616 fatbike fork - rake questions
So I've been considering fork options to shave a bit of weight off my muk. What I realize now is that I don't fully understand the impact of rake on my bikes handling. What I do know is that for the most part I like the way my Muk handles (and in this particular area, steers). If I could make it steer just a tad quicker without making it twitchy, I would be interested in that.
I'm currently looking at this nice fork that 616 makes and it seems to me to be a good option, lighter than the Salsa Enabler (about 1/2 pound) but still made of steel. Then I see the geometry figures and realize I better get my brain wrapped around how that change in rake will impact my ride before I go and order one.
Fat | Six 1 Six Bicycle Fabrication
So can anyone explain to me in simple terms what kind of a change I could expect going from a fork that has 45mm of rake to one with 38mm of rake? If I'm understanding what I've read, the 38mm fork will make the bike more responsive in the area of steering, but just how much, I have no idea. Is 7mm of change in rake considered a drastic change or is it more of a subtle difference?
FYI, most of the riding I do is a combination of moderately technical single track and abandoned railroad bed riding. My muk is my only ride so it also gets used on lazy family rides on paved bike trails.
If the head angle doesn't change, then reducing the fork rake (offset) will increase the trail figure. The fork are virtually the same length, so the head angle will be barely affected.
I have some opinions which differ from current thinking so I won't bore you with them, but if you read Bike Geometry in Wikipaedia, you'll get a better understanding.
As little bike as possible, as silent as possible.
Latitude: 57º36' Highlands, Scotland
Thanks Velo. I did review the areas on Wiki's site that pertain to rake and how it impacts steering so I understand the jest of it. You did peak my curiosity when you said you have opinions that differ from current thinking. Might those thoughts be fatbike specific or in general? The reason I ask is because it would seem to me that alot of suspension geometry gets thrown out the window on fatbikes when you start dealing with air pressure down below 10 psi. I often find the sweet spot for single track is in the typical 8 or 9 psi range. With my 220+ lb body, that translates into a fair amount of squish where the tire / trail contact patch is concerned.
I went with a Carver O beast on my muk and I will not look back. The front end is quicker and a heck of a lot lighter. I trail ride mine with no worries. That might be an option you should look into.
More trail does not equate simply to faster steering, excessive trail causes very wonky unpredictale steering. The 616 frame looks to be built around 73degree HTA, thus the low rake. Putting a 38mm rake fork on a slacker fatbike with around 69 degree HTA will make for very long trail, I wouldnt reccomend.
Run the numbers at;
Bicycle Trail Calculator | yojimg.net
I would recommend not going with less rake on the Muk's slack Head angle. The resultant increase in trail and flop will cause the steering to be prone to self steer at slow speeds, where, as you lean into a turn, the front wheel will want to turn more sharply than you are turning. Basically, any time you are going through a turn, you are actually having to pull the handlebars back towards straight, just to maintain the turn radius.
A 38 mm rake is definitely more suited to a steep head angle. 73* sounds about right with a 29" tire.
With the 69* Muk, I would actually try to get about 50-55mm of rake, to help quicken the steering a little, and make it more balanced at slower speeds. I am not sure what is out there in that range other than the Fatty fork, and maybe a Jeff Jones, or a northpaw. The Fatty probably weighs as much or more than the Enabler. The Jeff Jones truss forks are awesome, and would be light, as well as improving the steering.
I do like the steering feel of a slack head tube, with a long rake fork, and I love the cornering ability at speed that the slack angle gives.
I wonder if 616 would custom fabricate their fork with a rake to my specs? Might be worth checking out.
That would be nice if they would do that. there might be an extra charge, especially if they normally use a fixed jig. They are made there in Michigan, and I'm sure they could do the increased rake with the same fork legs. they would just need to contour the ends that attach to the crown with a little more angle, and weld it up jigged to 53-55mm offset(rake).
The extra 5mm of height will also give you a little slacker angle(only by about .3 degrees)
I'd like to see a light steel fork like that with curved blades, to take up a little vibration, and shock, especially when running a bit of pressure in your fat tires. Maybe you could just get the rake up to about 55mm by taking a stock 616 fork and bending the legs forward 17mm.
edit: I mean have a professional frame/fork builder bend a radius into the lower fork legs of the stock 616 fork, to get another 17mm of rake.
I would also recommend Waltworks to make a nice custom fork.
I am sure there are a bunch of other custom frame builders who could also do a fat fork in a custom rake, maybe even getting the same blades 616 uses.
I bet 616 would do it, though.
I prefer steep headangles. I've done a bit of testing with various dimensions on my own bikes and I like a steep headangle (72-73º) with plenty of trail. My opinion is that is the recipe for a nimble bike that stays stable when you get caught in a rut. (You can get nimble with slack HAs, but I don't think they handle ruts so well). Bear in mind my opinion is entirely based on empirical testing with very little science involved, just how scared I could make myself.
Originally Posted by Chromehorn
However geometry on a fatbike can vary much more than on a normal bike. For example, your steering head height can vary depending on your tyre pressure and your tyre profile can change dramatically with rim width.
I'm interested to see that the new Specialized Fatboy is using 71.5º (if it is really the same as the Crave) and not the more fashionable slack angles. The reviews suggest that it steers well.
As little bike as possible, as silent as possible.
