CWIC Rds 3&4 – 11th November 2012

In brief…

Mini Formula

Adding the wing to the Blitz GTi shell made the straight line stability a lot better. The track had a long, fast sweeper which meant I needed to gear up to 22/33, and in Round 2 I started to get some traction roll which I solved by reducing the shock length (and thus droop) all round to 56.5mm. I also gradually increased the stiffness of the rear of the car as the day went on, and it felt more and more stable, although not really any faster. The attendance of a former Tamiya World Champion on the day meant that TQ was out of reach, and I lined up third on the grid. A big mistake by me, rolling off the track at the last corner, cost me second place in the first final (finishing third), and backmarker troubles cost me second in the second (finishing fourth). The car was good by the end though, the setup – m05_2012_11_11_cwic_setup – is once again moving closer to where I was at the end of last indoor season, which is normally a good sign that you are close to the limit. Think I might drop this class though, two cars is just a bit too much to concentrate on and the results haven’t fallen in my favour so far.

17.5 Blinky TC

It was more of a power track this time around, the long sweeper was taken flat, and there was a full-throttle mini-straight after the first chicane too. I wasn’t happy with the HPI Flux Pro at all after practice and Round 1, and after asking the owner of the fastest car on the track what motor he was running, I knew what I had to do. He was running a Vampire – the very motor I cooked a few weeks ago – so I went straight across to Ian@MMR and handed over my Visa card. Again :(. Not only did Mr B. reveal his motor, he also told me his timing and gear ratio without prompting – thanks Les. His suggestion of 3.7:1@10deg timing felt a shade overgeared to me, but nonetheless I set my fastest laptime of the day so far in Round 2. For Round 3 I went to 3.9:1@10deg, but that was undergeared (the sweet spot in 17.5 is very small) and although I was leading the race, a mistake by me and the lack of raw pace meant TQ was out of reach. So again I lined up 3rd on the grid. For the first final I went to 3.9:1@15deg, and I was very happy with the pace, with plenty of rip off the line and completely matching the other cars for top end. The race was running in grid order with barely 2 seconds covering the top 3, until another car hit the track markers hard and pushed them out into the middle of the sweeper. Pole position man Jay got caught up in the melee, but me and Les got through, and raced nose to tail until the buzzer, with only a few tenths separating us. Sadly for me, it was Les in the lead, but it was a great race. The second final was a bit easier, Les and Jay came together on the first lap, and gave me a comfortable gap which I held until the finish. So, two wins and two seconds from this class so far this year, a great start. The ARC R10’s setup remained unchanged from the last round.

Not sure that I can make the next meeting, so it will be interesting to see who scores the big points while I’m away!

Dave

Blitz Mini GTi

 

My advice – run the rear wing! Made a huge difference to straight-line stability and general “plantedness” at last nights club meeting. Would have qualified 3rd in TC with my Mini time. Wish I had put it on at the CWIC…

CWIC Rds 1&2 – 21st October 2012

I’ve been really looking forward to this series – great racing, great company and it DOESN’T RAIN (unless the roof of the old sports centre gives out, I remember a couple of buckets were needed last season)!

Here’s this years fleet…

I do like to have matching schemes, so I painted up a fresh Blitz Mini GTi for the M-05. I re-masked the side windows to look more like a real Golf, the standard masks are way off, and I think the car looks a lot better for it. Still messed up the body hole positions, even after what seemed like hours of measuring and re-measuring – self adhesive body washers came to the rescue once again!

The ARC R10 touring car will be running in the new 17.5 Blinky class. The M-05 is in the Mini Formula class which is more or less the same as last year although touring-style shells have been formally banned (only a couple of people were using them anyway).

