Going down a hill quickly

[McFrancis]

I don't know if you have seen this but it is an interesting read from the GT40 forum. https://www.gt40s.com/threads/gravity-racer-part-iv.55915/

The guy has been running a gravity racer for a few years, Your racer is definitely classic European looks and better in my eyes, whereas our man over the pond has gone down the streamliner route.

[Mark (smokey mow)]

Brilliant thanks.

 

I’m not sure how the streamliners will fare on the jumps and obstacles we have here.

 

It was one of the reasons I’ve opted for moped wheels rather than skinnier bike wheels.  I wanted something more robust that wouldn’t fold under at the first obstacle.

 

It’s a compromise as I’ll be trading an increase in rolling resistance but hopefully it’ll mean I’ll make it to the finish.

 

I’ve also tried to base my build so it’ll fit the regulations for different events. There’s no common set of design regulations so I’ve downloaded several and attempted a fit all.

 

For the Colchester race the maximum width is 1.5m and a weight of 100kg, but I’ve designed for 1m and under 75kg as this is what some of the smaller events require.

[Nic Chase (NICO) - Shropshire & Mid-Wales AO]

Following this with great interest. 
Nice to have a different build thread. 
 

Your usual extremely thorough job Mark. 

[stephenh]

Mark, I've no experience of "modern" downhill racing, but many decades ago I did several soapbox derbys, and we used an old car chassis, so with car wheels and tyres. We got over the high rolling resistance by blowing up the tyres to much higher pressures than they would normally be at. This seemed to overcome rolling resistance quite well. 

[Mark (smokey mow)]

@stephenh that was pretty much my plan. One of the advantages for using moped tyres is that they have a much more rounded profile compared to car tyres so the contact patch is relatively flat small.

[stephenh]

Quite agree, Ours was just a one-off low budget afair, for one event really. Way back, a Round Table club in Derbyshire used to organise a soapbox derby on the drive of a large Country House near Melbourne. I was then a member of the Uttoxeter club, and we entered for several years. If I remember correctly, one year I came 2nd. overall (as driver) with a lad from our then local approved school for naughty lads(I kid you not!!) as riding "mechanic". One one run we rolled on the hairpin near the bottom. He thought it was great fun!

[BigSkyBrad]

 

[XTR2Turbo]

Excellent 👍👍

[BigSkyBrad]

68mph - with cars coming the other way!

 

 

[Captain Colonial]

On 13/04/2023 at 23:04, BigSkyBrad said:

 

Almost got taken out by two separate dogs between 5:00 and 5:09 🤣

[Mark (smokey mow)]

Not much progress to report while I wait for my steel order to arrive so today I went on a research trip to watch the Gt Dunmow soapbox race.

 

 

 

 

 

 

 

[Nic Chase (NICO) - Shropshire & Mid-Wales AO]

Some original cars there.
The No.1 car looks vaguely familiar 🤣

[Mark (smokey mow)]

A big pile of metal arrived this morning  so since it’s my day off it was straight to work assembling the chassis.

To appease the neighbours I ordered all the tubes pre-cut to length so there was no cutting or grinding to worry about.

 

 

Quite quickly the base came together 

 

and then very quickly the shape of the frame started to appear.

 

 

I’d had a jig made to hold the suspension brackets in place so they were centred correctly and all parallel.

 

 

After half a day it’s almost there but annoyingly I’ve now run out of gas so can’t do any more for a couple of days.

the more observant will also notice that the metal for the wishbones is missing. I’ve raised this with the laser cutters as it was ordered and paid for but missing from the delivery.

 

 

 

 

 

[Mark (smokey mow)]

Thanks to the long weekend I’ve got the bracing welded to the top of the chassis.

 

Whilst it probably looks a bit OTT for the job it’s also there to mount the steering rack and the upper wishbone mounts as else they’d hang off the edge of the chassis unsupported.

 

With that all done I trial fitted the steering rack and column as I was a bit worried about the angle on the UJ, as it was it seems to work ok which is good news.

 

 

I’ve also heard back from the laser cutters and they’ve found the missing bits from my order so those should be arriving tomorrow or Wednesday.

[Mark (smokey mow)]

The good news is the remaining part of my steel order was delivered today so not wanting to waste any time I got the wishbone tubes fitted on the jig.

 

 

I’m very happy with the fit here and having everything laser cut has saved me a lot of cutting and fettling time and means I get get straight to the welding.

The other missing parts were the diagonal braces for the front of the chassis so these were fitted at the same time.

 

after a test fit of the seating position I wasn’t comfortable with the position of the rear tube behind my back. It was limiting how far I could lean back and would limit any options for fitting a fibreglass seat in the future.

 

out came the angle grinder and this was changed to a pair of diagonal braces.

 

That’s all the welding finished now so next task is to paint.