Front end geometry help needed

bissona

Active member
Patreon
Oct 14, 2018
137
106
Guernsey
Am hoping to make some mods to my 2018 Jam2 Ltd that will speed up the steering, but don't want to order anything before getting some input from those more knowledgeable than me!

For reference the bike is a standard XL Jam2 with a 27.5 x 2.6 Minion DHF on the front, usually around 21psi. Rider weight is c.90kg.

The problems I'm having are;
  1. the front tyre really wants to wash out when cornering on looser surfaces
  2. it also has a distinct desire to flop to either side on turn in
  3. I'm getting juddering on the front under hard braking, as if the Recon RL's 32mm stanchions really aren't up to the job
  4. the steering is slower than I would like, although not by a huge margin.

If I ride with my weight a long way forward then things improve, especially in terms of grip, but it's not viable to ride like that all of the time. I also rarely ride DH at speeds that would take advantage of the bike's fairly long geometry, so would like to change to slightly quicker steering all round.

On motorbikes I would usually start with increasing rear ride height, but this obviously isn't an option on the JAM2, so I'm looking at fork offsets to start with but open to any and all suggestions.

ETA: I'm currently looking at the RockShox 35 Gold as a possible option to try (not too pricey, fair bit beefier than the Recon RL), but the offset question remains puzzling, despite/because of this article by Worldwide Cyclery... Increasing offset decrease trail but " While a slack headtube increases stability, along with longer reach, you start to lose that front end feel and thus, losing traction. By reducing the forks offset, you are bringing the wheel back under the mass of the body, helping keep that traction, while still having a longer wheelbase and slack head tube angle."
I get that in terms of gut feel, but my mind keeps telling me that the increase in trail can only result in floppier steering. Officially confused!
 
Last edited:

MattyB

E*POWAH Elite World Champion
Jul 11, 2018
1,274
1,301
Herts, UK
"(Fork offset is...) The distance between the front axle and the fork’s steering axis — the line running through the centre of the head tube, about which the fork rotates when steering.

Fork offset is made up by the forwards sweep of the fork crown and the placement of the axle in front of the lower legs.

Fork manufacturers now offer some forks with multiple fork offset options. RockShox produces 37mm and 44mm offsets in 650b, and 42mm or 51mm in 29in forks. Fox forks have similar numbers.

Fork offset affects the trail (see below). Longer offset results in less trail, which makes for a quicker but twitchier steering feel. Conversely, shorter offset forks increase the trail, which makes for more stable steering especially in steep corners or bumpy sections.

The fork offset also affects the front-centre (shorter offset means a shorter bike), as well as the distance between the rider’s hands and the front axle. For this reason, increasing the fork offset can feel a bit like a shortening the stem, in that the front wheel is further in front of the hands."
 

bissona

Active member
Patreon
Oct 14, 2018
137
106
Guernsey

Thanks for digging those out. They're all good articles, but the most apt part was in the first one, near the bottom:

"Shorter offset also reduces the ‘floppy’ feeling that can occur when tackling tight corners, where the wheel can feel like it wants to tuck under."

That's exactly what I've been fighting, so will keep shorter offset on the list.
 

Jam2rider68

New Member
Sep 24, 2019
47
56
Tewkesbury
run tire higher pressure 30 to 35 psi plus size tires have weak side walls and move around a lot in corners. I got a jam 2 plus and run my Maxxis on 35psi and bike handles superb on all surfaces.
 

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