Mini Formula

Not much to say. I’ve only run the car once since my disaster at the SPC, and that was at a very small club meeting where all I learned was how many different ways I could get punted by lapped traffic (the answer is LOTS). So I was starting from scratch again with the diff, and the diff is perhaps the biggest part of a Mini’s setup. I started the day with the broken-in 3Racing gear diff running 100k oil – in practice it was far too stiff and the car wouldn’t go in a straight line, cornering was OK though. After the second round it was clear that the track conditions weren’t going to get grippy enough to make the diff driveable, so I put in the standard-build TA03 diff. Too loose, and I was losing a lot of speed out of the corners. Despite this I did end up in an unexpected TQ due more to other cars difficulties than my pace – the top 5 in the class were all setting pretty competitive times, so I knew the finals would be tough. For the first final, I went back to the gear diff, but the lack of stability made it very difficult to race with, and after a shocking first lap with a body tuck, I could only finish 5th. For the second final, I mixed some 7k oil in with the 100k oil, which did seem to make the diff easier to drive, albeit perhaps a bit too loose. I still had a terrible first lap and ended up 3rd at the buzzer. There is definitely more to come from the car, 30k and 60k diff oils were ordered as soon as I got home, hopefully one of those will be the answer!

17.5 Blinky

Again, I haven’t really had any meaningful running with the car on carpet yet, just a couple of races at the same punt-fest club, which ended up with me cooking my good motor and ESC. That hasn’t stopped me tinkering with the car – the main change I’ve made since the outdoor season is to fit Tamiya shocks to the car, simply because they are easier to rebuild. The XRAY-style bladder-in-cap design of the ARC shocks just wasn’t working for me, I couldn’t get a consistent build from shock to shock, and drilling the caps actually made it worse because there was no air pressure to stop the bladder from collapsing as you screwed the cap on. I’m using ARC springs though, no complaints about them whatsoever.

I’ve also tweaked the steering setup, mainly to get more even throw and linearity. I’ve fitted a Tamiya servo saver (which requires taping the servo in place rather than using the outside servo mount), fitted with an M’s Racing servo horn. This is 4mm longer than the Tamiya part at 20.5mm centre-to-centre, and it means I can get the steering linkage at a right angle to the horn, and still get even throw to full lock. I have also swapped the 3mm spacers on the rack for 0.5mm spacers to give me straight steering links (which requires a short ballstud).  My EPA’s are now around 90% each way, whereas with the original parts I was at 90% in one direction and 120% (the maximum on the radio) in the other.

Finally, I’ve swapped all the screw-through balls for ball-studs which have a wider range of movement – temporarily using Tamiya parts on the inside camber links. The ball-studs are 1mm higher, so I have reduced the camber link spacing by that amount.

Anyway, onto the racing. Practice was fine, setting good laptimes albeit with a car that felt a little slow at 4.06:1 with the HPI Flux Pro motor timing at 15deg. Towards the end of the run I dropped the steering rates down to 90%. For round 1 of qualifying proper, I quickly realised that the rates change was a mistake as I clattered various barriers, so I put them back at 100% and had a solid run from then on, managing third in round. I had geared up to 3.82:1 for that run, but the car felt very sluggish through the infield, so I went to 3.94:1 for the second round of qualifying. I also felt the car had a little too much understeer, so dropped down to a white front spring from the red. The car was still a bit slow, but I put in a solid run to snatch provisional TQ by just under a second. I left the car as it was for the final round, and another solid run saw me secure TQ with a slightly faster run of 26/307, which would have been good enough for the top half of the C-final in 13.5 boosted.

The first final started well, and I managed to get a small cushion over the second placed car as he battled with the other cars. Unfortunately, about three minutes in I found a lapped car facing the wrong direction in the middle of the track, and I couldn’t avoid him, and those two seconds lost were just enough for Jay M. to turn a clear deficit into a clear lead, and I couldn’t do anything about it. The slow speeds of 17.5 are a blessing and a curse – it makes for close racing, but it also makes it hard to shrink a gap!

