Regarding Aero Forks and Aero Frames

posted Oct 14, 2010, 7:35 AM by Donald Vescio   [ updated Sep 12, 2011, 8:12 AM ]
Recently, I entered into a discussion regarding the aerodynamics of a specific fork, Oval Concept's A900; this discussion quickly evolved into a more wide ranging examination of fork, wheel, and frame interaction.  Many of the concepts that I touch on may be of use for one who might be looking for a new TT/Tri frame and fork.



Very true--these are nice, stiff forks [Oval's A900] that test fast, despite the fact that there a some who claim that the fork's slots are oriented the wrong way. I found the fork to be faster (say 10-12 seconds for 10 miles) than a stock Cervelo Wolf fork, and I've also found it to be a bit quicker than the 3T, depending on the front wheel used. I think that it would be a good match for many second generation frames, like a 3C, Planet X, etc. 

The Oval is an excellent match for my P3C and I plan on using one in my 3T for the track; it didn't match as well as QR's stock fork on my CD 0.1, but the CD 0.1's downtube has a sideways bend that might make the frame more sensitive to fork design.

If you need a new fork and the price is reasonable, worst case, you most likely will have a better build than the stock fork that you have. Now if you have one of the current generation frames, like Giant, Specialized, etc., with the highly integrated front end, then maybe not so much.

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What I've found, in general, is that forks the have a close profile to the wheel/spokes tend to do better with deep rims; spokes that have a wide profile work really well with composite spoked wheels and many deep rims. I think that the vents are for marketing, looks; the Oval, does, though, work very nicely with the Hed trispoke. 

It's interesting to look at Kestrel's and Quintana Roo's forks--both bow out away from the wheel, providing lots of space. This is a different design philosophy than 3T's Funda, but it seems to work. When tested in aggregate, complete builds with the same equipment, I found that Kestrel's Airfoil was as fast as my P3C (at least within the margin of error), despite the Kestrel having a downtube that is shaped very poorly. It seems to me that the key is the front end of the bike.

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Yes, you're right--I've run Cobb's Time Bandit fork (7:1) ratio on my P3C, which looked *very* ugly, but it was fast. So ugly though that I actually was glad that it isn't legal under UCI rules ;)  Cobb's fork is very, very quick on most bikes, but it looks out of place on any frame that I've ever seen it on.

The other post regarding the back of the fork crown is dead on, at least from my experience. There does, though, seem to be a couple of different ways to address the issue. One is to blend the fork crown into the frame to minimize any discontinuities; the other is to intentionally design a large gap between the fork crown and the frame. My Quintana Roo CD 0.1 is designed as the later. In the case of the CD 0.1, I was shocked to find that the bike performed best--extremely well--with the stock fork and rear mounted brake. Some smoke trail testing showed interesting airflow dynamics. For most fork and frame designs, mounting a brake behind the fork is a disaster--but there are some exceptions, like QR (and I suspect a couple of others).

Re: front disc--when I run a front disc on the track, I tend to run a narrow spaced fork so that the fork blades are as proximate to the wheel's surface as practical. I know that there is a lot of debate as to whether large composite spoked wheels (IO, Hed) create pressure waves when rotating through a fork. Not having seen one of GB's frames up close and in person, I suspect that the design was optimized for a specific set of wheels, with the additional assumption that using an alternate wheel, like an IO, might not be optimal, but still acceptable. 

It's interesting that among some of the fastest forks that I've used have been simple steel forks with a 3:1 or 4: 1 ratio (Hooker; there's a guy in England who's making similar forks, can't remember his name). Think of the old Schwinn Varsity stamped steel forks--very heavy, very weak, but very fast.

I'd even go so far as to say that the Oval is one of the fastest UCI legal after market forks out there. For non-uci aftermarket forks, I'd go with Blackwell's Time Bandit (ugly, but fast) and Kinesis' old aluminum aero fork (heavy, but fast). Unfortunately, you only can get these two forks used these days.

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You know, I've pretty much come to the conclusion that the differences between today's top UCI legal aftermarket aero forks are not that huge--given the parameters associated with a fork's design, and UCI constraints, there just is so much that one can do with profiles. So, if one has a 3T or an Oval, I'd worry mostly about handling characteristic than absolute aerodynamic purity, as they both will be close enough in performance not to matter too much. 

Now, if one is *not* constrained by UCI, *many* more possibilities open up. I suspect that we'll continue to see the trend toward integration of fork and frame design on higher end TT bikes--one would buy a "system," rather than a frame and fork.


EDIT: This, then assumes that a fork's design might be factored into wheel selection. We're already seeing this on the rear end of TT bikes--push to narrow the profile of chainstays, toroidal rims might not fit....

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EDIT: Should have begun with this--I agree that it's the front end of the bike that's most important; the rest of the frame follows, so to speak, on many different levels.

At the very top, pointy end of frame design, I'd say that there mostly likely isn't a huge performance difference between a Shiv, Speed Concept, the new Felt, etc.,  provided that sound principles are used. If sound principles are not used, if a bike is designed for style, then performance will suffer. For instance, Argon's 114 has a highly integrated front end (good), but it has a massive diamond-shaped downtube (not so good). I would suspect that it would be a few notches below the bikes above. The rectangular profile of some of Kuota's frames look great, but won't be all that fast.

With UCI design constraints (and even though most don't ride under UCI rules), there's just so much that one can do by shaping tubes and working on profiles. I think that we're starting to see a maturity of development in TT frames/forks, though there are some manufacturers who are trying to think outside of the box by looking seriously at airflow management.

I'd say that if one buys a top model from one of the big five manufacturers, I wouldn't worry about differences between frame aerodynamics--they's all be pretty aero. I'd focus more attention on optimizing body position, etc.

Note, thought, I am not saying that my old '80s custom round tubed frame would ever be as fast as my P3C, CD 0.1, etc.--what I am saying is that if one buys Cervelo's, or Felt's, or Giant's, or Specialized's top TT frame, they'd be at parity with the other top performing designs. 
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