Hard to give solid feedback because this isn’t a back to back test. But initial impressions are that the small changes in suspension geometry have given the car a LOT more rear grip.
Normally at the Chippenham club track I would be running 85% steering lock on the Scythe and feeling as though it was on the edge of oversteer. With the Scyton, I was running 90% steering lock and feeling on the edge of understeer.
Made a small change during the night to increase the front droop by 0.5mm to try and take some of the edginess away from the steering. Also had to raise the body up to clear the bumps in the carpet – not sure which change caused the improvement, but the car was better afterwards!
Was I any faster? Not really. I finished in the same sort of place I usually finish with the same sort of margin to other drivers who haven’t changed cars (although there was one brand new XRAY 2011 which seemed to have given it’s owner a little extra pace). But I was more confident in the car, and that can only be an advantage.
Basically it’s a hybrid of the TOP Photon suspension and the TOP Scythe chassis and drivetrain.
Why have I done this? Well, partly because I can (since I own both cars), and partly because I hope it will take the best bits of each car to make something even better.
I like the simplicity of the Scythe; I like that the chassis has no undercuts beneath the motor and that the top deck is not countersunk for more consistent flex and tweak; I like the security of the servo mounting; and I like its light weight. But I have reservations about the suspension (borrowed from HPI). The hubs are incorrectly moulded with unequal toe; the roll bar drop links are tight and the geometry is prone to oversteer (probably due to the short camber link at the rear). The Yokomo shocks are tricky to build consistently with the supplied diaphragms (Yokomo do now have some black rubber Tamiya-style diaphragms but I haven’t tried them).
I like the suspension on the Photon. It is much more accurately moulded; strong; and operates smoothly. The rear geometry is better balanced, with a longer top link. The Tamiya-inspired shocks are plush and easy to build. But the chassis is over-complicated with too many components to come loose in a race, and arguably too many adjustments that you don’t need. It also has an undercut under the motor (on my 6-cell chassis, although the LiPo chassis has been changed), and a countersunk top deck which makes de-tweaking impossible. The servo mounting is far less secure than on the Scythe, requiring smaller mounts and a lot of extra spacers.
Handling-wise, the Photon has a slight understeer tendency, the Scythe has a fairly strong oversteer tendency. I’m hoping that the Scyton will be somewhere in between.
Overall, the conversion is fairly straightforward, although there are a few small dimensional differences. This is how I did it… (all measurements approximate)
Starting at the front, the Photon arms have a forward sweep of about 2mm compared to the Scythe. To compensate for this I chose to move the ackerman plate forward by one hole to give straight steering arms again. The Photon ball ends are too big to fit on the Scythe steering bellcrack, it is the only part of the conversion that requires the old Yokomo ball ends. The Scythe crank would be equivalent to the middle length and a theoretical “8” insert on the Photon. The Photon C-hubs are 0.5mm lower, so I removed 0.5mm from the inner camber links. The Photon suspension arms need a 0.5mm spacer and a couple of shims on the inner suspension shaft. Shocks and anti-roll bars are a straight swap, although I have spaced the top shock mounting forward by about 4mm, and the anti-roll bar drop links foul the inside steering knuckle on full lock at full compression (shouldn’t be a problem on the track).
The rear arms are swept forward by 1mm. When you take into account the extra 0.5mm spacing required on the suspension shaft, the wheelbase ends up about the same, but the weight distribution has been moved back a tiny bit (0.5% or so!). The rear lower arms are 0.5mm shorter than the Scythe, but the top link is mounted 2.5mm further out on the hub, which should generate a better balance overall. The rear hub is 2.5mm lower than the Scythe, simply use a 2mm spacer though and it matches the front Photon hub geometry. Again, I am running 0.5mm less under the inner camber link. Photon driveshafts (46mm front and 44.5mm rear) appear to fit the Yokomo diffs perfectly.
As you can see the Scythe comfortably fits brushless and LiPo technology. The LiPo has to be mounted outboard for balance – so the NiMH-shaped centre bulkhead of the Scythe is not a problem. Car needs about 15gm ballast to reach 1350gms with no fancy lightweight parts.
I’m expecting the car to have a bit less steering than the Scythe, so for that reason I have put a basic Scythe setup on the car (0.5mm under the blocks, 2.5 front arm sweep), rather than the setup I have developed to tame the steering with the original suspension. Testing will prove whether the setup is right or not. Looking forward to it.
Not 100% happy with these but may be of use to others…
These are what I developed at the Carpet Thrashnal on the 15th November. They were good enough to qualify 5th in 13.5, and finish 5th and 2nd in the finals. There is definitely a lot more to come.
The initial setup is what I threw on the car before arrival. It has a few tweaks to the roll centres, shocks and bars compared to the kit setup. It proved to have a bit too much steering, so during the day I stiffened the oil, springs and bars, and raised the roll centres to get the final setup, which was well balanced but a bit edgy at the limit once the grip came up.
Unfortunately I ran out of parts before I could try any stiffer suspension or chassis settings! Will be trying the foam top-deck and 1.4mm bars at the next meeting – possibly more. These parts seem to be working well in other hands.