The second final also started well, but I was a bit untidy through the first couple of laps which let the third place qualifier get onto my tail, and then get neatly past as I clipped a barrier trying to defend. A lap later, he lost a little time with a backmarker and I managed to sneak back into the lead, before my rival hit the sweeper hard and put himself out of contention. From then on, it was a clean run for me, winning by three-quarters of a lap. All in all a very pleasing start to the Blinky championship.

Here is a setup for the TC – cwic_setup_21_10_2012 – the Mini wasn’t right, so not worth sharing.

Looking forward to the next round in just under three weeks time!

Dave

Measuring diff tension & why my gear diff let me down…

Here is my setup for measuring diff tension…

Very simple really – a kitchen scale, the diff itself, and a 2mm Allen key as a lever to press on the scale. Simply slide the key through the outdrive up to the bend, place the opposite end of the key on the zeroed scale, and turn the opposite outdrive smoothly with your fingers. The exact figures are not important – what matters is the consistency of your measuring method from diff to diff.

Here are a couple of figures from my tests…

  • Tamiya M-05 kit gear diff with ceramic grease – 7gm
  • Tamiya M-05 ball diff (1/16th turn loose) – 35gm
  • Tamiya TA03 ball diff (standard greases and build) – 70gm
  • 3Racing gear diff (new built) with 3Racing #2k oil – 40gm
  • 3Racing gear diff (run in) with Fastrax #7k oil – 35gm
  • 3Racing gear diff (run in) with Fastrax #100k oil – 55gm

There is more to these figures than meets the eye. Although the gear diffs and ball diff can have very similar tension at a constant speed, they have a very different feel. The oil-filled gear diffs have a distinct inertia and are harder to get turning, and harder to change direction. The ball diffs are very constant and responsive to change. You’ll have already noticed this on the track – the gear diff is slower to react, which makes it either smoother or sluggish depending on your opinion. It’s also no surprise to see that the kit diff offers almost no resistance, and that the TA03 ball diff, with it’s heavy springs and strong bolt, is considerably tighter than the touring-car-derived M-05 ball diff.

Another key observation is that the 3Racing gear diff, once run in, is significantly freer in it’s action. As you can see, the thin #2K oil supplied with the diff gives a tighter action when first built than the thicker #7k oil does when the diff is run in. This is basically because the o-ring seals on the outdrives are of a very hard material and completely the wrong size. My new advice is to run the diff in as much as you possibly can before you put it in the car – break those hard o-rings in so that they don’t give a false impression. I wrote earlier about my woes at the SPC – the diff had changed so much after a year of running-in that the decrease in weight over the front tyres caused by a simple change of wheelbase caused it to diff-out (and spin-out) at will.

I’ve rebuilt the diff with #100k oil.  I’m yet to test it, but interestingly the diff does not feel 15x tighter than it does with the #7K oil, and the scales show that the difference is only about 50%. You need to make quite large steps in the silicone oils to get big changes in the diff behaviour – even the #500k oil I am using in my ARC R10’s front diff is not exceptionally stiff.

Dave

Costly mistakes at Cotswold…

Often it seems that stories of failure are as educational as stories of success…

The SPC (Super Production Cup) came to the Cotswold track yesterday. Last year, I had a great time, winning the Mini class with my Xevo Triumph, and I have been looking forward to this race meeting all summer.

Weather was a lot better than forecast, with only a couple of showers disrupting proceedings.

This year I was competing in Mini Formula, and my trusty M-05L has been setting good times at club meetings all season, so I was pretty confident of a good result.  That confidence turned out to be misplaced…

A brief shower before qualifying had dried from the track before my first race, but the grass and painted kerbs were still damp. I clipped the kerb through the fast kink at turn 4, and before I knew it I was facing the wrong way – the first of several spins and trips through the grass as I tried to keep the car under control while the rear tyres dried off. This left me almost a lap down on the leader, but I managed to find a rhythm and set the quickest laps of the race on my way to finishing second in the heat and third in the round.

