longer with a massive clearance and looking quite silly with fenders fitted.

So either you go with a short travel 26" fork and get a slacker headtube angle, or maybe look into a 24" suspension fork

Have you though of what effect a suspension for is really going to have on that bike? It's got massive big fat tires already, they look similar to Schwalbe Big Apples, having used a 20" bike with Big Apples, there was no need for suspension, as the tires do a great job in soaking up all the bumps along the way, unless you are going off road adding suspension isn't going to do much for the bike more than the fat tires you already have on.

@LarDasse74 - When I said "soak up some kerbs", I didn't mean just charge them down, I already hop the bike as it is. Just want a little more cushioning when I bring my 13 stone back down to sea level.

@dabac - cheers for the suggestion on 24". I know someone has actually put a 20" fork on theirs, but I am looking for more info about said fork...

@jimc101 - The tyres are Schwalbe Crazy Bob's and I am running them hard at a full 95 PSI, which makes her a fast but hard ride. I tried them at 60, and yes they did soak up a lost more vibration, but the going wasn't as good. The frame is really solid too, so it transmits everything to the handlebar and saddle. I am considering the BA's as they will probably roll better than the CB's.