Sorry about the poor image quality – I don’t have full Acrobat so re-scan the sheets into the computer!
I’m now running the latest car from the TOP stable, the Photon.
In time-honoured fashion I will run through the build with a few badly-lit pics and tips that you may or may not find useful.
Building the Asphalt Rubber Pro version of the car, I was very impressed by the quality of the parts, a clear step up from its forerunner, the Scythe. The drivetrain in particular is outstandingly smooth. Mouldings are very precise; press fit where they need to be; free running where they need to be. With two exceptions, no fettling is required to build a car that works as it should. So despite there being a lot of screws and a lot of components, the quality means it goes together very easily.
The two disappointments are the spur gear adaptor and the shock pistons. More on these later.
General build tips apply as they would to all model cars. Use quality hand tools, tighten parts evenly, use a little threadlock on metal-to-metal fixings, and lubricate accordingly. This car uses a lot of M3 set screws and some M2.5 screws which need a quality 1.5mm driver. I chose TOP’s own driver (#PT-021025). The car is quite unusual in that it comes with no lubricants, fluids or tools, so a well equipped pit box is an asset!
My car is from the 3rd batch to enter the UK, and has a few updates compared to the manual and addendum.
First change from the paperwork is that the car now comes with steel spool outdrives, which are held in place by long 15mm screws. These take a lot of tightening, but it’s worth it. I’ve got two race meetings under the car’s belt now, at the first one I did a lot of crash-testing (oops), and the spool was completely unharmed. The alloy driveshaft bent though! Changed to the steel driveshafts for the second meeting, and although I crashed a lot less, there remains no damage.
The rear diff is truly superb. If your kit is anything like mine, you may be concerned by the slightly coned appearance of the diff plates and the inclusion of steel balls instead of an expensive alternative. Well you needn’t be. This is the best diff I have built. It can be confidently set to rotate smoothly with zero slip. I think the Belleville washers on the screw may be the key (most diffs use a coil spring).
On the subject of Belleville washers, the addenda suggests removing a couple to stop the driveshafts fouling the diff screw. This is no longer an issue as the kit now comes with shorter driveshafts – more on that later.
Other great features of this diff are the precision alloy outdrives, the press fit dust covers, and the easy external adjustment.
The first problem part is the spur gear adaptor/layshaft pulley. For whatever reason the pegs on the adaptor have been moulded at the wrong centres for “standard” spur gears – this includes the Kimbrough included in the kit, and my preferred Kawada gears. TOP acknowledge this in the manual and suggest reaming out the spur gear. I chose to go the other way, and just trimmed the outer edges from the adaptor pegs. This does mean that there is less material for the screws to thread into.
The good news is that spur gears with the correct sizing are being developed right now.
The quality of the pulley mouldings is very good, and the layshaft runs true. However there was a little lateral movement on my car which I decided to shim out after my shakedown run – 0.2mm on each side should do it.
The steering crank is possibly the most adjustable on the market today. It has a big impact on the amount of steering the car has. The longer the crank, and the narrower the insert, the more steering you have. The kit setting does not generate a huge amount of steering, one of the first setup changes I made was to move the crank to its longest setting which was a significant improvement.
When building it, a tip I have been given (but not yet used) is to superglue the nut into the bottom for easier adjustment. You also want threadlock on there, loose steering mid-race will not be a happy experience!
This is the first appearance of the 5.3mm diameter ball nuts that the car uses. They are robust and smooth, but because they are built onto long set screws, they need careful threadlocking to stop them working loose. A 3mm hex driver fits into the ball end to tighten them.
Two important tips at this stage. One is to carefully threadlock the layshaft mounting screws in place – I know of two UK team drivers who have had it come loose mid-race. Second is to spend a little time on the belt tension. I found the front belt way too tight in the default setting, so much so that it would bend the front of the chassis up. Three notches looser was right for me.
By the way, a good tip I was given on using threadlock is to just run your finger over the thread to spread it around without having a big blob that might goop up something you don’t want to. On this build I gooped up a driveshaft with threadlock, and in the past I have gooped up chassis screws which then refused to come out!
This stage is where a lot of the smaller-than-usual M2.5 screws get used to fit the top deck. Take care not to overtighten or strip these – a good hex driver is essential. This is also the stage where the steering crank is bolted in – after my shakedown I found it had a little free play, a 0.2mm shim cured it.
TOP responded quickly to the problem of driveshafts hitting the diff screw and the kit now comes with a pair of 44.5mm shafts. These must be fitted in the rear and use the blades. The kit also comes with alloy 46mm driveshafts for the front instead of steel – I bent an alloy one during my shakedown (admittedly I did crash a lot), so I have now fitted the steel ones.
Another part that isn’t mentioned is the pair of shims that come in the driveshaft bag. These are for the rear axles and take out a little bit of play.
The steering hubs need careful assembly. The kingpins are screws going through collars – if you tighten these down fully, the steering will lock up. So, they need to be backed off 1/4 turn. To make this work on the upper kingpin requires careful threadlocking of the ball nut, otherwise it will work loose.
