Huge Understeer Problems On Certain Rc Buggies

[Live Steam Mad]

Hi Everyone,

I used to own a Schumacher Cougar 2 that would not even remotely corner properly.

It's taken me 18 years but I think I may have finally figured out what the understeer problem was being caused by on my Schumacher car (Tamiya and Kyosho cars don't seem to have such problems). In fact, my Cougar 2 never could corner as well as even my old Kyosho Raider!!

I have traced the problem to the so-called CRASH BACK front end (it's called Crash-Back in RC circles, my local friend Mike Taylor (who is currently taking a long break from RC which is a great pity) calls this "Kick-Up", he says "it helps the geometry on off-road racers", he seemed to be implying that this Kick Up term applies to 1:1 desert racers. Anyway, Schumacher put in a "Crash Back" front end in the Cougar 2 so that racers would still be able to race when other cars would have otherwise had their front suspension wishbones ripped off. The Crash-Back suspension design consists of simply mounting the front wishbones and C hubs, uprights and kingpins etc., on an ANGLE that slopes BACKWARDS.

You can see the severity of the angle by looking at the angle that the front shock tower makes with the vertical. The angle was suprisingly big on my own C2.

This is all very well at taking a heavy front impact and surviving it (compared to a car with non-angled front wishbones / uprights) but it has a massive drawback, which is this ;-

When the car tries to steer to the left, the INNER half of the front tyre, which is normally in contact with the ground when the steering is straight, is lifted off the ground!! You can verify this in your own mind by imagining that the car is positioned so that it is held vertically with the front end pointing upwards. Any attempt to steer to the left makes the stub axle which the wheel is on point downwards, which lifts a lot of the tyre off the ground, leaving only the very outermost edge of the tyre contacting the ground. Same principle applies when you steer to the right.

The result is, when I was using the only tyres that I ever had (a pair of 2-row large spikes tyres, a pair of 1-rib tyres with a separate row of large spikes, and a pair of road type tyres which are completely flat) that there is very little cross sectional area touching the ground when cornering heavily. This is precisely what we want to avoid at all costs, of course.

It is true that there is more weight (force) acting on the smaller cross sectional area of tyre, which you would imagine would add grip and compensate for the smaller contact area, but this is offset by the heavy loss of friction that having only a small contact area makes. You are far better off by having a much wider area of contact with the ground (which does lower the pressure on the ground admittedly) but of course the massive increase in friction caused by the much larger area of contact makes the cornering ability go up hugely. This is exemplified nicely by watching how a Morris Minor on narrow cross-ply tyres corners, they slither and slide around when cornering with a feeling of very little grip, compared to today's hot-hatches with their wide tyres which generate very good grip in cornering and would make a Morris Minor look rather feeble in the handling department.

The only 2 solutions that exist to this problem with, for example the Cougar 2, would appear to be 1) use spacers to heavily cant the front suspension angle forwards, which linearly reduces the angle of the crash-back (but of course sacrifices some crash-survivability), and 2) to use what I never thought of doing, a ROUND section tyre like a normal Dark Impact or Top Force buggy style of tyre which would make a significant improvement in contact area it would seem.

The Tamiya buggies (Top Force, Dark Impact, etc.) and the Kyosho buggies (Optima etc.) don't have nearly as big an angle of crashback as the e.g. Cougar 2 does (about a difference in magnitude of 4 or 5 times if I remember right on my C2 whose front shock tower was reclined back a LOT). Hence they (at least in 4WD versions) do NOT seem to understeer badly like my C2 did.

A final thought for you... Avante managed to avoid this issue altogether it seems, as looking in the 1989 Tamiya catalogue at the large picture of it seems to show that the suspension has almost zero crashback angle, so the front tyres will be in large contact with the ground even when cornering hard, and it has a BUILT-IN (I and Radius arm type) crash back suspension (which is incredible to watch in the demonstration in the Tamiya Vintage Video of the Avante, the front suspension collapses when you pull on it hard, which is achieved with the metal arms and the geometery) which is very ingenius. Avante solved the problem once and for all in my book LOL.

