HyperDynamic.co.uk

Scrapheap Challenge

time dedicated:100s of hours

location: Luton & Hitchin, Uk

year: 2007

Scrapheap Challenge index

Preparation Diary

Competition Diary

8/7/7 planned: assemble design. More trials

Actual: developed design. Mark built a model out of welding rod. Agreed to build the back section in steel and attach the main chassis when we know the loads/length. We made the driver sit behind the rear axle to simplify the chain routing. Elastic now attached straight to a drum on the axle – no freewheel. We struggled with the frame design and axle supports. Built a small amount of the frame and stripped a bike. Went scavenging for pipe clamps for the bearings but only found a plastic chair, some steel angle and some inner tubes. Dan fell in love with the front of a dirtbike/scrambler.

Things we still need:

                -elastic

                -4 bearing supports (50mm pipe clamps)

                -2 more bearings

Did some load tests on the inner tubes at home. Stretches ~ 5 times. Load stress  ~4Mpa (max (about the same as the bungee) The two inner tubes should consume 12kJ. Load measurements suggest ~85% energy release. Bounce test shows only ~50% after one bounce (elastic at least 80%). Also did wheeley calcs. Should be just about ok.

The rod model, assuming one heavy steel tube was the main frame member

Shaft made by Mark Conduit axle in 50mm bearings with a bike hub/cassette for gearing/freewheel. all held together with studding. not perfectly concentric but does the job in scrapheap style!

proposed front wheel/fork/frame with 30m pink elastic in foreground (background: 2 bike frames we might use + the rear axle attached to the rear wheels)

Mark & Dan assembling the rear axle support

the rear axle & crank support nearly finished

Me & Dan's Haul after going for a scrounge round two of Luton's industrial estates - Two van tyre inner tubes, a car inner tube, some tyre liners, a plastic chair (background) and a steel frame made from angle Dan Also fell in love with the front fork from a scrambler bike we found but it was way too heavy to use. At one point on the scavenge we thought we were going to get nicked - there were a couple of security cameras on the industrial estate but none of them were pointing our way. we thought it would be ok 'cause we were only getting things out of skips. One of them started barking "YOU ARE BEING RECORDED ON CCTV AND THE POLICE HAVE BEEN ALERTED! YOU ARE BEING RE...." in an automated voice. We laughed to start with, then get worried, then worked out that it was coming from a completely different part of the estate, it was just so loud we thought it was coming from right next to us. The police still haven't caught up with us so I think we're ok :-)

8-15/7/7 Planned: collect rest of parts

9/7/7 hacksawed the aluminium frame up into approximately the right size pieces so that it would fit on Mark’s trailer. It looks really good but it’s a bit heavy. Not helped by it being partially full of water!

10/7/7 started developing a more representative cad model and started to cut up one of the inner tubes – it’s hard work!

11/7/7  further developed cad model

                cut up rest of inner tube

                did a pull test-pulled the bike 50m slightly up hill (40mm x 1mm x 25m, stretched less than what it could do) the bike moved about 50m. two inner tubes should definitely get us to cover 100m. but why does the model say one inner tube should be quick enough? maybe the efficiency is very low?

heavily adapted ali frame, with tubes to cover the elastic

+ very strong, reasonably aerodynamic, lots of traction

- requires lots of work, front bit flexible?, might wheely

  12/7/7 rang the Hitchin comet again, yet again they said they would ring me tomorrow - we'll see.

            Rang Serena - we should get their bungee by midday Saturday so we should be able to do some trials on Sunday.

spoke to HR at work about doing a raffle to raise money, they agreed and will provide a cool prize!

14/7/7 The bungee arrived from Scrapheap Challenge. On the phone Serena had suggested it was approximately 1.5"DIA x 30' long, it was actually more like 14mm x 27m! Obviously the first thing to do is stress test it.  Max load was ~600N (really strong), ~500J per meter max energy storage. This should be enough to propel us to 30mph and a sub 10s ET.

15/7/7 Planned: get rolling chassis

Actual:

We got a rolling chassis!

First thing we actually did is some more tow tests with a bike and the bungee. It turned out to be really difficult to stretch the bungee to it's full potential. If you put the same load into the bungee as we can into the inner tubes (ie as much as Mark can pull), the inner tubes are actually nearly as good as the bungee. This gives about a 15mph speed.

Mark had a 'slight' accident with the bungee. I tied it to the bike using some kite string, thinking it would be plenty strong enough. Mark wrapped the other end of the bungee around his waist, holding it together with both hands. When the elastic was nearing full stretch, the string at my end chafed on the frame and snapped, propelling 30m of highly stretched elastic at high speed towards Mark's groin! Luckily his hand took most of the force, saving his dignity, but he still collapsed to the floor in pain.