The new suspension mounts give a lot of scope for adjustment of roll centre (6 positions, 0.5mm apart) and toe (0.5 deg or less at a time). Although there are a lot of parts, the quality is very good and it all goes together easily to make a solid yet free suspension. Threadlock is important here.
On the front suspension, a 2mm spacer is supplied instead of the published 1.5mm one. I chose to use it on the rear of the shaft.
Building turnbuckles is bloody hard work! Especially ones made of hard plastic like the new TOP ones. Fortunately I have a couple of tools that make it a bit easier. One is an M3 tap (TOP #PT-081030), another is an old Yokomo (Lunsford?) turnbuckle wrench which is just the right size. It is still difficult to thread the left-hand threads though. My finger is still recovering a week on!
The anti-roll bar links are really excellent – the smoothest ones I have come across yet, which is sure to have a positive effect on the handling. But they are also quite a fiddle to assemble, so again wise use of the tap and threadlock pays dividends.
Another small disappointment with the roll bars is that the aluminum shock tower mounts do not hold the smaller diameter bars in place properly. I am working on a solution to this.
Here we find the second big disappointment on the car. The shock pistons.
The finish on these is a little rougher than other parts of the car; they have an unusal design with a groove around the centre; and they move up and down on the shock shaft if left unshimmed.
From talking to others it would seem that the pistons themselves are fine if you clean them up a little. But the movement on the shaft is a definite no-no. It builds a shock that clicks and has inconsistent damping. You will need 3mm shims (TOP do some, #PA-100305), maybe as much as 0.3mm worth.
I have chosen a different solution. I have rebuilt the shocks with Tamiya 3-hole pistons (#53573), and to avoid using shims I have also fitted Tamiya shafts (#53575). In my opinion, the quality of the pistons is higher, and the shocks feels a lot better for it. I also fitted some Tamiya o-rings (from #53574) and a Yokomo bladder (#YS-8DMH), although that was mainly because I had them to hand and they made the shock a little easier to build – no performance gain was intended.
The shocks also benefit from a thin 10mm shim (TOP #PA-101012) between the bladder and top cap to improve the seal, otherwise they may weep.
Otherwise, assembly is very straightforward, the shock bodies and other plastic parts are very good quality. One thing to watch out for is that the shock bottom will not thread on fully – so don’t try! The exposed threads are hidden by the spring cup.
Because the Photon uses the same bulkheads front and rear, the front shock mount has this slightly unusual reversed layout. Works perfectly though – again, all it takes is a little careful threadlocking and the mounting is very secure.
One small detail is that the front bumper needs trimming down a little. The bumper is built for a 3mm chassis, the car has a 2.5mm chassis, so it creates a lip at the front of the car which needs to go. The Scythe used to have some optional fine bumper shims for a very professional finish, unfortunately the Photon does not, but it is hardly an issue.
One of the most noticeable features of the Photon is its deceptively simple LiPo tray. Just mount the LiPo in the usual place, and stick weights in the tray until the car is balanced. No ugly lead on the pack itself!
This solution was great… until the BRCA dropped their weight limit to 1350g. I’m running relatively heavy electronics, and I couldn’t get the car to balance using the tray unless I added 60g to it – which took the car up to 1440g!
Instead, I have chosen to mount the LiPo further out – 16mm to be exact, so that the centre of the LiPo is in line with the tray mounting screws. I have made a little plasticard tray to give the LiPo some protection and to help me put the pack in the same place again and again. With this setup the car is perfectly balanced at around 1380g.
Many other Photons are near 1350g with standard parts, and don’t need to run the LiPo so far out, or choose to use a little ballast in the tray. Others don’t bother with balancing it at all and still compete. Run whatever you are happy with!
Another thing that is a bit of a fiddle on the Photon is mounting the servo. For some reason the mounts are way off fitting my Futaba S9550, a common low-profile servo. A healthy selection of screws and shims is required. This setup is using the larger TOP mounts, 5mm of extra spacers, a large M3 washer, and an M3x15mm screw. It has proven to be very secure, and the steering arm is lovely and straight.
A servo saver is highly recommended. Running without can break the steering crank. I’ve never believed the claim that servo savers make the car sloppy in the steering – I can’t tell the difference between a good saver and a solid servo arm (although I do know a weak saver when I drive one). This is the Tamiya High-Torque saver (#51000), which I was very happy with in the Scythe. The outer hole on the saver is in the same position as the kit arm so the geometry is correct.
At the end of Step 37, TOP claim that the Photon is ready-to-race… well, they did overlook a couple of parts that are in the bags. One is the wheel nuts – I’m sure you can all manage to fit them 🙂 – the other is the antenna mount. As far as I can tell there are two spare holes for it, one on the lower deck near the motor and one in the middle of the upper deck. The upper deck worked out better for me.
Wiring up these cars is almost as time consuming as building them. After much wielding of the Weller, I settled on a fairly straightforward layout. There is not a great deal of room between the servo and the motor, even with a small ESC it seems that only a micro receiver could fit on the lower deck. My receiver is on top of the servo, but it only weighs 15g so it is not making much of a difference to the CG.