SRB Superlight - vintage restomod

[BuggyDad 6375]

10 hours ago, Mad Ax said:

In my experience, left hand wheelnuts come off more often than righthand ones, which means the braking torque actually puts more shock through the drivetrain than the acceleration torque.  That makes sense - even with a super hot motor, there's some lag in accelerating the drivetrain, but when you land from a jump with the wheels spinning, the sudden stall in the wheelspeed is much sharper.  Even spinning the wheels going over bumps in the track surface will put a lot of stalling torque through the drivetrain.

I understand that on bicycles the reason some things, most obviously the left pedal, use a left hand thread is an odd phenomenon that explains why things loosen seemingly the wrong way, precession:

https://en.m.wikipedia.org/wiki/Precession_(mechanical)

I had wondered whether similar applied to the left wheelnuts on an RC car. Regardless though, it'd surely be solved with left hand threads (accepted manufacturing cost impact) on the left. I don't think I've ever had a wheel on the right come off. 

[Mad Ax 10826]

28 minutes ago, BuggyDad said:

...an odd phenomenon that explains why things loosen seemingly the wrong way, precession...

ooh!  learning!  Cool, that explains a lot, and is probably more correct than my "braking shock forces" theory

It does feel counter-intuitive - you'd think a right-hand thread on a left-hand wheel would tighten up, not loosen off.

[Mad Ax 10826]

Righto!  More updates.  This was actually work completed last weekend, I've just been too lazy to do any updates this week.

My first problem was that I wanted to add some anti-squat, but I'd made my standoffs too short and there wasn't space to lift the gearbox without impacting the top deck.  So, first solution was to cut some new standoffs.

Then I used a scrap of FR4 from making the top deck to make an anti-squat rotation plate

This isn't the perfect way to do this, as it's going to change the front geometry also, but it's only a tiny adjustment at either end.  I doubt it will even be noticeable, to be honest.

[Mad Ax 10826]

The next problem was one of wheelery.  Tyres are open at Revival, so we can run modern rubber.  That means we need to run modern wheels.  I know it detracts from the visuals a lot, SRBs look lame with big wheels on, but sand paddles will have virtually no traction at all on astro (especially if it rains) and the Hornet spikes can be too aggressive.

So, this means I need a hex conversion.  I've got a few sets of these HPI Super Star adapters, but they aren't deep enough - there's still some axle poking through the adapter which stops the wheel sitting properly on the hex.

But it was OK for a clearance check.  Clearance is fine at both ends, no need to mod or lift the body.

Wondering what to do about the wheel fitment problem, I grabbed my re-re SRB to see if it was different.  Indeed it was - the drive flower on the vintage SRB is bonded to the axle, but on the re-re it's a conventional pin drive.  That means the SRB-style pot metal drive flower can come off, and the slightly deeper Frog-style drive flower can go on.  This pushes the adapter out far enough that the wheel sits on properly.

albeit with a bit less thread than I'd like

Still, this should work fine with knurled flange nuts

[Mad Ax 10826]

So, I placed an order with RaceCraft RC for some re-re style axles, to which I can fit another pair of frog-style plastic drive flowers.  I don't know why I have all these Frog parts, but fortunately I have enough drive flowers and enough HPI adapters to do both Revival cars.  Which is nice.

There's another reason why I want the Frog flowers on my superlight - but you'll have to wait for that, as my takeaway is about to arrive.

[Dadio 127]

Nice build ! 

I think the modern loosening of front arms a rear UJ's is both caused by the same thing , old school front arm swivel pins were softer so the set screw bit in more and stayed tight , the modern ones are hardened steel , sure the don't bend but the set screw bites in less so is prone to loosening , same with the old brass UJ's , just a little more give so the set screw locked better , the modern UJ's are again hardened so probably a similar problem , that and the thread lock back in the 80s was probably better , deadly toxic and instant cancer but better LOL 

[Mad Ax 10826]

OK - on to the next part of the build!  I'd noticed while I was doing all the stuff with the wheels and adapters that the shocks were loose.  It seems the little clampy clippies that hold the shocks and rear cage on don't fit well enough to stay still, so they were wobbling around.  I also noticed that the short rear shocks I had previously fitted weren't really doing the job that well - I couldn't get full deflection on the rear arms, there wasn't much adjustability in the spring and I don't know if I've got any others to adjust.  They're probably way better than the stock torsion spring and leaky shock from the 80s, but I figured I'd see what else I had.

And boy, did I have something else!  But that would mean more travel, and for that, I'd need some new shock towers.  They would serve the dual purpose of bracing the rear cage mount so it doesn't wobble do much, and allowing me to install a longer shock.  I probably can't use the full deflection of a long shock, but it will do a better job of damping than a short one.

I started by making a template from plasticard.

[Mad Ax 10826]

Then I started cutting the main part on 2mm FR4 sheet.

