Frog Man

Hello,

I want to build a small/light aluminum/chromolly framed ebike for an efficiency challenge but don't know what tubes to use.

Like this but with bicycle components.


My competition will use road/mountain bike frames so I need a BMX sized frame that is lighter than a road/mountain bike frame.

Here are the details.

Power: 500w mid drive (average 370 watt over 1 hour)
Rider Weight: 50kg rider during race (100kg rider [me] everyday)
Average speed: 40-55km in an hour (we won't know until the race).
Pedels: I need pedals to make it street legal ebike but the pedels will be removed for the competition.
Wheels: I am thinking low rolling resistance 20" BMX wheels since that will mean better aerodynamics.

I have a heavy fabrication friend with a lathe and welding stuff and the skills to put it together but I need to come up with the design.

Let me know what you think? Advice etc. Thanks.

Edit: Rule Book: https://www.hunterevfestival.net/uplo..._class_web.pdf

CliffordK

Look at recumbents and velomobiles. Do the rules specify the type of bike? The recumbents get you very low, and you can pedal (it would be hard to pedal in the photo you have).

Frog Man

Unfortunately there are two rules that make recumbent bicycles ineffective. 1) You have to be able to easily stand on the pegs. 2) You can't be enclosed by the vehicle (rider visible from side and top). This is a shame because recumbent faired bicycles would be king (and comfortable).

Picture for fun.


I'm thinking I should design a gravity bike except lighter since gravity is the enemy of an ebike.
Example BMX frame gravity bike.


It'll be like this but at 40-55km per hour
https://www.youtube.com/watch?v=KJweO0c_8u8

unterhausen

it seems like a reasonable idea. However, you didn't really ask any question. There is a sources thread at the top of the forum. I suggest looking at Nova and getting whatever you can from them. If you can't use a bicycle tube, get some 4130 tubing from Aircraft Spruce or Wick's.

Andrew R Stewart 

I would be careful about how light/thin walled tubing you use, especially at the motor mounts. Unless there is a weight requirement or significant hills having a couple more pounds in the frame will quite likely be a good design. Not only to better handle the power but also to allow for more heat cycles and grinding away as you figure out that some bracket really should be over here instead of over there.

Your OP is a bit confusing as to what you'll be using, or not using. I looked at the rules and while there are a lot of dimensional and power capacity ones there is very little about other components. Sounds like a lot of fun. Also sounds like there'll be a lot of student brain power (and their parents/teachers/mentors help). My experience with these design competitions is that first efforts don't usually finish well in the standings. Good luck. Andy.

Frog Man

Not having too many rules is good for creativity.

The biggest advantages I can bring to the competition will be a fairing and a small bicycle frame. If nothing else fails I should win because of that. However I am thinking about every advantage I can gain on the competition (I don't want to underestimate them). I'll be competing in the open category which mean's rich adults and companies.

I do need the vehicle to be reliable though, I only get one race per year and there is money to be won.

A road bike is light weight because of the skinny/thin components but it is too big, I want a small road bike frame (BMX size). Here is questions.

What size tube is suitable for a light weight road bike?
Can I scale down the size of the bike frame to achieve both light weight and small size without catastrophic failure?

I'll look at the sources thread and 4130 tubing.

Edit: this is the track, there is a hill, lots of turns.

CliffordK

Also consider Shapiro's in St. Louis

Don't be overconfident on a bike that hasn't been made yet.

One race on a track shouldn't be too demanding on your bike. If you use it on the road for commuting, or year-around use, that would be different. Also, perhaps considering if you plan to use it for several years, or once and done.

If you look at a website like Nova, you can get an idea of tubing sizes being used. Many of their kits are double butted for full sized bikes. Cutting them down to a small size would loose at least the butting on one end.

A custom Chromoly frame might be lighter than a department store BMX frame (although there are quality racing BMX frames).

As far as "scaling down", there are 2 ways to do it.

SHORTER
SHORTER & Smaller.

Your frame should scale down well by just making everything shorter, but I wouldn't, say do a 50% scale, using smaller, thinner tubes. Just use regular diameter and thickness tubes. 0.035"?

Oh, and if this is an annual race, look for photos of previous contenders.

unterhausen

you probably don't care much about how long the bike is, so road bike tubes or mtb bike tubes should work fine. Bike tubes are butted, so there is a limited range of tubing that can be cut off. It's one of the most annoying things about designing a bike for me. For this bike, I assume you can design around the tubes. I would probably go with 9/6/9 tubes, unless the person building has a lot of experience with lightweight tubes. You can always get bigger tubes

Frog Man

Maximum length is 2.4m, I'll have a fairing though. Frame length will be determined by the minimum space required for me to get into position with adjustments made for the shorter smaller race rider.

I just learned that butted tubes exist.

As for the competition they are all using regular bikes without much effective variation.

fietsbob

Your Uni has a Mechanical Engineering department doesn't it? You In It ? You have a Faculty advisor?

How about Buying a Moulton triangulated truss space frame if You are not starting from Scratch..