Latitude: 57º36' Highlands, Scotland
Jones offers curved-blade fat forks.
Originally Posted by autodoctor911
Give BlackSheep a call...or WaltWorks
If Huffy made an airplane, would you fly in it?
I do also like the feel of the steeper(72-73*HTA) frames with a fair amount of trail when going through diverse terrain at slower speeds, but when it comes time to shred some turns at high speed, like only a 4" tire will allow, the slacker angle feels more balanced to me, but I ride with my handlebars quite high, and most of my weight on the rear. that may be different with a lower bar, and more weight on the front.
Originally Posted by Velobike
I think a good compromise for a fat bike for all situations would be anywhere from 70-71* with whatever rake/trail is needed for about 30mm of flop. I think the flop has more to do with how it feels at slow speeds than the trail, because it seems the slacker head angles feel better with less trail than the steeper ones, which would make flop about the same.
One thing that stands out when making trail calculations is how fast trail increases when going from a 26x3.8/4.0 tire of about 29" diameter to a 26x4.7-4.8 tire of about 29.8" diameter. This would explain why when I rode a pugsley and a moonlander back to back, the self steer tendency on the moonlander felt considerably worse than the pugsley, even though the frame geometry was identical. The Big Fat Larry tires give about 3/8" more trail compared to a regular Larry on the same frame. that's with putting them on front and rear. If you put a wider tire on just the front, the trail would go up even more, since the head angle is affected.
For an apples to oranges comparison of sorts. Having ridden several Big Twin Harleys, a Wide Glide and a Road King to name two with opposite geometry. The WG has a 56* HTA and the RK has a 74* HTA, the WG has 5.2" of Trail and the RK has 6.7", an inch and a half more.
Originally Posted by autodoctor911
Would I be too far off to say that the WG could be likened to the Krampus and the RK, a Pugs;...with a stretch? At any rate, for me, the RK wins hands down! At parking lot speeds the WG was a chore to handle, and the RK a pleasure. At high speeds the RK had, at lest before a frame redesign, a tendency to get wobbly.
Despite the lower trail of the WG, the RK felt lighter in the front with more, and it is the heavier of the two. Which leaves me to wonder how to decide upon a good trail figure. Will be interesting to see CKs report with the Jones Truss on his Pugs.
Of note also is that the WG has a 80/90 - 21 front wheel while the RK has a 130/80(?)17, ie fatter wheel, to spread that weight over a larger area.
Hope I have not lost anyone by going too far afield, but bicycles and motorcycles are both single track designs, so what applies to one should be transferable to the other in a broad sense.
I guess what I am trying to infer, is that fat bikes by nature should accommodate steeper HTAs than is normally the case, but at what trail figure...85+?, 100+? Wonder if any will go in this direction?
CORRECTION: The HTA for the RK is 64*, and when I have more time, I will try to determine the fork offsets.
Last edited by Sand Rat; 07-17-2013 at 04:31 AM.
After plugging in some numbers to where I arrived at the stated trail figures...WG = 80mm in front of the steering axis, the RK = 12.7mm behind the steering axis.
Just when I believed that I had a basic understanding of trail and handling...
As I value low speed precision over high speed stability, and want to be in an upright position on the bike, I realize that in so doing the front end could be unloaded to the point of being undesired. However, this could be countered with say, the Jones loop bars and a little different riding discipline?
I gave the illustration of the WG and RK as a point of reference, of where my experience lies, so as to give a point of reference for a reply, if it helps.
So a question for VB...if I recall correctly, you like to steepen the HTA but like to maintain the stock trail. Are you doing so as a means to play it safe? The reason I ask, is because with the WG & RK, is room for an increase in trail being suggested? An increase in trail, ie, a reduction in offset does bring the front axle back under the rider to 'some' degree, helping to counter the unloading of the front by a more upright position.
If a short wheel base is desired, the real estate could begin to get a bit cramped, for fitting 29+ wheels. But there are ways to counter this also, if one is willing to go there.
Of course we are talking custom.
A long winded way to ask for a reality check on my trail/handling understanding, as the OP brought back to mind where I once stood. Hope this has been of value to him and perhaps others, to point out how one value can not be changed without affecting other areas; name your cure, pick your poison, so to speak.
Well, am I still sane, or do I need to go back to school?
Update - I finally got around to contacting 616. The fab department said they could make a fork that was essentially the same as my Muk's. I didn't get into specifics on geometry just yet, just wanted to find out if they would make something that was different than their standard fat fork. Waiting to hear back on pricing which I'm sure will be impacted greatly by materials.
More details. $300 for a custom 4130 CroMo fork, whatever geometry I want, and powder coated whatever color I want. From what I can tell, this fork should weigh just a bit over 1/2 pound less than a Salsa Enabler. Sounds like an ok option for me anyway as I just can't mentally make that jump to a carbon fork.
I'm still a bit torn on whether I want to change the rake though. I like how my muk rides currently. There are some rare occasions where I find myself wanting it to steer just a touch quicker on tight wooded singletrack. Maybe I just need to get a bit better at using my body weight to get the bike to turn quicker. What I don't want is to lose slow speed stability. Would it be safe to say that medium speed steering quickness may come at the expense of slow speed stability if I change the rake from the current 45mm to say 50mm?
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