Oh well you live and learn I suppose. If terry.sc and others can confirm that the crash back suspension of a large angle causes this problem then I would be grateful. Or at least help me to pinpoint what the problem was with the C2, if it wasn't the crashback angle causing the loss of contact area (I can't think of anything much else that it could be).

Anyway the bottom line is that I raced at Kidderminster 2007 TC meet with a Top Force (it's been 15 years since I last raced an RC car of any kind) and for the first time ever I had a car that performed like it should and cornered like it should on kit tyres. Thank Goodness. I recently bought a Kyosho Optima, I hope that handles well with little understeer? Also does anyone know how the Kyosho Turbo Optima Mid is in the handling department? Does it understeer much?

BTW I posted this here in the Vintage forum as this is where I hang out most of the time, and for me it only applies to old buggies since I only ever race old buggies (I liked the look of the Sand Viper though very much, I think there was one running at Kidd. 2007?). It's a general point that applies to all manufacturers...

Cheers,

Alistair G.

[markbt73]

The huge caster angle on 2wd buggies is there to make them more stable in a straight line, and make the steering less "twitchy" around center. And it actually improves the contact patch of the front tires in a turn. Try turning the wheels, then lean the buggy in the way it would be leaning under a high-speed turn, and you'll see the outside front wheel (the one that does all the steering) gets closer to vertical the more you lean the car.

If you want to add steering, do it by decreasing front toe-in (or even going a degree or two toe-out); it's much more effective than decreasing the caster. Did wonders for my old RC10, which had 30 degrees of caster.

I don't know why 4wd cars have only a few degrees of caster; my guess is that they're inherently more stable in a straight line, with the front tires tending to pull them into understeer, so they don't need the help.

[Live Steam Mad]

The huge caster angle on 2wd buggies is there to make them more stable in a straight line, and make the steering less "twitchy" around center. And it actually improves the contact patch of the front tires in a turn. Try turning the wheels, then lean the buggy in the way it would be leaning under a high-speed turn, and you'll see the outside front wheel (the one that does all the steering) gets closer to vertical the more you lean the car.

If you want to add steering, do it by decreasing front toe-in (or even going a degree or two toe-out); it's much more effective than decreasing the caster. Did wonders for my old RC10, which had 30 degrees of caster.

I don't know why 4wd cars have only a few degrees of caster; my guess is that they're inherently more stable in a straight line, with the front tires tending to pull them into understeer, so they don't need the help.

Err no, I didn't mean the caster angle, although I accept fully your point above about the cars needing having some caster angle (pointing in of the top of the wheels) which when the body rolls to the right, e.g. when making a tight left turn, makes the OUTER portion of the RIGHT hand front wheel (which is now off the ground when running straight!) come back down so that the whole of the tyre is now making contact with the ground.

Instead I mean the amount that the front shock tower is raked backwards by (called crash-back in RC).

When you add on your very good point of body roll however, this makes a car with a large angle of crash back have the same thing happen to the right hand wheel when turning e.g. into a tight left turn i.e. it makes the whole of the tyre contact the ground.

Ironically however with my Cougar 2, the batteries were down the centre of the spine of the car (longitudinal) and not transverse mounted, and I seem to remember that there was very little body roll, if any. This, combined with the large crash back angle, meant that the tyres could then not contact the ground properly when turning!! If only it had had the cells the other way around and the receiver and servos and ESC quite a bit wider apart across the chassis, it would have increased body roll and thus increased grip!

I noticed that the Top Force that Harvey used to race (from "Boats and Bits" model shop, Earlestown, Newton-Le-Willows) used to lean heavily into the corners when cornering hard, and you could hear the front tyres scrabbling for grip, it was a wonderful sound, and it cornered beautifully. Although it had quite a lot of body roll as I say, it also had round profile tyres and so kept in pretty good contact with the ground at all times, more or less.

Thanks for the tip about the decreasing the toe-in to make the car corner more sharply, I'll try that with 2WD buggies if I ever get one again!

Cheers,

Alistair G.

[sosidge]

A buch of stuff.

I'm afraid you have the wrong end of the stick.

The Cougar does not have a crashback front end - only the older TopCat and 4wd Cat/ProCat had that system. It is where rubber o-rings link the front wishbones to stop crash damage. It does not help handling and serious racers would remove the rings and replace with a solid link.