Round 2 started better, and with the sun having moved over the track the shadows near race control were gone and the grip was much better. The back end of the car was a bit flighty, especially when changing direction, and a spin into the grass on my last lap cost me nine seconds. I was lucky to set the fastest time of the day so far, by just one second from Jamie Carpenter.

Round 3 followed a rain shower. The racing line was 80% dry, but the area around race control was still damp, and my car was very tricky indeed. The runners with Tamiya S-grips in their pitbox were a lot faster than my Sweep-slick-shod car, and I realised that if the finals were held in these conditions, I would be well off the pace. So, with the aim of increasing rear grip, I made the bold decision of rebuilding the car to “M” wheelbase with the HPI Civic shell to put more weight over the rear tyres. I also put some new Sweep 33s on the front, as the previous set were very worn on the shoulders and I wondered whether they might be biting too much on turn-in.

Round 4 was dry again. But the car was diabolical. The back end was extremely unstable, to the extent that it would do 360s on the apex for no apparent reason. I pulled off after a couple of laps. Fortunately for me, the track was a little slower than Round 2 (probably because of the earlier shower), and my TQ remained.

Before the first leg of the final, I decided to change back to the old front tyres – assuming that the new boots were the problem. I was wrong – the car remained undriveable, and after two laps I pulled off again.

I made an extra close examination of the car before the second final, and noticed that the pin in one of the front CVD driveshafts had come loose, and was only secured in one side of the axle. This is the kind of thing that can cause handling problems – so I repaired it. I also spotted a couple of small tears in the rear tyre sidewalls – so I re-glued them.

Confident that I must have solved the problem this time, I lined up on the grid for the second final. I got through the first two corners OK, but when I got to the tight hairpin at turn three, the rear end kicked out, and I couldn’t carry the speed through to the exit, losing the place to Jamie. By the start of the second lap, I was losing more places as the back end jumped around, and after a few trips into the grass I finished back in 8th place. The car was still basically undriveable. All my changes had been for nothing.

After the race meeting finished, I decided to have a few practice runs to see if I could get to the bottom of the problem.

First of all, I lowered the rear ride-height and disconnected the anti-roll bars. It made no difference.

Then the answer came to me. The 3Racing gear diff was the problem. I had a suspicion that it was getting looser over time, which would explain the oversteer problems as the inside wheel “diffed out” during cornering, causing the outer wheel to slow down, turn tighter and swing out the rear end. Changing to medium wheelbase would only make it worse – because although re-distributing the weight to the rear should improve rear grip, taking weight off the front would only make the diffing-out worse, because there is even less load on the inside tyre during cornering.

So, I rebuilt the car with a TA03 diff, which turned out to be quite a lot tighter than the 3Racing diff with 5K oil which hadn’t been re-filled for many, many months. The handling was much improved, although there was still some diffing-out, and the car had very poor traction out of the corners. A switch back to the “L” wheelbase and the Mito shell gave me back the Tamiya Mini I had become used to – great traction out of the corners, and a pleasure to drive. A change to Sweep 40s on the front end seemed to be another small improvement, and a newer set of Sweep 25s on the rear was a small improvement again. You still needed to be careful from a standing start before the rear tyres got cleaned off by the track, but after that I could run lap after lap at a good pace.

So what made the 3Racing diff loosen up? Well, I have yet to strip it down, but I did see some thick oil around one of the bearings when I took it out of the car, so it may have leaked a little. It’s also true that the standard o-rings used to seal the outdrives are very hard and a very tight fit. These have loosened up a lot with running, to the extent that the diff probably needs a thicker oil again to get the same feel that it had when first built.

Congrats to Jamie on the overall win. If the SPC is at Cotswold again next year, I will be back with a bag full off diff oil!

Dave

I love the smell of tarmac in the morning…

Considering my passion for RC, it’s surprising how rarely I actually go for a practice and a practice alone. Today was one of those days.

We’ve had wonderful week of early-spring weather, and the Cotswold Model Car Club track at Kemble was in great condition, clean and dry. Ambient temperature was in the high teens.