I also had and incident during the inner tube tests. Once the inner tube had pulled the bike up to speed it went slack, dropping to the floor. For some reason it stuck to the wheel, got lifted up into the mudguards and jammed the wheel. I stopped VERY quickly. Luckily I didn't come off the bike and I think part of the inner tube snapped rather than completely locking up the wheel. Future tests will have to try to get the slack elastic out of the way some how (hopefully all future tests will be on the actual vehicle, so it won't be so much of an issue.

When Mark AND Dan stretched the bungee, it was much faster, like 25mph. So not quite 30mph but still very quick. Tests didn't last long enough for the gps to record the max speed - we need to get a bike computer to get better measurements. The acceleration is brilliant though, very smooth and sustained. We've easily got enough to cover 100m now, probably in about 11 seconds time, as it's definitely faster than us on a bike with pedal power alone(13s). We've not done any trials with combined pedal and elastic power yet.

At this point we were thinking about just using lots of inner tubes, as using elastic seems a bit like cheating.

Next we laid out the bits and decided which bits of the frame needed cutting (great, lots more hacksawing :-(   We also though of a relatively quick way to attach the bike frame (front fork) to the main frame, making it even longer.

While I was doing that Mark assembled the rear axle with the newly acquired pipe clamps so that we could decided where to put it.

With the frame cut down and the bike frame roughly attached we tested different positions for the rear axle so that it was easy to attach and the pedaling position was comfortable. we settled on the cranks being high up with the seat slightly higher than the axle.

Next up was gusseting the frame to hold it together in it's new shape and getting the rear axle fastened in place.

All that took us to about 5:00pm, so we did some quick rolling tests in the car park to check for turning circles etc, take photos and have a laugh. Turning circle is not actually that bad. the rear wheels will turn a circle of about 2m DIA. This does mean that the front wheels turn a ~15m circle though. The frame seems to be stiff enough for sitting on and general turning but god knows what it will be like at 30mph with over 800kgf in the elastic! During the roll tests the wheels start to rub on the frame and point at all sorts of silly angles - it looks like both the studding has bent and the joint between the tubular axle and the wheel hubs is acting like a ball joint, allowing the whole thing to bend. It looks like the inserts we discussed early will have to be made.

Then we started to load the trailer. Then it really hit us how massive this thing is. It dwarfed the trailer, which is full twin axle car trailer! We ended up taking the bike frame off to get it to fit sensibly. The dragster weighs so much I think we still need every bit of elastic material we can get our hands on, so it looks like it will be bungee & inner tubes.

Still left to do:

Plenty to do but we should have enough time (3 weeks but 2 weekends)

15-22/7/7 Planned: sort out charity stuff, costumes?,, decorations/paint scheme, get extras. Discuss issues

16/7/7 Started making the axle inserts on the lathe at work. I think they should work really well. Dad phoned me - he's found some elastic leftover from when a trampoline was resprung at his school - about 7m of 14mm elastic. this should just about offset the weight of the frame against the bike frames weight, meaning we'll go as fast in competition as in the bike trials - Thanks Dad.

17/7/7 Mark managed to get a disk brake off the front of an old moped. it looks ideal in size and seems to work ok. It matches up perfectly with the centre hub.

 18/7/7 ish - Me and Mark drilled the disk so that it could be bolted to the centre hub.

22/7/7 Planned: fit extras, paint, test & modify

 Actual:

Things went very slowly. I concentrated on getting the front brake working and some steering while Mark and Dan fitted a seat, strengthened the rear frame, attached the handle bars at the back and added the rear brake.  I also attached the elastic guard tubes. I tried to wire them in place as the mount points at the back were two far from the ends to get a spanner to the nut on the inside. The front end was bolted. I also laid out the elastic.

By the end of the day we had got most of what we wanted to get done finished and were reassembling it to try to squeeze one test run in. We'd only had time to fit one pin to the rear axle, which was through the centre hub/disc brake. We decided to use a 3mm split pin to hold it in place. while we were tightening the wheel inserts the pin sheered. We'd hardly put any force into it. We next tried a bolt, which did exactly the same. Next up was a drill blank. We loosely attached the wheels and tried to wind up the elastic (only one strand). This went really well, and the wheels accelerated quite quickly and spun quite fast (while held off the floor, we didn't actually move) Since the drill blank had held quite well we decided to attach everything properly and go for a test run. When we were putting the last turn on the wheel hubs to prestress the axle, the axle snapped in half! game over for the day.

