Homemade Motorcycle Tail Section

[Frank]

I've been involved in land speed racing for a number of years, mostly with the Loring Timing Association, which races at the old Loring AFB in Limestone, Maine. I've run a number of bikes and have gone over 200 mph on my electric Hayabusa (E-Busa) with OEM bodywork (with the exception of the front fender). I've been thinking about improving aerodynamics for quite some time, have asked questions here, looked at the Template, etc. and have been paying attention to fast bikes using commercial bodywork, mostly LSR fairings from AirTech. These fairings are quite good - possibly *too* good - which is what led to this project.

There seems to be a lot of experience with the commercial fairings indicating they are extremely susceptible to any sidewind component. There have been crashes and scary moments as described by the riders. I don't have any data on Cg/Cp locations, but the tails seem pretty big and I would expect this relationship to be okay, plus AirTech has spent time in the wind tunnel and I'm sure are quite aware of how important this is.

My theory is that the fairings are excellent airfoils which can develop lift fairly easily with anything but a dead-on head or tail wind. One of the NASA simulator programs (I think I used FoilSim) showed it wouldn't take much to develop 100-200 pounds side force. Accordingly, I decided to build something that was less than optimal, hoping to avoid the crosswind susceptibility issue while simultaneously improving performance.

Does this make sense to you guys?

[Frank]

Here's some pics of what I built (taken at the 1-mile point).

[IMG]LTA 2019 Harvest Event by Frank, on Flickr[/IMG]

Commercial fairings provide a cut-out for the riders legs and feet; I'm sure this is better than what I did, but so far I'm very happy with the results.

I should add these runs were made using a lower voltage battery pack than the pack I used for the >200 mph runs.

[Frank]

There was a sidewind all day of almost 10 mph. What was interesting was the run with the tail was not only quicker (by 5 mph) but the bike felt more stable. You can see by the little flap at the back of the safety vest I was wearing that airflow looked improved, even when the vest itself is flying around (it needs to be adjusted better). The most interesting thing was the coastdown tests I did: I computed speed for 15 seconds (based on motor rpm) with regen shut off. The bike slowed down 13 mph more without the tail! I'm planning to make it streetable this summer and get real-world range and consumption data.

Thoughts, comments, etc. appreciated. Happy New Year!

[IMG]LTA 2019 Harvest Event Graphs by Frank, on Flickr[/IMG]

[freebeard]

Thanks, but no comments?

Nothing but respect for what you are doing.

It's hard to see from the single pic, but if that tapers to a vertical edge, it's way too fast. It would be [partially] filling the wake. A truncation would give you 80-90% of the full result. In my imagination, a vertical ducktail spoiler, or Gurney flap, on each side would 'spoil' the crosswind effect. Possibly a shuttylecock effect. Prove me wrong?

I don't have a reverse tricycle yet, but I've been thinking similarly. I think the foundation will be 'turnpike cruiser' style fender skirts that clip together at the back to form a closely fitted wheel spat. The upper part as you might expect. HTH.

[Frank]

Hi freebeard,

It does taper to a pretty sharp point, about 16" change in width over the 30" length which I think is about 14* per side. I chose a sharp closure thinking I might try some VG's to keep flow attached. One reason I chose not to implement full leg cut-outs was that the initial width could be less. The width at the back of the front fairing is wider (22"?) so it's in the shadow of that. Part of the plan for next season is to tuft it and figure out how to take some pics and if necessary try some VG's to see if they do anything.

I'm limited by rules in that the back of the tail cannot be more than 10" from the back of the rear tire, and the height from pavement has to be between 4" and 40". Much lower than that and you can have trailer loading problems and interference issues when cornering (don't ask, lol).

The surface finish is pretty rough: is it worthwhile to try to smooth it out? It's been laid up with biaxial 1708 and polyester resin. I suspect the rough finish might keep the boundary layer area energized but don't really know. Reynolds number is pretty high. So much to try to understand. Thanks for your comments.

[freebeard]

If it's a wedge that's widest at the front, consider more of a bullet shape with a leading edge fillet to promote re-attachment with some taper leading to a boxed cavity.

I'm not sure about surface roughness. I believe dirt and duct tape don't have much effect, it would involve Reynolds number. 'Attached turbulent flow' is a term aerohead has used.

[Frank Lee]

Originally Posted by freebeard

Thanks, but no comments?

I forgot about the contest.

[aerohead]

From a safety standpoint I don't want to recommend anything that the Vetter Challenge folks haven't 'proved' already.
Gusting wind could force an 'out-of-lane' correction which could cause a collision in traffic.
If you had the means,grad students at Cal Tech's wind tunnel and Mathematics department might take on your project,if you could provide a model,as they've done for land speed record bikes.
A trike would be far simpler challenge.

[Fat Charlie]

Originally Posted by freebeard

Thanks, but no comments?

I didn't have anything to add, but am interested and subscribed. Simply adding thanks shows that people are seeing and appreciating it while keeping the S/N ratio up.

[Frank]

Thanks everyone - I'll post a "plan view" some time and update next spring once I get some consumption data.