Then I trimmed it down to a more reasonable size and re-marked the holes to follow the arc drawn by the shock when the suspension arm is at the midpoint

Test installation

[Mad Ax 10826]

Made a matching tower, and filed them both down to the same shape

Countersunk screw allows the shock to get as close as possible to the tower, as there's a clearance issue between tyre and shock

This looks like it means business  

[Mad Ax 10826]

I took it out for a quick test-run on the concrete and tarmac at the back of my workshop.  Sand Paddle tyres give terrible traction on concrete, especially when it's cold, but even so, I was having trouble with turn in.  In typical SRB fashion, with all the weight over the back, there's nothing to keep those skinny smoothies on the ground.  A solution I've learned from the Revival crew is this:

Self-adhesive wheel weights!  A recent Revival rule is that additional weights are limited to a max of 50g, and must be commercially-available stick-on wheel weights (not just strips of lead, for example).  This SRB now has an extra 30g up front, and the handling is drastically improved.  It turns in well, stays planted and turns around the front axle, just like it should, but the better shocks at the rear are helping to keep the back end under control.  I haven't even set them up properly yet - they still have very soft springs and thin oil, and they're catching on the tyre too.  As mentioned earlier, the Frog-style drive flowers are going to push the stock wheel out a little further, which I hope will fix this problem.

For those who are interested - the shocks are from an Element Enduro Scale Builder's Kit 2, and they are exceptionally smooth.  Stock oil and springs are soft for crawling, but I can get stiffer springs and have plenty of racing oils lying around to rebuild them later.

I'm half wondering if the shock towers are a step too far for the rules - Super SRBs need to compete in a later class, but this one doesn't have the full catalog of Pukka Parts installed, so it will be totally outgunned in that class (especially with me driving it).  But if the scrutineers don't like it then it's no problem at all, I can fall back to the shorter alloy shocks - with some fresh oil and some TRF touring springs they'll probably be good enough.

There's still a little bit of work to do on this one, like rebuilding all the shocks and getting the modern wheels on, but most of it is now cosmetic - since I have to run an original(ish) body, I've decided I'll run a driver too.  He'll be getting painted over the next few weeks, plus the body - while looking pretty nice already - need some race colours.  Traditionally my colours are silver and metallic blue, this body was painted silver by the previous owner so I think I just need some blue panels and custom stickers, and it'll be done  

[Pablo68 1158]

Great thread and even greater project. Enjoying watching this thing take shape.
Being perhaps a bit too fair to Tamiya, they were known at the time for making buggies that handled quite a lot like their 1:1 counterparts. I can say with experience that Beetle front ends were quite light. The front brakes and general handling was always a lot better when I had my tool box in the front under the hood/bonnet.

[Mad Ax 10826]

To continue the saga of the SRB Superlight, we must cast our minds back, through the mists of time, until the morning of the 7th of May, 2023, when I arrived in the workshop at the early hour of 8:25, to begin the process of dust sheet removal after my wife had run a table-making lesson two days previous.  (In fact you can see one of her previous tabletops underneath the dust sheet in this photo)

This table was made by her candidate.  2 days after being poured, the resin was still drying - I had to be careful not to get any swarf in it.

And this was my workbench, which had been covered by a dust sheet, but only 30 minutes after they'd started power sanding.

Dust.  Low in fat.

[Mad Ax 10826]

30 minutes of dusting later, and I was ready to resume work on the SRB.  In fact it was pretty much finished last time I played with it, but I wasn't happy with the rear shocks.  Even with the wider wheelbase made by using re-re axle shafts with HPI drive flower adapters, the wheels still hit the spring on full compression.  In fact there's a fair bit of black rubber dust gathered on the shocks and suspension arms from a very short run around the block.

To get around this problem, I hoped I'd be able to move the shock tower inboard a little, but that would mean angling the shocks forward instead of backward.

First, I rough-cut a new lower mount

Then screwed on a solid link to try to work out the best arc for the shock top mounts

[Mad Ax 10826]

This rough example would look very nice when tidied up

but, alas, no matter how I arranged it, I still couldn't get the shocks to clear the wheels.  Lame.

Back to the drawing board, then, and time for a change of tack...

can you tell where this is going yet..?

well, from the looks of it, it's going nowhere...

 

[Mad Ax 10826]

OK, new plan...

In the words of Duke Nukem - aah, that's better!

It's not as pretty as my vertical plates, but it works

Plus those brackets that I cut earlier and didn't need are handy for attaching body mounts

The drawback - no rear cage.  It doesn't really matter, I suppose, for a superlight race car - the cage is only cosmetic.  But it did look much better with it installed...

I do have to wonder at this point if it's gone beyond what will be allowed under the Revival rules, but we'll see - I've kept all the parts to go to the previous version or even back to the smaller coilovers that fit on the stock mounts, so I'll be able to run it in some form or other.

Not a lot more to do on this one now, besides put some appropriate oil in the shocks, fit the modern rubber, and repaint the body in some nice race colours.

