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Th effects of fork offset and trail on handling.

Orfen

Monkey
Feb 22, 2004
259
0
UP, michigan
Do you notice a difference when riding forks that have different offsets?

Longer trail gives you a more stable bike at speeds, but also a sluggish steerer. A short trail gives you that quick handling feel but the bike might be a bit twitchier at speeds.

Just like a slight variation in head angle, chaning the Trail by a few millimeters really affects the bike's handling characteristics.

what is the norm for DH? anyone know? slack (rake) head angle with short trail better? steeper head angle with longer trail?

discuss..
 

yopaulie

Monkey
Jun 4, 2009
165
7
NH
I just went from a Avy DHF 8" ti to a 2010 Boxxer wc and the fork offset between the two is completly different. To be honest at speed I really didnt feel much of a change but when it came time to turn the steeper head angle and shorter offset made a big difference. I think that if you start to go below 64 the bike will get floppy and not turn as well, conversely if you go above 66you start to get xc ish(bad on the steeps).
I dropped about 3 pounds off the front end and that is huge but the shorter offset and steeper head angle made my bike come alive(Glory 8).

And yes I did join if for no other reason than to reply to this post.:biggrin:
 

ridingsupreme

Monkey
May 12, 2008
125
0
Santa Cluas lane
My bike adjusts via/ HT sleeve. Steeper head angles are deafinatly better for turning on most "trails" but there is a loss to stability at speed as it also shortens the wheelbase. Also if you go through a rough section you may find it being more work to hold a straight line and keep yourself from going OTB. Lastly it makes the bike feel more nimble and give more output to the rider so naturally its gonna take more input. Trade offs you know?

On the other hand having a slacker HA you gain much more stability and it shouldm't require as much to make it through something a bit rough or anything you may hit at speed. also note this lengthens the wheelbase. I find slacker makes drifts at speed much more controllable and safe as well. also this requires a little less input through rocks or anything high-speed or technical makes things nicer for anything steep/fast.

but hey don't take my word for it! you know whats up so whats to discuss! :) Ride!
 

yopaulie

Monkey
Jun 4, 2009
165
7
NH
I think it comes down to the longer the offset you get slower steering but more stability. A shorter offset gives you faster steering and less stability. It seems to have the same type of effect that a steeper/slacker ha would.
How it exactly relates to "trail" in terms of a technical explanation I haven’t a clue. If you have the chance try both extremes as that is what really matters, how it feels on the bike.
 

ChrisKring

Turbo Monkey
Jan 30, 2002
2,399
6
Grand Haven, MI
Don't be too quick with the thought that more trail equals slower steering. It deffinately is more stable with more trail. However, more trail on the same frame also equals more weight on the front wheel which will result in more bite (less tire scrub).

Anyway, there is a lot more discussion on this topic in MX forums due to the availability of aftermarket crowns (clamps) with different offsets.

I can't say for sure on a DH bike how much I can tell the effects of changing just the trail since I do not have multiple offsets options to quickly play with. Sure, I can adjust fork height and can surely tell a difference. On my MX bikes (KTM 250SXF, 450SXF), they come with ecentric steer tubes so that you can run either 18mm or 20mm of offset by simply turning the steerer 180 degrees in the crowns. It takes about 15 minutes to change. Anyway, I prefer the 18mm offset in all cases since it increases trail (stability) and increases front wheel weight bias (turning traction). Some people dislike less offset due to fork dive under braking resulting in the fork "knifing" or "tucking" which results in a high siding off the bike. Josh Grant reportly has tested over 10 different fork offsets with many different suspension settings to arive at what they are using. I haven't heard of anyone in DH that does the level of testing that Pro MX'ers do. I really wish we could get a couple of the lurkers here to post up on this but I am sure they won't.
 

offtheedge

Monkey
Aug 26, 2005
955
0
LB
The rider position and stem length will have a huge effect on the desired amount of trail as it pertains to the bikes center steer. If you are over the bars, then the effect is less noticeable. If you are back off the seat then you are increasing the function of trail and center steer. This is one of the obvious reasons DHers would run short stems on heavily offset forks. Add in the grade of the hill and it is easy to have a perceived center steer well in front of the front wheel.....lucky for me my slow reaction time compensates for the bust ;)
 

ohio

The Fresno Kid
Nov 26, 2001
6,645
6
SF, CA
The internets are definitely the wrong place to get the answer to this. To be clear, before anyone else sticks their foot in their mouth:

1a) Increased offset = decreased trail = faster handling despite longer wheelbase.
1b) Decreased offset = increased trail = slower handling despite shorter wheelbase.

2a) Increased/steeper head angle = decreased trail
2b) Decreased/slacker head angle = increased trail

Please, continue.
 

William42

fork ways
Jul 31, 2007
3,756
426
The internets are definitely the wrong place to get the answer to this. To be clear, before anyone else sticks their foot in their mouth:

1a) Increased offset = decreased trail = faster handling despite longer wheelbase.
1b) Decreased offset = increased trail = slower handling despite shorter wheelbase.

