Ask a Mechanic: 29er Forks – 46mm or 51mm Offset?

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Question: I am looking at ordering a Rock Shox Pike and saw that Art’s has 46 and 51mm offset versions available. Which one should I get? I have a Santa Cruz Tallboy LT2. From: Trevor

Fork offset, a complete mystery to many, is really rather simple once equipped with a bit of background knowledge.

Answer: You are not the first, or the last person to ask me this question Trevor, as it perplexes a lot of people. In order to understand which offset is right, it is helpful to understand the theory that gave birth to the two offsets.

As headtube angles have slackened over the years to add stability to bikes at high speeds, there arose a need to remedy the accompanying wheel flop that makes climbing difficult and singletrack a little more cumbersome. Add a 29″ wheel to the mix and it magnifies the problem. The solution is to increase the offset (sometimes called “rake”) of the dropouts on the fork to push the front wheel out further. As the offset is increased, steering feel is quickened. This is why early 29ers had really steep headtube angles. Case in point, the original Intense Spider 29er had a 73-degree headtube angle! With the small fork offsets, the only way to make the steering feel light was to use a steep head angle.

The smaller of the two offsets you are considering (46mm) is already a big increase over the offset used in the past. Early Rock Shox forks had a 25mm offset and many current 26″ forks use 40mm offsets. More recently Gary Fisher helped to push the envelope of standard fork offset with his G2 geometry, featuring 52mm offset Fox forks, that aimed to make 29er steering feel as light a 26″ bike would feel with a similar headtube angle. Hence the 51mm offset Rock Shox forks that we have now. Does this mean that you need a Fisher or Trek/Fisher to benefit from a 51mm offset? No. It’s a good choice for anyone that is looking for a quicker-handling bike. In fact, 51mm offset forks are standard spec on the Intense Carbine 29 and the Specialized Enduro 29.

There’s something to be said for the usefulness of the classic black-and-white diagram.

However, there is a point where you can have too much of a good thing. 29ers already have long wheelbases, especially the new crop of long travel 29ers. Adding 5mm of offset to quicken the handling makes an already long wheelbase even longer. The added wheelbase is nice on the steep stuff or in the rocks, but when you get into tight switchbacks, it can be a handfull. Extremely slow speeds are another area where the 51mm offset feels weird with headangles slacker than 68.5 degrees. Bottom line: you can use increased offset to cheat the feel of a steep head angle only so much before you create the problem you are trying to fix, or move the problem to another area of the riding experience.

Here’s my two cents. I think we need to let go of the idea that the 26″ steering feel is “right.” The truth is that it is just established and isn’t right or wrong. The virtues of 29er steering should be judged on its own merits. After riding long travel 29ers with both offsets, I’m currently in favor of the shorter 46mm offset. I like the maneuverability of the short wheelbase in switchbacks and I like the slower, more stable steering it offers on flatter trails. I also like that it places more of my weight on the front wheel to keep it hooked up in the corners.

So, the short answer to your question is that if you want your bike to handle more like a 26″  wheeled bike at singletrack speeds and are willing to give up some performance on switchbacks, go with the 51mm offset. If you want your bike to handle the same way that I like mine to handle, go with the shorter 46mm offset.

If you still can’t decide, just keep in mind that all we are talking about is 5 millimeters of difference between the two, so don’t expect a night and day difference between them. Whichever one you choose, you’ll have the perfect geometry at some point on nearly any trail you ride.

Daniel Slusser is a professional bicycle mechanic with over ten years of experience. He holds a bachelor’s degree in business administration from HSU and a master’s degree in history from Cal Poly. When he is not riding, wrenching, or writing he enjoys spending time with his wives and two children.