imagine for a moment what you are asking of these little cars when you turn the steerwheel or stick on your transmitter , we expect it to change direction on command without too much drama , so, the tyre has to grip consistently, the suspension has to hold up the car and absorb the bumps without going too deep into its travel, the diff has to allow the wheels to turn at different speeds while providing forward drive smoothly despite the wheels being in a state of constant grip then no grip then grip etc.
if you consider for a moment that all three of the above listed components are working against each other it becomes obvious that the set up of these items becomes an essential part of making driving an rc car enjoyable , its a great moment when every thing starts to gel , the car becomes so easy to drive that the only concern is tyre wear ,
tyres are often limited by club choice so you will end up trying to get the diff and shocks set up for the tyres and track , although you can usually trim and shape the inserts to give more even tyre wear .
lets first imagine a high grip track and tyres that are perfect ( will grip on all track corners) , if we run the car ride height too high the car will grip roll so running the ride height low would seem the obvious choice , problem with that is that the shocks may bottom out from chassis roll , if this occurs in very high grip conditions the car will also tend to grip roll , we could fit firmer springs to reduce the amount of roll and stop the shocks bottoming out , if we choose this route then thicker shock oil will also be needed , the next problem is that thicker oil and firmer springs bring their own issues , if the track has any rough spots the suspension may be too stiff to absorb them and thus the car becomes difficult to drive , lets go back to the standard set up and instead of changing oil and springs we just go for one less hole in the shock pistons and run the diff tighter , this will combine to reduce roll and also bring a little slip at the rear end which will also reduce roll by removing some of the bite from the tyres , its often the small adjustments that bring the greatest rewards !
all too often i see drivers deviate massively from the kit set up , we should remember that rc cars are often heavily tested under race conditions prior to release , having said that most cars are tested on indoor conditions , and if you plan on running say a 2wd buggy on an outdoor UK grass track then some deviation from kit set up is usually required ,
all my 2wd buggies use ball diffs (sealed ,geared diffs are preferred if you only run on one track!) , i always use silicon nitride (Si3N4) ,often referred to as ceramic diff balls , i use them in the diff ring gear and the thrust race , i get mine from a Chinese supplier on eBay for less than the cost of those branded items and get lots more for the money , they are more expensive than steel balls but, they last for ages , i am still on the original set i bought 5 years ago ,i only replace the diff rings and they build up super smooth every time , they are so much harder than the diff plates , steel balls can flat spot easily and as soon as that happens they start to destroy the diff plates and a rebuild is required , i had to rebuild my diffs after two or three runs with steel balls , its more like 20 runs with the silicon balls and the plates just need a wet sand to restore them , run your ball diff as loose as possible without slip then try it a little tighter and keep adjusting until the car is settled and quick ,although the ceramic balls allow a far tighter diff without killing the thrust race , use clear silicon grease on the ring gear balls and plates and black molybdenum grease on the thrust race ,
you can buy the silly little tubes of grease supplied by a well known rc car maker or save yourself an absolute fortune and get a tub of clear silicon grease and a tub of black grease from your local motor factor , the branded rc stuff is specially formulated to empty your pocket , I've tested them , under a microscope they look the same , they have the same thermal threshold and weigh the same because its the same damn stuff !!. the clear grease can also be used on the 'O'rings in your shocks.
although this is all just skirting around the actual set up such as static camber vs active camber change , roll centre adjustment , weight bias , aero dynamics etc will all effect each other and that is why i suggested earlier that kit set up is often the best place to start then do small adjustments one at a time and see what it does to the actual car you have , sometimes a change at the front of the car will effect the rear of the car and vice versa .
handling ,
so, you have your car set up stock as the build manual suggested , you've dropped in a pack and hit the track/street , i will start with 2wd cars as,lets face it ,they are harder to drive and far more fun ,
i bought a few 2wd buggies from ansmann/team c a few years ago , cost vs spec they are astounding value on paper at least ,and , as it turned out they are superb , i would go as far as to say that the big names felt a bit threatened and have had to move their respective games on a bit!
you can find the specs on line so i wont go too far into them, with the kit set up they had no front end control unless i came right off throttle and dabbed the brake to load up the front end a bit, not ideal for racing , to get more front end control i moved the rear shocks to the outboard position top and bottom and added a turn to the (excellent) rear diff and backed off the slipper clutch a little , this all helped a bit , i then added about 30g of weight to the front end and all of a sudden i had on power steering again , to bring it up a bit more i went over to two hole pistons in place of the stock 3 holes in the rear shocks and this was the end game , now the car was really turning off and on power .