What the angled front end of the Cougar is called is kick-up. This increases the castor angle, promotes dive under braking, and improves bump handling.

These are always employed on competitive 2wd cars to improve the handling. The Cougar had ample steering for racing.

[Live Steam Mad]

Thanks for the information, much appreciated. Any ideas then why the Cougar 2 understeered so badly? Or was it just my example? I think I remember terry.sc e-mailing Mike Taylor telling him that terry.sc's Cougar 2 or 1 also understeered.

If so, what would you recommend to cure the bad understeer on the Cougar 2 (as built by me when I was 15 LOL not 33, stock kit spec)? I ask incase I get a 2WD car which understeers. My C2 is long gone thank goodness. Mike had it for a short while and he said it had the most understeer that he had ever seen on any buggy!

Ali. G.

[madaz75]

Err no, I didn't mean the caster angle, although I accept fully your point above about the cars needing having some caster angle (pointing in of the top of the wheels) which when the body rolls to the right, e.g. when making a tight left turn, makes the OUTER portion of the RIGHT hand front wheel (which is now off the ground when running straight!) come back down so that the whole of the tyre is now making contact with the ground.

Pardon my confusion, But I do believe you have your Steering Geometry Terms a little mixed up.

What you're referring to as Caster, is actually Camber.

Here is a quick link to a Glossary of Automotive terms: LINK

[94eg!]

Kickup is found on all 2WD buggies it's also known as negative anti-dive geometry (because it promotes brake dive). This angle by itself can create the vehicles steering caster, or more caster can be added with angled uprights. When hitting a bump, the front tires move backwards as well as upwards which REALLY helps out with impacts, allowing the cars to ride up walls & barriers & other cars extremely easily. It also helps smooth out bumps. It's not found in such extremes on 4WD cars because wheelbase is extremely important when driving all 4 wheels. If you hit a bump and the wheelbase is severely shortened, it will result in a loss of traction due to the wheels suddenly traveling at different rates. At lesser angles, it can be used as a tuning device for 4WD cars.

Chances are you could really get your car to perform better by reducing rear traction instead of trying to increase the front. Try using harder rear springs, stiffer rear dampers, longer rear camber arms w/ less rear camber, tighter rear diff, or adding/stiffening rear sway-bar. There might just be too much mechanical grip in the rear.

[Live Steam Mad]

Pardon my confusion, But I do believe you have your Steering Geometry Terms a little mixed up.

What you're referring to as Caster, is actually Camber.

Here is a quick link to a Glossary of Automotive terms: LINK

Oh yeah, quite true, sorry about that.

So the previous poster should have said Camber, I understand now.

And the large amount of "Kick-back" on the Cougar 2 causes the C2 to have an equally large amount of Caster. This all works when there is body roll, but there was none or very very little on my C2, so the tyres would not contact the ground properly when cornering and hence the bad understeer.

Thanks to the guy who just posted before me here, I now understand that my C2 would have had much less understeer if it had a large amount of "dive" (where the front of the car goes down a lot) when cornering, because as the poster says it cancels out the kick back / caster angle to a potentially large extent, and thus allows the tyres to properly contact the ground when cornering under heavy braking, but alas from memory the suspension at the front end on my C2 was quite firm and I never saw it dive to any extent when braking, I think that this was yet another case of my cheap ESC at the time having brakes that were not strong enough to create enough dive to remove the kick back / caster when cornering / braking.

Hey well at least I am learning some really good things here, many thanks to you guys for the help, I really appreciate the light being brought to this, it helps me understand things much better. I can also now see that the suspension design of 2WD cars can be very complex with many factors to think about!

Out of curiosity, can someone please post a brief list of some 2WD Tamiya / Kyosho cars which have nicely low understeer in stock kit form?

Thanks,

Alistair G.

[sosidge]

All 2wd cars understeer. With oversteer they would be undriveable as you need to keep the driven wheels gripping otherwise you will go nowhere fast.

To generate more steering on your Cougar fit some softer front springs, I expect the Schumacher spring tuning kit is still available. Or use tyres that are better suited to the surface.

