44mm vs 51mm Fork Offset
Many clients use our custom builders to quickly spec the parts they need for frame swap projects. That’s awesome. We’re glad to dial in everything from frame swaps, factory builds, semi to full custom bikes with our clients. We have had A LOT of clients ask about utilizing previous generation offset forks to their new bikes. Let’s take a look at the advantages of the lower offset forks, particularly the 29″ 44mm versus 51mm offset.
I guess let’s start at the end.
29″ bikes have reached an amazing level of performance. Modern 29’s pedal amazing while being much more behaved on the descends. We see smaller riders enjoying the current generation of big wheel offerings. Compare this with the days when Small 29’s weren’t even made because they scare riders out of the sport!
The early all-mountain 29ers were romantic. But like your early heartbreaks, when you look back, they really weren’t that good!
Early 29’s didn’t really want to change direction. Their trail capabilities came a lot from the wheel size’s ability to simply monster truck over terrain.
So what’s the big change? Two major points are confidence and control.
Riders feel like pilots rather than passengers on bikes like the Ibis Ripmo, Yeti SB130 or SB150, Alchemy Arktos 29 or Transition Sentinel.
How’d we get there? Well the bikes mentioned all have excellent suspension platforms and cutting edge geometries.
Math… OK, we all talk about numbers in MTB. But we’re going to look at a little bit of math to see what the numbers we talk about really generate.
(as a side note – cockpit concept has played a big role in this but it’s another subject for another day. Today it’s forks…)
Bikes have gotten longer, lower and slacker. Ah, well, yes slacker than back in the day. But less slack than they could be if 29″ forks didn’t adapt.
Similar concept with length. The 44mm offset helps balance top tube length versus wheelbase. Let’s see how 44mm versus 51mm offset shifts geometry design.
Trail, Mechanical Trail & Offset
On the front end of the bike some of the major “personality” factors are trail, mechanical trail as well as wheel base.
Trail measures the distance between the steer axis’ contact point with the ground to a line dropped from the center of the rim.
Mechanical trail is a perpendicular measurement of the contact points.
The factors effecting trail (and wheel flop – but we’re not going into wheel flop today) are wheel radius, headtube angle and fork offset.
Trail is critical to a bike’s balance of maneuverability as well as speed capacity and tracking. There is a window of accepted personality for All Mountain / Enduro setups and more XC / Trail setups.
Less trail makes a quicker handling front end. Increased trail provides a more stable platform. Finding the balance is critical to have a bike that’s playful and fun through a variety of speeds.
Decreasing offset will increase trail.
44mm vs 51mm Offset
Here is an illustration of trail measurements between the 44mm vs 51mm offset fork.
On the Alchemy Arktos 29, with a 65.5 degree headtube angle, the 44mm offset generates 119.5mm of trail while the 51mm is 6.5% less at 111.8mm.
Mechanical trail has a similar difference. 44mm offset has 108.7mm while the 51mm offset comes in at 101.7mm.
This means that the 44mm offset will have a bit more stable platform.
If we wanted to design the same amount of trail into the 51mm offset fork we would need to decrease the headtube angle.
In the case of the Arktos 29 it would require approximately a 64.4 degree headtube angle to equalize the trail for a 51mm offset fork.
Equalizing Trail with Headtube Angle
As illustrated above we now have the same trail and mechanical trail measurements. However the illustration shows the next query for a designer. Changing the headtube angle modifies one of two other things in the bike’s geometry.
Top tube length or wheelbase. Which happen to also be quite chatted about bike geometry numbers!
So you can decrease top tube and reach as shown above or you can stretch the front wheel further in front of the rider.
Using the 44mm offset fork with a 65.5 degree headtube angle and the 51mm offset with a 64.4 degree headtube creates about a 20 to 21mm increase in the wheelbase.
Increasing the wheelbase and slackening the headtube limit a bike’s ability to load the front wheel and track correctly. This creates a bike that requires a more front heavy riding style, such as VPP suspension.
Wrapping it up
Modern bike geometry is a complex balancing act for designers. The new standard of 44mm offset forks has helped create more intuitive, fun to ride bikes. The Alchemy Arktos 29, Ibis Ripmo, Transition Sentinel as well as Yeti SB130 or SB150 have blurred the lines of full speed capacity, nimbleness and playfulness.
Think of it in terms of your personal bike setup. Would you notice a 20mm stem modification? Probably!
So if you’re building a new bike go with the offset that the designers spec’d. If you’re looking to be a bit slacker, etc there are options on fork travel that are appropriate on many of the current bikes.
If you’re doing a frame swap – well if it’s in the budget go for the new fork offset. Sell the old fork with the previous frame. If your budget is stretched your 51mm older fork will “work” – but it does give up some performance.
When I work with clients we often pencil out the “good money after bad” concept a bit. How much value does the old fork have on the used market? Is the steer tube long enough for the new bike? Does it need service? Many times you check off the boxes and suddenly “updating” the previous fork to fit is in the neighborhood of the new fork you’re shopping.
Questions? No problem. Our expert sales team is here to get you dialed in on the best spec, setup, tune as well as pricing in MTB. Chat, call or email to go over your build.