On the plus side we had:

• a BBQ lunch

• stiffened the axle (not nearly enough though)

• attached brakes

• bound the elastic

• finished the steering

• strengthened the rear frame

• finished the seat -  and it was really comfy

• added safety catches for the bungee

• pinned the centre hub

• mounted the elastic covers

• fixed the freewheel clearance issues

• fitted the chain the correct length - no need for a tensioner.

unfortunately though:

• the brakes caught badly and didn't stop very well

• the axle needed completely remaking

• the seat position made peddling difficult

• we only have one weekend to fix everything

• the elastic was bound to the wrong length

• the tubes weren't attached very well

Still left to do:

22-29/7/7 Planned: discuss PFMEA

Actual:

22/7/7 We decided to go for a solid stainless shaft. since the wheels only had 11mm holes in them, we settled on a 10mm shaft with ends turned down and threaded. John ordered it for us from? During lunch at work I managed to drill out the middle of all the hubs to accept 1/2" shafts, so we decided to go with the larger size. Dan collected the 1/2" stainless from John that he already had.

Mark found out he needed to go to America, flying out on Tuesday. He should be returning on Thursday.

23/7/7 I turned the ends of the shaft to fit the wheels and threaded them. While turning the shaft it 'whipped really badly and bent almost in half. I managed to straighten it but because all the components had been reamed 1/2" they would only slide on from one end. The shaft was also 10mm too short! Everything fit so well on it though that I drilled and tapped the ends so that we could use bolts to compress everything, rather than nuts. This setup was obviously so much stiffer we thought it would be plenty strong enough.

26/7/7 found out Mark might not be coming back from America until the day of the competition, so we might have to do it on our own. .

28/7/7 I bought a cheap bike computer from Tesco and made a parachute from a bin liner.

29/7/7 Planned: test & modify

Actual:

This went much the same as the weekend before but we got slightly more testing in.

I noticed that the wire I'd use to attached the tubes had broken, so we had to reattach them. John gave us some rivnuts that made it a lot better and actually took much less time time to fit than the two bits of wire.

Both me and John dismantled the rear brake caliper, trying to get the pads to line up and run freer. we realized that the little spring were there to hold the pads off the disk, not hold the pads in. This made a massive difference to the brake drag. Because the axle was slightly bent from the lathe accident, the disk wobbled, catching the pads. we tried loosening the bolts holding the calipers, which helped but it was still catching quite a lot. The front brake was equally difficult as the cable run was so long that the weight of the wire pulled the brakes on. Pulling on the lever just took up the slack so there was no real force to it. We added springs at the front to hold the pads off, which stopped the pads dragging but meant it took massive finger strength to pull the brake on. the stretch in the wire meant it still hardly had any force in it.

We also pinned all the components to the shaft with 4mm drill blanks . The centre hub was pinned with an m6 shoulder bolt.

At this point we decided to go for it an do a test run. We wound up the elastic half way and let go. Almost nothing happened. The dragster slowly limped forwards for about 10 metres. At some point during this test or slightly afterwards we heard a ping and noticed that one of the drills had sheared. We replaced it and carried on.

Next we wound the elastic up all the way and let go. this time the dragster moved a bit better (and slightly up hill) but only peaked at ~14mph and moved about 50m. there seemed to be not much thrust and a lot of drag. Peddling didn't help much. because of the fixed gearing and high drag it was too difficult to move away from a standing start under peddle power, but above ~13mph you couldn't peddle fast enough to help any. This wasn't helped by the seating position.

This test was disappointing because of the speed (or lack of) but it did prove a lot.

• the steering worked ok (provided you didn't let go of the handle bars - the wobbles started and built quite quickly)

• the brakes could hold back the elastic it that configuration

• the drive principle works

• the elastic was enough to get us to move (and reconfiguring it should get us 100m)

• the seat is comfortable

• the bike computer is good but there is so many things that could potentially break violently close to your face that you don't have time to look at it during a run

• the brakes are good enough to come to a gentle stop

• the frame is about stiff enough

• the axle is now strong enough to take our weight

• the elastic needs encouragement to wind nicely on the axle - but it will do it.

So we stopped for lunch and decided to redo the elastic to get more out of it. As originally bound, it was about 2.5m, meaning we could stretch it to double it's original length within the tubes, giving about 500kg total stretch. If we redo this so that it is bound to ~2m, we can get x2.5 stretch and a total pull of ~800kg. The extra stress and longer pull mean we get almost double the energy storage.