Rear motor 2wd cars are designed to be driven with braking in a straight line, turning in off-power while the front tyres are still loaded, mid corner the car will balance out and the weight will be over the rear again, ready for maximum traction on the exit. They don't steer particularly well on the throttle, certainly no way near as well as a 4wd car does.

[terry.sc]

Hmm, a lot to got through here.

I have traced the problem to the so-called CRASH BACK front end (it's called Crash-Back in RC circles, my local friend Mike Taylor (who is currently taking a long break from RC which is a great pity) calls this "Kick-Up", he says "it helps the geometry on off-road racers", he seemed to be implying that this Kick Up term applies to 1:1 desert racers.

Mike was right, it is kick up, not crashback and it is to help the suspension and is related to weight transfer and has no connection with the steering geometry at all. Decent off roaders allow you the option of changing the caster angle separately from the front wishbone kick up. It does not apply to full size off roaders.

Anyway, Schumacher put in a "Crash Back" front end in the Cougar 2 so that racers would still be able to race when other cars would have otherwise had their front suspension wishbones ripped off. The Crash-Back suspension design consists of simply mounting the front wishbones and C hubs, uprights and kingpins etc., on an ANGLE that slopes BACKWARDS.

The Cougar was the first off roader from Schumacher which didn't have the crashback front end, which allows the suspension to swing back in an accident and was held in place by large O rings.

This is all very well at taking a heavy front impact and surviving it (compared to a car with non-angled front wishbones / uprights) but it has a massive drawback, which is this ;-

When the car tries to steer to the left, the INNER half of the front tyre, which is normally in contact with the ground when the steering is straight, is lifted off the ground!! You can verify this in your own mind by imagining that the car is positioned so that it is held vertically with the front end pointing upwards. Any attempt to steer to the left makes the stub axle which the wheel is on point downwards, which lifts a lot of the tyre off the ground, leaving only the very outermost edge of the tyre contacting the ground. Same principle applies when you steer to the right.

The result is, when I was using the only tyres that I ever had (a pair of 2-row large spikes tyres, a pair of 1-rib tyres with a separate row of large spikes, and a pair of road type tyres which are completely flat) that there is very little cross sectional area touching the ground when cornering heavily. This is precisely what we want to avoid at all costs, of course.

The tyres designed for the front end of 2wd cars are actually rounded, like the rib spike and 2 row studs you mentioned. The caster angle (the angle the kingpin leans backwards) means that when it steers the wheel deliberately leans in which gives the steering more bite when turning into the corner.

The only 2 solutions that exist to this problem with, for example the Cougar 2, would appear to be 1) use spacers to heavily cant the front suspension angle forwards, which linearly reduces the angle of the crash-back, and 2) to use what I never thought of doing, a ROUND section tyre like a normal Dark Impact or Top Force buggy style of tyre which would make a significant improvement in contact area it would seem.

The Cougar front tyres are already rounded, for example the rib spike is designed to run on the rib in a straight line and lean the spikes over to dig into the grass when cornering. The Cougar was designed to use spacers behind the front shock mount so you can adjust the caster angle of the suspension, which for a 2wd buggy the ideal angle is usually around 25-30 degrees.

The Tamiya buggies (Top Force, Dark Impact, etc.) and the Kyosho buggies (Optima etc.) don't have nearly as big an angle of crashback as the e.g. Cougar 2 does

All those chassis are 4wd and sending drive through the front end means a totally different suspension geometry is needed. Their 2wd equivalents, the Astute, Dynastorm and Ultima, all have a large caster angle, which due to their design also translates to a large kick up angle.

A final thought for you... Avante managed to avoid this issue altogether it seems, as looking in the 1989 Tamiya catalogue at the large picture of it seems to show that the suspension has almost zero crashback angle, so the front tyres will be in large contact with the ground even when cornering hard, and it has a BUILT-IN (I and Radius arm type) crash back suspension (which is incredible to watch in the demonstration in the Tamiya Vintage Video of the Avante, the front suspension collapses when you pull on it hard, which is achieved with the metal arms and the geometery) which is very ingenius. Avante solved the problem once and for all in my book LOL.