I was joined for the day by my 3-year-old son Elijah. His first visit to a track. The long-ish drive meant he started the session by sleeping, but was quite keen to be technical director by the end of the day, telling Daddy when it was time to stop driving, finding lost tools and rearranging the power cables…

Anyway, onto the practise itself. My intention was to get a better setup on the motor and speed controller. I’ve usually been hugely dissatisfied with the feel and performance of my 13.5 boosted setup, and the rare occasions when I’ve been happy have only left me even more confused. My latest hunch that I have been dramatically under-gearing the car and wasting the natural torque of an electric motor, smothering the car’s punch out of slower corners and giving a “two-speed” effect where you are waiting for the boost and turbo to kick in.

My motor is a Speed Passion V3 with the timing on the 10deg mark. My ESC is a Hobbywing Xtreme Stock. To get a good baseline for the gearing I started in “blinky” mode (508_no_timing). At 4.86:1 (the highest I could go with the available pinions) the car felt surprisingly quick around the big Cotswold track, with a lot of punch. I could have even gone a tooth or two bigger on the pinion. The laptimes don’t lie though – slow 22’s are nearly 4 seconds off my best with boost.

I tried the car at 5.35:1 next. A smoother feel all around the track (in part due to a better gear mesh), but the laptimes were in the mid-23’s. A further change to 5.94:1 resulted in a car that was simply slow, revving out before it had even reached the exit kerb of some corners. I didn’t want to sacrifice any more bottom-end performance, so I decided that this ratio would be a good basis to start boosting from.

So, a return to the pits, a charge of the batteries, a re-application of the tyre additive and a lift of the lid of the laptop. I updated the ESC software to the latest stock profile (930STOCK) and left the settings at their defaults. Put the car on the track, and straight away I was running laps in the slow-19’s and quick 20’s. The boost alone was worth nearly 3 seconds a lap. And the car felt good, really good. A quick increase of the braking strength to 75% gave me a car that was very driveable indeed, a slight dullness in the lower midrange but barely worth worrying about – perhaps gearing up again would be the answer. Motor temps were quite high, but the car coped with about seven minutes of intermittent running without problems.

And then the LiPo swelled.

Coming to the end of a short run, I felt the car slow down as if it was about to dump. I brought it to the end of the lap and pulled in. Took the car back to the pits, started to brush it down, and untaped the battery pack. Which promptly expanded to what seemed like twice its usual height, splitting the hard-case wide open. I quickly moved it to the other side of the pit area, just in case something dramatic happened.

In the end, nothing did – in fact, after half-an-hour of waiting for the YouTube-style fireball I put the LiPo back in its protective sack and drove back home, dropping it off at the council dump on the way.

I’m surprised that the LiPo swelled so dramatically. Even though it is almost three years old, and is only rated at 30C-ish (it’s an older Intellect 5000) I would have still expected it to be up to the demands of a 60a rated speed controller. I don’t think it was over-discharged as the ESC’s low voltage protection was at 3.2V per cell and hadn’t kicked in on the track, and the LiPo’s resting voltage once out of the car was closer to 3.5V per cell. Hopefully just one of those things.

So with the rebuilt TOP Scythe out of action for the day, I grabbed the Tamiya M-05L for a quick run. With no additive on the tyres and its club-night carpet setup it was a real joy to drive, a bit flighty at the back end perhaps but such an addictive flow through the corners. These front-wheel-drive Mini’s are so much fun around almost any track that I’m debating whether to bother with the touring car at all this summer. It needs a LiPo now, and BRCA legal ones are not that cheap.

I’ll just sign off with another note about the day. Despite the good conditions, the tyres (used Sorex 32RYs) did not generate any heat in themselves at any point in the day. This probably contributed the Scythe’s at-times awkward combination of understeer and tail-happiness. There’s usually a lot more grip when you have had 100+ cars running on the track over a big race weekend.

Dave