2a) Increased/steeper head angle = decreased trail
2b) Decreased/slacker head angle = increased trail

Please, continue.
Listen to this man.
 

Sandwich

Pig my fish!
Staff member
May 23, 2002
18,171
2,315
01776
I can't speak to the technicality side of the discussion (ohio) but I honestly felt a huge change coming from my Avy-equipped Brooklyn to my new boxxered Sunday. There's about one inch of difference in the top tube, and something like 15-20mm of offset difference (the avy is like 45mm, the boxxer is what, 25?). The biggest change is in slow, tight situations. The Sunday feels much less "floppy" and I've made far fewer mistakes in picking lines and such. Both bikes felt pretty similar at speed, perhaps because of the slacker angles on the Sunday.

Unfortunately, it's difficult to place the blame exactly, since the differences between the two bikes and their builds are huge. It would be great to see an eccentric head tube or at least a definitive test between forks.
 

bdamschen

Turbo Monkey
Nov 28, 2005
3,350
123
Spreckels, CA
I'm a fan of more trail. That way my runs are longer... the downside of course is that you get more tired. Less trail really is no good since then you have to ride your bike on pavement or maybe even a sidewalk.
 

bushrider

Monkey
Jul 4, 2006
146
0
NYC
I've ridden boxxers, shivers and Avalanche forks mainly.

My understanding is boxxers have about 40mm of offset and the avy forks have about 70mm.

I think the effect of the offset/rake depends on how you turn.

I have in certain situations (mainly lower speed tight corners) noticed that a fork with more offset like the Avys has a tendency to flop or tank slap. What I mean by flop or tank slap is that when steering around a tight corner the fork has a tendency to over steer suddenly when your turning the bar. Its like your pulling on the bars with your hands/arms to turn the fork and then the fork all of a sudden turns too far often causing the fork bumpers to hit the frame (tank slap).

I do not have a solid conceptual understanding of the effect of rake/offset. However my impression is that a larger offset is a benifit to handling when turning at high speed where the turn is executed by the tires contacting the ground at an angle rather than the turn being executed by turning the handlebars. I know from extensive time riding both an Avalanche DHF8 and Boxxer WC that the Avalanche feels more stable on highspeed corners and feels like it has a tendency to flop/tank slap in tight corners where as the boxxer seems to handle fast and tight pretty equally.

I currently ride an Avy and have found that by focusing more on leaning the bike in lower speed situations that the flop/tank slap effect is minimized.
 

ohio

The Fresno Kid
Nov 26, 2001
6,645
6
SF, CA
Bushrider, thanks for that. It's a very good additional point is that what I wrote is applicable when you're moving at a sufficient speed that the trail creates a caster-effect and self balances the bike. At low speeds, the effects of these geometries are very different. Others probably know better than me, but I guess the threshold speed is around 5-8mph... about a jogging pace.
 

MttyTee

Monkey
Jun 20, 2007
209
0
Back on the east coast!
I'm not sure this subject really has a concrete answer since there are multiple variables which can all produce very similar results in the end, and even though the end results are similar, different set ups of the variables can make small improvements in other areas that may work better in different scenarios.

Total fork offset, head angle, valving, springs and tire diameter all play together to equal what's really important, a dynamic trail measurement. Meaning what is the trail measurement at some point while you are riding. Like static sag, static trail measurements only give you an idea of what is going to happen when you are actually riding.

Example, super slack head angle and minimal offset will give a large static trail measurement which should be super stable, but without proper springs and valving when you hit your front brake going down a steep hill and try to turn the trail measurement will decrease too much and the bars will want to be ripped out of your hands or the front end will knife. CK, this is most likely what the opponents to less offset are feeling to some degree. As with any math equation you can't adjust one variable without adjusting another and expect to get the same results.

The amount of stuff the test in MX is crazy. Most factory teams, including "factory" satellite teams like JGR have the ability to adjust the offset at the clamp or at the axle, 1/4 degree angled races to adjust head angle as well as just about any other wacky thing (I've seen clamps that have an angle to the fork tube, relative to the steerer tube).

That's my 2 cents
 

MttyTee

Monkey
Jun 20, 2007
209
0
Back on the east coast!
This thread got me interested.

I found this website that goes into a lot of depth on the subject:

http://wotid.com/tls/content/view/24/53/
A little linking from that site got me here
http://www.msgroup.org/forums/mtt/topic.asp?TOPIC_ID=10229
I've never seen this put into graphical form, but this is an interesting fact that many may not know. Basically the slacker your head angle the less steering input you have, one of the reasons choppers and super slack bikes steer like poo and feel lazy. Interestingly the positives for offroad use is that deflections at the wheel equal less bar twitch, but as with anything there is a balance. My personal opinion is modern DH bikes are getting too slack and I'd like to see more companies, factory and/ or aftermarket, offer different offset clamps.
 

Sandwich

Pig my fish!
Staff member
May 23, 2002
18,171
2,315
01776
so i was just looking into this via the googlenet and thought I'd bump it. I was wondering if anybody else had further insight into the effect of offset on steering as we progress into the era of really slack head angles, and this thread came up.