The Avante suspension had a lot worse problems than the front caster angle. Again it is 4wd so not relevant to the discussion of 2wd buggy suspension. Part of the suspension problem with the Avante was the fact it had no caster angle, so the tyres didn't lean in when it steered so it never steered as hard as other cars around at the time, which is the opposite of what you have said.

I recently bought a Kyosho Optima, I hope that handles well with little understeer? Also does anyone know how the Kyosho Turbo Optima Mid is in the handling department? Does it understeer much?

Kyoshos cars are proper racers, so you can make it handle exactly how you want. You can make them understeer or oversteer, whatever you prefer.

[Live Steam Mad]

Thanks for the clarity Terry, wow this is a complicated topic, no wonder I couldn't understand it when I was 15 I'm only partly sure I understand it now, and that's only thanks to TC, without you guys I would be lost!

Cheers,

Ali. G.

[terry.sc]

I now understand that my C2 would have had much less understeer if it had a large amount of "dive" (where the front of the car goes down a lot) when cornering, because as the poster says it cancels out the kick back / caster angle to a potentially large extent, and thus allows the tyres to properly contact the ground when cornering under heavy braking, but alas from memory the suspension at the front end on my C2 was quite firm and I never saw it dive to any extent when braking, I think that this was yet another case of my cheap ESC at the time having brakes that were not strong enough to create enough dive to remove the kick back / caster when cornering / braking.

With the Schumacher Cougar 95 and Fireblade they developed a unique front end that was designed to keep the caster angle the same when the chassis squats and dives, thus fixing the problem of the caster angle changing. Works brilliantly off road but totally unsuitable on flat surfaces.

Out of curiosity, can someone please post a brief list of some 2WD Tamiya / Kyosho cars which have nicely low understeer in stock kit form?

Any of the more performance oriented ones if you have the right tyres and suspension adjustments. With a 2wd buggy it is easier to give it more steering than to try and fix a 2wd which oversteers everywhere.

[terry.sc]

Thanks for the information, much appreciated. Any ideas then why the Cougar 2 understeered so badly? Or was it just my example? I think I remember terry.sc e-mailing Mike Taylor telling him that terry.sc's Cougar 2 or 1 also understeered.

If so, what would you recommend to cure the bad understeer on the Cougar 2? I ask incase I get a 2WD car which understeers. My C2 is long gone thank goodness. Mike had it for a short while and he said it had the most understeer that he had ever seen on any buggy!

Yes I had problems with understeer on my Cougar 1, but that was mainly down to my driving style. Most people find my cars very dificult to drive because they are always set up with tons of steering and I personally couldn't get the handling twitchy enough for me. Everyone else I knew ran them and had no problems.

The cure for more steering on the Cougar was to fit the long wheelbase rear wishbones. I know making the wheelbase longer doesn't sound the best idea to give you more steering, but by moving the rear wheels back you end up with less weight overhanging the rear and more weight on the front end. The Cougar 2 front wishbones were swept back to put more weight on the front end to improve the steering compared to the Cougar 1.

[terry.sc]

The amount of kick up on 2wd buggies is something that came about due to the development of the chassis layout. To put the power down in a buggy designed for a relatively low grip surface you need a lot of weight over the rear wheels for traction, which is why the motor hangs out the back of the cars. The battery pack is placed down the centre of the chassis to reduce the polar moment of inertia to help it change direction when cornering to give them more steering. To get the front end of the 2wd buggies to turn in to corners while being stable in a straight line means a large caster angle. The large amount of kick up came about as a by product of the large caster angle as the manufacturers just angled the wishbones back to get the caster angle right.

The kick up has a couple of advantages. The first is the most obvious in that it helps the front suspension rise as it hits a bump, as well as helping on landing from jumps if it is coming down slightly nose down. The second isn't as obvious. As the buggy brakes for the corner the kick up means as the suspension compresses it increases the weight transfer towards the front of the car to give you more steering into and through the middle of the corner. As you open the throttle coming out of the corner the kick up helps the weight transfer towards the rear to help rear traction out of the corners.

4WDs are different as the driven axles mean all these effects would be greatly exaggerated if they ran with the same amount of kick up as the driven axle can load up the suspension under braking which can't happen with the undriven axles of a 2wd.