take a look at this video from the ETS series ,
http://www.youtube.com/watch?feature=player_embedded&v=EnfBvB7U7Ho
you may notice that the track has massive grip levels on almost all corners , on a few the drivers are almost having to come to a complete stop as the track gives up more front grip than the drivers might like which has the result of the cars almost pointing at the track markers as the front end is 'hooking',as the event runs the drivers will of course learn this and will compensate with set up changes and altering their driving technique.they may even scrub tyres and use the reduced grip that scrubs(semi worn tyres) give , depending on the chassis they even opt for cutting some of the pins off the tyres, a fairly common practice.
its fairly unlikely that most club tracks will have quite so much grip but, there may be the odd corner that does , with 2wd you will get a large difference between on power and off power steering , i like to set my car so it over steers slightly when off power and has a hint of under steer when on power and almost total under steer when fully on power , this is achieved with the diff and weight distribution , as i mentioned earlier i run 1 full turn more than the initial set up point and 30g of weight added at the front end ,
if i was lucky enough to run on a track such as the ETS event has then i would be running far harder springs than the ones i use for the grass track i run on, but i may run the diff a little looser to kill the off power steering a bit ,also, judging by how much front end bite those guys are getting i would try removing the front weight or at least take it down to 15g from the 30g.
something i do to all my cars is fit titanium turnbuckles and apply a tiny amount of silicon grease to the ball studs and cups , the Ti turnbuckles are light but also very tough and any unsuspended weight you can reduce should help to keep your tyres in contact with the track surface , alloy items are light and stiff but are also weak, aluminium has a tensile strength that is only about 30% titanium , i tried stainless steel but broke two on the first run with a traction roll due to its brittle nature, the Ti items have not let me down yet , i try not to add any hop up parts unless i find a stock part that is weak or doesn't do the job it was made for , most supposed hop ups are just bling, replacing plastic dampers with good alloy items is a worthwhile hop up though as they are tougher and more consistent , choose parts carefully and save your money .
today i was out with one of my team c 02/ ansmann x-pro (and a few other names) buggies on the small turfed area i have worn a small track into, the surface ranges from short tight turf with moss to bare mud ,
this buggy has had no changes made apart from ti turnbuckles. ceramic diff balls and thicker oil in the dampers,otherwise it was the kit set up , i use 47.5wt and 3 hole pistons with the kit springs , the car was spinning out and swapping ends on the bare mud corners but was quick on the mossy corners without grip rolling , the shocks haven't had a service in about 4 years !! , i decided to give them a clean out and fitted new 'O'rings and shaft guides , while i was at it i changed to 30wt oil to see if i could get a bit better grip on the muddy corners ,
so, all cleaned up and silky smooth i headed back on to the grass , sure enough it had stopped swapping ends on the muddy sections but the downside was on the mossy sections it was grip rolling on and off power so i was having to tiptoe round the mossy sections , the car had much more grip but was slower over all.
this is what i was expecting , i could go down to 2 hole pistons but i will instead go for 42.5wt oil and run the diff a little tighter and save this car for dry conditions ,
i did this experiment to try to put across that even one small change to the shock oil can ruin the handling , while before the change i could simply drive slower through the slippy mud and still get round the track quickly ,messing with my proven set up turned the car into a marshals nightmare,
upper arm mounting and camber change ,
so i grabbed one of the other buggies that is set up for these intermediate conditions the track had,
it has the upper arms fully extended , inner most hole on the inboard mount and outer most hole on the outboard mounts , i also have 2.5mm of spacers under the front inboard mounts , and use the lower mounting holes on the rear uprights, this raises the roll centre and also gives positive camber change , the car has 1 deg negative rear static camber(top of the wheel is Lent inward toward the car) and 0deg front static , but, with the upper arm set up the wheels actually go into positive camber(top of the wheel leans outward) as the suspension compresses , in effect removing grip as the chassis rolls in turns , this car has 35wt and 3 hole pistons all round , it was way easier to drive and gripped without grip rolling ,
on the muddy sections the whole car drifted but i could easily catch it with the throttle and power the car through the turn to give great speed really early onto the straight ,
as the tyres started to bite on the mossy sweeper at the end of the straight the chassis starts to roll and the positive camber change dials out the tyres grip by reducing the tyre contact area and also rolling over onto the smooth sidewall of the tyre , this reduction of grip from the tyres also stops the shocks being fully compressed in high grip turns so the car has far less tendency to grip roll , remember as soon as the shocks run out of travel in a turn with off road cars all the cornering forces go into the tyres , when this happens the suspension can no longer absorb bumps and the g forces seen by the chassis spike , its this spiking that gives the chassis no where else to go but on its roof.
more soon :-)