For the next test we wheeled the dragster out to the main road for more room and a gentler slope. John and Graham also came along (they'd been helping all day anyway). On the way out we heard pings and noticed that the pins had sheared. the fixed rear axle puts loads of load on the pins under turning. We replaced them but they went again even quicker. We ended up drilling all the holes out more and replacing them with m5 bolts.

This was now strong enough for the first serious test. We stretched the elastic until there was slightly more force in it than the previous test and tried to hold it on the brakes.  It just crept forwards so we decided to hold it back manually until ready to go. Because of the force in the elastic, the frame started to bend slightly. not enough to worry about it folding up, but enough for the steering ropes to go slack. Once let go it accelerated better than before and I think it reached ~16mph (slightly down hill). it probably covered about 100m, taking about the same time as we did on the bike with no elastic assist (guessing about 15seconds). At maximum speed though the steering really got the wobbles on and probably sapped quite a bit of power. I also noticed that the brakes didn't change much between pulling on the levers or not - there was still loads of drag. During this test run another pin sheared, so we replaced that and decided that we could no longer do corners with somebody sat in it.

While pushing the dragster back we noticed that the rear axle had been pulled forwards on one side, rotating the bearing mount and bending the shaft badly. It was clear that we needed to beef up the bearing mounts.

For the next test we decided to strengthen the frame (to reduce steering rope sag and put off the frame folding up for a bit longer. We also removed the rear cassette and disk brake to reduce the rolling drag. I also added some plates to encourage the rope to coil neatly as it was a slightly dangerous and tough job to 'encourage' the rope to sit right. We had to use long pry levers to push it around.  John also welded in some additional bearing support plates.

We set out to test it again. It was obvious how much less drag there was just when pushing it around. We started to wind the elastic up, aiming to go to full stretch this time. The rope guide plates worked ok but weren't smooth enough to do the job on their own. When we got to about 3/4 stretch, things started to get quite difficult to hold the dragster back. All of a sudden there was a large bang, bits of plastic and dust flew every where and the dragster shook. It wasn't immediately obvious what had happened - we thought a rope might have snapped. The elastic had shot forward and hit the end of the tubes with massive force, shattering them and firing bits everywhere. Graham, who was holding the steering at the front at the time was very lucky to not get hit by any high-speed shrapnel.. After inspecting the vehicle and the ropes we realized it wasn't the rope that snapped, it was the pin holding the tube the rope had been wrapped around.. This meant that the tube and the very sharp looking rope deflector plate must have spun round at fantastically high speed, allowing the elastic to fire forwards. Anybody with their hand near the plate (like me and John had seconds earlier) would have had lacerated hands.

We wheeled the dragster back to the workshop and started to talk about extra safety measures and how to strengthen the pins. It was only then that we noticed that the shaft had been snapped in half at the pinned joint that sheared. Talk immediately changed to bigger shafts and how much work we had left to do.

so jobs left to do:

So the job lists going down but they're getting harder to fix.

30/7/7 Dan managed to find some scrap 25mm steel rod at work. We had both independently realized that two rear axles would probably be better than one fixed one. At lunch I managed to drill out the centre of both rear wheel so that they would accept 19mm shafts - same as the inside of the bearings. I turned the shafts down at the ends after work and roughly assembled it:

This should give us several major benefits:

• nothing should break when we go round corners

• we shouldn't be able to bend the shaft

• only two pins (through the wheels and we can easily go bigger diameter)

• simpler maintenance

• less drag (they spin much freer than anything before (because they're truer?)

downsides:

• we now only have a front brake that doesn't work very well

• we have nothing to hold back against the force of the elastic (just manpower?)

• if the elastic develops more force on one side, it will try to turn us.

• we no longer have peddling as a backup.

• it's a bit heavier

Things that have to be done with four days left:

• pin the rear axle

• add guards

• add effective enough brakes

• test

Not too much but it would be nice to paint it etc. Current thinking for the brakes is to attach the front brake cable to the peddles so that pushing on the peddles provides lots of braking force. We might go for simple pads on the rear that push against the ground or the tyres.

1/8/7

finished axles & made pins

2/8/7

did some painting and did some tests - disaster - we snapped the drive rope! after looking around for a replacement we ended up using the mainsheet off my Laser.

3/8/7 Planned: final trial. Load trailer

Actual:

• Painting

• pin the rear axle

• add guards

• added brakes (rope connecting front brake to rear peddles - strong enough to lock the wheel up!)

• test (still not to full tension, a mistake?)

4/8/7 Planned: compete

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