[Live Steam Mad]

The amount of kick up on 2wd buggies is something that came about due to the development of the chassis layout. To put the power down in a buggy designed for a relatively low grip surface you need a lot of weight over the rear wheels for traction, which is why the motor hangs out the back of the cars. The battery pack is placed down the centre of the chassis to reduce the polar moment of inertia to help it change direction when cornering to give them more steering. To get the front end of the 2wd buggies to turn in to corners while being stable in a straight line means a large caster angle. The large amount of kick up came about as a by product of the large caster angle as the manufacturers just angled the wishbones back to get the caster angle right.

The kick up has a couple of advantages. The first is the most obvious in that it helps the front suspension rise as it hits a bump, as well as helping on landing from jumps if it is coming down slightly nose down. The second isn't as obvious. As the buggy brakes for the corner the kick up means as the suspension compresses it increases the weight transfer towards the front of the car to give you more steering into and through the middle of the corner. As you open the throttle coming out of the corner the kick up helps the weight transfer towards the rear to help rear traction out of the corners.

4WDs are different as the driven axles mean all these effects would be greatly exaggerated if they ran with the same amount of kick up as the driven axle can load up the suspension under braking which can't happen with the undriven axles of a 2wd.

Very good explanations Terry, especially in that part above, thanks for that, it was clear and interesting.

Ta,

Ali. G.

[Live Steam Mad]

Yes I had problems with understeer on my Cougar 1, but that was mainly down to my driving style. Most people find my cars very dificult to drive because they are always set up with tons of steering and I personally couldn't get the handling twitchy enough for me. Everyone else I knew ran them and had no problems.

The cure for more steering on the Cougar was to fit the long wheelbase rear wishbones. I know making the wheelbase longer doesn't sound the best idea to give you more steering, but by moving the rear wheels back you end up with less weight overhanging the rear and more weight on the front end. The Cougar 2 front wishbones were swept back to put more weight on the front end to improve the steering compared to the Cougar 1.

Ahh, I think you hit the nail on the head here Terry, as I was reading this first line it all suddenly felt very familiar, YES I am ALSO one of those guys who likes to have the car run dead straight for 50 yards and then to hit the brakes rediculously hard and then have the car turn practically within it's own radius, and then slamming on the throttle to get it away again in the other direction

Yes, I like the car to twitch round like a wild cat into the corners, I HATE understeer Which ever buggy I used to examine, the first thing I would look at is how much steering lock I could get out of it!!

Most people who can drive much "better" than me [well by better I mean smoother, I was actually quite competitive and did well, until the speeds went to rediculous levels as everyone upgraded their motors and batteries and ESC's!] did not have a problem with their Cougar 1's, and I was the only one having problems, but I was also the only person at that time with a Cougar 2 at the local track (it was new out) and I instantly wished that I had bought a Top Force Evo...

The reason I went with Schumacher at the time was the good review of the C2 in Radio Race Car, and also because of the results that the C1 and C2's were getting in championships, and finally because of my (by that time) perception that Tamiya was all about low tech buggies made from plastic with heavy ABS bathtubs. When I drove Harvey's Top Force Evo (which he let me do because I spent quite a bit at his shop, and only months after I had got the C2) I was actually a bit dissappointed to find that on Tarmac with stock kit tyres, the TF Evo was also understeering (as I percieved it) but not anywhere near as much as my C2. Of course at that time I did not know about all of the much better cars that Tamiya had, and the Evo was much more expensive than the C2, and by then this "perception" in my mind about Tamiya's cars put me off them. That changed pretty quickly when I reaslised a few months later that I had made a huge mistake and that I should have bought a Tamiya Top Force.

At Kidd. 2007 I drove the a buggy for the first time back on a track in about 17 years and WOW did I ever have a lot of fun, it felt GREAT! For once, I actually found that I was just about satisfied with the handling of a buggy with my TF, which was stock kit except for GPM alloy motor mount, Tamiya Super Stock RZ motor, and GPM alloy front lower gearbox. Marvellous machine, finally I am happy!

Cheers,

Alistair G.