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This is a bit off the cuff. I was watching a video of a performance analyst for cyclists. I'm not that into cycling, but I've been riding more recently and this piqued my interest. One of the things he mentioned off the cuff was that in terms of different racing categories, mountain biking / cross country style bikes are notably unoptimized in certain obvious areas. One being the front fork. Where on a road bike there's no suspension and all the reason to reduce drag, the fork will tend to get an aerodynamically optimized shape. On a mountain bike the fork gets a beefy spring or air based suspension, which seems to just get left as a cylinder.

Now I've got a 3D printer and some spare time, limited CAD experience, so I figured this would be an interesting project. Can I design a plastic part to reshape the front fork into a more aerodynamic shape?

A couple things I've found:

Here's what I'm assuming. I should look at streamlined bodies (the classic tear drop shape) for reducing drag since I'm not aiming for lift. In fact if the design produces lift it's probably a detriment unless it acts as a stabilizing force (during a turn?). Second, since a bike travels relatively slowly compared to a plane, I should be concerned about drag induced by the surface of the shape, so in theory I may want to create a "stepped" airfoil.

Now personally I don't go out mountain biking on steep dirt trails much, I do tend to stay more on flat ground, and so I can picture myself wanting to cruise at around 22 mph if I'm putting some effort and 12 mph if I'm being lazy. The front fork on my bike has around a roughly 8 inch tall, or 200 mm cylinder that I could attach to that's about 35.4 mm in diameter.

Where I'm at: I have Freecad installed and the cfdof analysis workbench, currently looking at an analysis of the NACA 0030 with a 108 mm chord. (35.4 rounded up to 36 and then I just multiplied by 3). I know I should've probably picked a NACA 0033, but I don't know how to make sense of what I'm staring at. There's U and P metrics, I'm assuming P is for pressure and U is for...flow?

Things I've yet to do: Assuming the right streamlined body shape, I should make sure the top and bottom of the airfoil aren't just flat, currently I'm just focusing on the "easy" steps first. Given that's the likely bad part of the current shape I'm assuming the brightly highlighted red edges are an indicator of some issues around that area.

ParaView NACA 0030 Analysis

Edit: Have looked at xfoil, entered what I assume to be the right reynolds number at sea level for 22 mph wind and produced this chart. xfoil naca 0030 oper v 139862 alfa 0

Struggled a minute to reinput a different reynolds number for 12 mph wind, and xfoil produced this... enter image description here I'm assuming here that the key metric is the Cd term, which appears to have dropped a bit between the two.

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  • $\begingroup$ There's an old but neat tool called xfoil. Give it a try. You can play with different geometries, angle of attack, etc and get drag coefficients. Might help you understand some of what's going on. $\endgroup$ Commented Oct 29 at 0:51
  • $\begingroup$ @Abel I think xtool is making an assumption that the shape is travelling like a wing. I'm assuming L/D is lift/drag. If I try shorter / stumpier naca shapes like 0035, L/D is no longer 0. Is there a way to tell xtool to ignore gravity / rotate the shape? $\endgroup$ Commented Oct 29 at 3:28
  • $\begingroup$ In reading more on xtool it appears to assume that the input shape has a chord length of 1 (meter). So I'm supposed to scale the reynolds number used, so the pictured plots are presumably incorrect. $\endgroup$ Commented Oct 29 at 6:08
  • $\begingroup$ There is no gravity in xfoil. Directions of drag is always in line with airflow. Direction of "lift" is just a force perpendicular to that - has nothing to do with gravity. Angle of attack is one way to rotate. Unit of 1 could be meters, feet, or any made up unit. One approach is to make sure you convert all length units to it (meters is usually easiest), not that it matters in your case since you just want to compare geometry. $\endgroup$ Commented Oct 29 at 7:44
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    $\begingroup$ What are the losses from the mis-shaped forks compared to tyre friction ? Seems a general analysis may allow you to find better areas to exploit. $\endgroup$ Commented Oct 29 at 8:20

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