[Big Jon 690]

On 5/18/2023 at 2:51 PM, Mad Ax said:

OK, new plan...

In the words of Duke Nukem - aah, that's better!

It's not as pretty as my vertical plates, but it works

Plus those brackets that I cut earlier and didn't need are handy for attaching body mounts

The drawback - no rear cage.  It doesn't really matter, I suppose, for a superlight race car - the cage is only cosmetic.  But it did look much better with it installed...

I do have to wonder at this point if it's gone beyond what will be allowed under the Revival rules, but we'll see - I've kept all the parts to go to the previous version or even back to the smaller coilovers that fit on the stock mounts, so I'll be able to run it in some form or other.

Not a lot more to do on this one now, besides put some appropriate oil in the shocks, fit the modern rubber, and repaint the body in some nice race colours.

I really dig this shock mount; it fits the spirit of the class somehow. Maybe the shocks are too new, they’re Associated V2 shocks which came out mid-90s. Really superb shocks, too.

It would be awesome if there was more vintage racing in the States. That looks like so much fun.

[Mad Ax 10826]

I'm a bit behind with updates on this one - I've put a bit of time here and there into this car to try to keep up momentum.  6 weeks from now I'll probably be sitting on my sofa, watching comedy and eating my post-Revival takeaway, having finished my last day's racing and driven 3+ hours home across the middle of the country.  And between now and then, I've got a 10 day family holiday and a vintage touring race meet in London, so I wanted to keep chipping away, little bits at a time.

I didn't get a lot of photos but I'll try to rectify that before the race meet.

Sunday 11th was warm - high 20s and dry - a little too hot for being stuck in the workshop all day, but at least I could have the roller door open and bask in the sunlight while I was working.

My wife had been busy doing some sanding and woodwork, so my workbench was covered with a dust sheet.

Under the sheet was my SRB Lightweight project

First point of attention was the springs and dampers.  The front shocks are generic alloy units, and the rears are from an Element Enduro, so naturally they're very soft.  Even with some ancient generic red springs, they're still a little too flopsical.

Fortunately the sizes are a near-perfect match for Tamiya DF-03 springs, so I bought a set from my local hobby shop.  This gave me various options for balancing the springs with the weight.  SRBs are heavy, so they need a fairly sturdy spring.  I went with Gold up front, which is the medium weight.

I also made a damper brace using some 4mm ID alloy tube from B&Q and some Tamiya brass top hat bushes.  I forgot to get any pics of it install, sorry!

[Mad Ax 10826]

I had some time the following day, so I turned my attention to the body.  The roof previous had some spotlights installed, and I wanted to fill up the holes.

I used a strip of scrap styrene, glued to the inside

[Mad Ax 10826]

Then filled the gaps with modelling putty

[Mad Ax 10826]

The following day the putty was cured enough to sand back.  Of course this cut into the lovely silver paint that the previous owner had laid down on the roof, so I will have to make plans to repaint this...

5 days later, after acquiring some masking fluid, that's exactly what I would do:

A day later, after removing the masking, it looked like this:

It's a bit scraggly around the edges in this photo because there was still some liquid mask stuck around the join lines, but I didn't want to get in with a knife to scrape it away as the paint wouldn't be fully cured yet.  I have now tidied it up and it looks much better - there's a few areas where it wasn't perfectly masked but it's more than fine for a runner.

I'll be racing this car along with my Hotshot, which was originally painted around 15 years ago in my old black and silver race paint scheme, so it seemed fair - since the base coat was already silver - to choose black for the alternate panels.  Now I'll have matching paint jobs on my main / dry cars  

Full race livery still to follow one this one, once I've had time to design the graphics

[Grumpy pants 5541]

I’d love to see a picture of the damper brace please?

[Mad Ax 10826]

2 hours ago, Grumpy pants said:

I’d love to see a picture of the damper brace please?

I'll try to remember to take one next time I'm on the project (Saturday or Sunday, unless I get some time in an evening).  It's really just a tube that goes between the front shock towers with some M3 allthread to tie it all together and mount the shocks.

[Grumpy pants 5541]

28 minutes ago, Mad Ax said:

I'll try to remember to take one next time I'm on the project (Saturday or Sunday, unless I get some time in an evening).  It's really just a tube that goes between the front shock towers with some M3 allthread to tie it all together and mount the shocks.

I’m pretty sure I used WO rear cage strut on my SS. 
 

 

[Grumpy pants 5541]

@Mad Ax what did you use to grind out the gearbox? 
 

You’re right about the roll bar/ hoop they are the same on all the SRB’s except the Ranger. 

[Mad Ax 10826]

12 hours ago, Grumpy pants said:

@Mad Ax what did you use to grind out the gearbox?

Ooh, now we're going back a bit...  Probably a Dremel with either a sanding drum or a grinding stone.  Neither are perfect and will fur up with melted alloy quickly, but it's hard to get a file in there to do it properly.