Fork geometries?

[BdaGhisallo]

How critical is the length of a fork to the front end geometry. I know one should always try to match fork lengths when swapping forks but I have been wondering about this after switching forks and feeling a much greater difference than I expected.

I have switched out a Reyolds ouzo pro in a 50mm rake and put in an ouzo with a 45mm rake. Now they both have the same nominal length, but is that only true when we are talking identically raked forks, right?. Will the 45mm ouzo put the front wheel further under the frame than the 50mm, raising the front end a touch, and effectively slackening the head tube angle? Is the lower headset race higher in the air than before?

The reason I ask is that after the switch the increase in trail and the slowing down of the steering was a lot greater than I would have expected.

If what I think has happened had indeed done so, can anyone hazard a guess at what I have done to the effective hta? It is a 72deg on paper, when paired with that 50mm ouzo.

thanks for all the help,
Geoff

[Dave]

Don't know why you would think the same brand and model of fork would have a different length. The length is measured the same, regardless of the offset. It takes a change of 7-8mm to affect the HTA by .5 degree and that would only change the trail by 3mm. The fork would have to be a lot longer to add to the change in trail that you've made. You didn't buy a cross fork did you?

Yes, there will be about a 5mm decrease in the wheelbase, but that would make the bike quicker, not slower. The change in the wheelbase is very small, compared to the trail, since the wheelbase is about 20 times larger than the trail.


I'd like to know how you evaluating this change. Usually it takes some pretty extreme cornering to notice a small change in trail. Casual riding won't tell you a thing.

[e-RICHIE]

Yes, there will be about a 5mm decrease in the wheelbase, but that would make the bike quicker, not slower. The change in the wheelbase is very small, compared to the trail, since the wheelbase is about 20 times larger than the trail.


he reduced the rake 5mm. that does shorten
the front center 5mm, but the bike will be quicker?
the steering will be slower, or, more accurately,
less lively, since the trail is decreased by adding
the newer fork measurement. no?

[Brian Smith]

When splitting hairs like this (not that there's anything wrong with splitting hairs for curiosity's sake), it isn't really enough to just use "nominal" values given by fork manufacturers for lengths and offsets. Their manufacturing variances can be up to 3mm in almost any direction, which is already more than half of your stated intentional change. If your original fork was a nominal 370mm/50mm fork that was actually 367/53 and the second fork is a nominal 370mm/45mm fork that is actually 373/42, then the trail change could actually have been a lot more than you intended.

I think instead of aiming for some idealized "value" for trail, you should give yourself a little time with any fork available for you to try, then reinstall whichever one produces your favorite effect.

It may be that your handling felt unstable for reasons not even found among trail-producing parameters.

[Smiley]

I went through this same analysis when I swaped my 365 mm span F1 with a 372 mm span Ouzo Pro , it did change the head tube angle for the better by increasing trail , thats on paper though . In real life the new fork felt much better .

[Dave]

he reduced the rake 5mm. that does shorten
the front center 5mm, but the bike will be quicker?
the steering will be slower, or, more accurately,
less lively, since the trail is decreased by adding
the newer fork measurement. no?

Here's what I said:

"Yes, there will be about a 5mm decrease in the wheelbase, but that would make the bike quicker, not slower. The change in the wheelbase is very small, compared to the trail, since the wheelbase is about 20 times larger than the trail."

I didn't intend to imply that the bike would be quicker overall, just that the reduced front-center and wheelbase contribute some amount of quickness, compared to the "slowing" from the increased trail. That's why I said the wheelbase, being 20 times greater would not be the major contributer to the change in the bike's handling, but it does work in the opposite direction from the change in trail.

[cpg]

When splitting hairs like this (not that there's anything wrong with splitting hairs for curiosity's sake), it isn't really enough to just use "nominal" values given by fork manufacturers for lengths and offsets. Their manufacturing variances can be up to 3mm in almost any direction, which is already more than half of your stated intentional change. If your original fork was a nominal 370mm/50mm fork that was actually 367/53 and the second fork is a nominal 370mm/45mm fork that is actually 373/42, then the trail change could actually have been a lot more than you intended.

I think instead of aiming for some idealized "value" for trail, you should give yourself a little time with any fork available for you to try, then reinstall whichever one produces your favorite effect.

It may be that your handling felt unstable for reasons not even found among trail-producing parameters.


Well said Brian. It's true that these manufactured forks can and do vary greatly from their stated lengths, rakes and even alignment. It amazes me when framebuilders strive to hold much tighter tolerances on their frames and then turnaround and stick on a fork made to such sloppy tolerances. Does that make sense to anyone else?

Curt

[CNote]

Does that make sense to anyone else?

No, it doesn't. It reminds me of when I met Chris Kvale, how he said that he had customers who actually replaced the forks on the custom framesets he had built for them with carbon forks, apparently to make the bike lighter. It boggles the mind.

[e-RICHIE]

Does that make sense to anyone else?
absoeffinlutely uh-uh.

[Dr. Doofus]

all hail the hand raked steel fork

as the good jerk has said front-to-back stiffness has an impact on handling -- there is probably a point of diminishing returns, but the stiffer front-to-back, the more solid and repsonsive* the bike will be, especially at speed.

Doof is no monster -- 325w at LT is good, but not monsterous. But on his interval loop, there is a section with a slight cross-tailwin where doof will get the bike up to 30+ and hold it there for about 7-8 minutes while at LT. At that speed, doof has given both bikes a simple test: look back behind you like Tom Boonen at the Ronde Van Vlaanderen. The ridley feels rock solid when doof turns his head to look back at the cows. The gunnar feels just a tad loose -- less stiff front to back is a chunk of this, in doof's humble opinion (the bikes have identifcal wheels, tires, setups, trail, sta, hta...).

ok -- where all this is leading

modern bikes are stiffer front-to-back than something from the 80s or 70s, for the most part. this does help handling, idho. a hand-raked steel fork would be the icing on the cake...fat round steel tubes, fat stays, steel fork...

ok, brian smith...you go make doof a 57x56.5 frame with some fat stays, 55mm of trail, and a 73sta...and a steel fork...doof will give you a couple of cases of ber for it, ok? then doof can try it out and tell everyone how well a modern steel frame witha steel fork handles.


deal, bro?



* responsive ain't nervous. responsive means it does exactly what you want it to, when you want it to. nervous means it has a mind of its own...idho.