Frog Man

I have decided to use a standard low cost 700c road bicycle with a relatively low head tube, for this first competition. I will spend my limited time and effort on upgrading to low rolling resistance tires and wheel bearings as well as work on the fairing construction.


Since I'll have some experience with using carbon fiber from this year I can construct an optimal carbon fiber frame for this application and maybe even optimize the fairing further. The competition will either build their own fairings to keep up with me or the event organizer will ban fairings (very possible). Either way the prize money will offset most expenses (and I can use the fairing for my motorcycle for even more savings).

I can't afford a Moulton frame.

Thanks for everyone's replies.

Andrew R Stewart 

Frog Man- I think you are making smart decisions in your first effort. Keeping to already produced products as possible will let you focus on other more unique aspects. I do think your search into tires and bearings will prove to be less advantageous then you're hoping for. And i'll add that your attitude (assuming you're serious and not making fun) that you'll win is a bit much. As if others don't already know about the advantages of fairings and are incorporating them into their designs... Really. Either way good luck and do let us know how you make out. Andy.

gsa103

Just for reference, a state of the art carbon road frame is ~750g plus about ~250g for the fork. Any steel BMX frame is likely to be significantly heavier than a carbon frame.

You can buy carbon tubes, some lugs, and you can hand-build a light carbon frame in any dimensions you need. Fab a prototype out of the cheapest steel tubing you can find, use it to refine the geometry then fab a matching carbon structure.

fietsbob

Or, Borrow from surfboard making technology...

make a frame core out of dense styrofoam then wet wrap it in carbon fiber and epoxy resin.

Low tech hands on DIY..

CliffordK

You have a lot of decisions to make.

The little mini-bike in your first photo looks cute... and is probably reasonably efficient.

Looking at the Hunter Valley EV Competition photos from the past, I'm not seeing any fully custom racing bikes with cowling, so you may well have a leg up on the competition if you build the bike, and do a good job on it. It will still have to maneuver safely at high speed.

You'll have a lot of decisions to make, and probably will end up with a few compromises.

• Cheap vs Sky's the limit.
  You could easily spend several thousand dollars on a fully custom bike.
• Racing only vs Street bike.
  It might be hard to put functional pedals on that little mini bike in the first photo. A functional bicycle drivetrain will add some constraints to the design. Even if you made it street legal, it may not be a comfortable ride.

A bicycle may, in fact, have some design constraints that your minibike would not.

For example, a pedal stroke is somewhere around 14" or 15" in diameter. So, with the constraints of the knees hitting the chest, and the legs straight, it raises the body somewhat. Even the knees are rarely fully flexed on the bicycle. So your bicycle puts the rider, maybe 2 feet higher up than the mini bike.

There are, or you can build high quality 20", 24", 650c, and 700c wheels. There is also a lot of debate on how significant tire width is as the wider tires bend less. However, your tire selection will depend somewhat on your total weight, rider + bike. 300 pounds? So, maybe 28mm, or even 32mm tires.

And, of course, the more weight, the harder the acceleration and hills will be.

Some kind of regenerative braking may also help on a course where you may need to decelerate for corners.

Do you have access to the race course for system testing?

I'm seeing the rules listing constraints on the battery size and capacity, but no mention of motor constraints. So, your goal would be to design a system that will use 90% of the battery power during the course (allow monitoring?), without completely draining the battery and be walking on the last lap.

Frog Man

I am not confident in making a foam core carbon fiber frame but I'll read about it. I think it is easier to reinforce a flimsy steel frame than reinforcing a carbon fiber frame. I'll look into it though.

Luckily I can probably test on the actual race course since I am 15 minutes away from it, I have sent them an email. With testing I'll have answers.

Hard to say what weight will be when I find out and test then I'll know what changes need to be made regarding tires.

There is equal prize money for most laps and fastest lap. Another team may blow all their power to get the fastest lap while I'll be rationing our power to maintain a convincing lap lead and then blow some power at the end of the hour to try and secure the fastest lap time prize money as well. There is little doubt that a team could use a higher c rated batteries to make it very difficult if not impossible to take both awards.

Will be interesting.

squirtdad

What is the goal? winning the the competition or having a practical EV to use after the competition from what I have seen in the past the winners of these things build very efficient vehicles that would not be terribly practical.

Looking at the rules it seems pretty clear that this is an EV competition, not ebike. Might be simpler to just build a EV

Frog Man

My goal in entering this event is to learn how to build an EV. I also believe there is a good chance of me winning this event if I apply what I know and learn what I need to learn. Price money will offset the cost of this experience. Also it would be preferable to be able to legally use the vehicle. Hence the need for pedals or getting the vehicle registered as a motorcycle. I have sent out some emails to find out if I can do that.

Frog Man

Street legal motorcycle will cost $5000-$7000 to get on road so I won't be going down that path.

squirtdad

I have no idea of quality etc but this may give you some ideas........ https://www.indiegogo.com/projects/s...lectric-bike#/