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Verskis

How to brake with an RWD onroad car?

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I guess there are many RWD onroad enthusiasts here, as the RWD M-chassis cars seem quite popular. So, a question about braking: how to do the braking, and how to setup the car to minimize the possibility of rear end spinning out during braking?

I just converted my Tamiya XV-01 to a mid-rear-motor RWD car (quite similar as M08, just a bit bigger. Picture below.) using Embieracing carbon chassis kit, and went to a local carpet track to test it. Otherwise the handling was pretty good, but the braking is really, really difficult. I get that with RWD car the braking is hard, but for example my rear wheel drive Traxxas Slash is nowhere near as hard to brake without spinning as this car. Slash is of course much bigger and more stable car, so I guess this one will always be more twitchy, but is there a special technique I should use when slowing down? Now I used 0% drag brake and 75% braking force, which results in pretty weak brakes with my weak motor and tall gearing. Still, the car is very prone to spinning out when braking, and even when braking first, then releasing the brakes and turning in only after that, it seems that the weight transfer is still happening and the rear end steps out too easily. Should I brake even earlier and just allow the car to freewheel a little before turning in? Or is the trick to add a little drag brake? 

And are there some setup tricks that could help calming down the car during braking? I tried to reduce rear droop to allow for less weight transfer to the front, but that didn't seem to have much effect. Stiffening the front springs and front rollbar seemed to help a little.

What about the diff setup? I had my diff really loose, would a stiffer diff mean more stable braking, or would it be actually even worse?

And front toe? I had very slight toe out on the front, should it be completely straight or even toe in? Although to me it seems that toe in would actually worsen the problem, as the rotating car would have even more weight on the outside front tire, which would be turning the car even more with toe in. Is my logic sound here, should I actually add more toe out?

 

 

xv-01_RWD.jpg

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I'm no RC expert, but I track my 1:1 car which is real wheel drive.  On that the setup I and others use is:

Rear - toe in, and a reasonable amount.  This causes a little tyre scrub in a straight line but will stabilise the car under braking.

Front - firm-ish to prevent too much weight transfer (which you've found) and neutral toe or a small amount of toe out.  This is to help give responsive steering not stability under braking.

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11 minutes ago, Twinfan said:

I'm no RC expert, but I track my 1:1 car which is real wheel drive.  On that the setup I and others use is:

Rear - toe in, and a reasonable amount.  This causes a little tyre scrub in a straight line but will stabilise the car under braking.

Front - firm-ish to prevent too much weight transfer (which you've found) and neutral toe or a small amount of toe out.  This to help give responsive steering not stability under braking.

Thanks for the reply. I need to get an adjustable rear suspension mount to be able to get more toe in on the rear.

 

There is obviously one very big difference on 1:1 and 1:10 RWD cars: the RC car does not have any front brakes. Therefore you should try to brake only with the handbrake on your track car and then share the best setups after that. Just joking :)

 

I actually have some experience from 1:1 vehicles with only rear brakes, as I raced karts some 20 years ago.  The braking needs to be done on a straight lines on karts as well, but there is no suspension so you can turn in pretty much immediately after you release the brake pedal. It seems that on the RC car the suspension is "delaying" the weight transfer effect.

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There is no possibility to change that. I had the same "issue" with DT-03.

You have to change driving style: brake earlier and prepare car for turn. It is not so easy as with 4WD.

In my Sanwa radio, I have ABS function. It works quite nice but I heard that it is bit "unhealthy" for motor.

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As above, if you're braking hard, you need to brake in a straight line, and turing will step the back end out.

Also with the Sanwa, as well as ABS, there's an option to have braking strength related to steering angle, so with a bit of fine tuning, you can slam on, and turn in, and the controller reduces the brakes for you.

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13 minutes ago, cyclonecap said:

Are you using race tyres with additive? If not that will make the biggest difference.

Yes, Ride tires, used as spec tire on a racing series here in Finland. Sometimes I use additive, if I want to get the grip immediately, but sometimes I don't bother as it doesn't seem to make much of a difference once the tires have  warmed up. 

Seems that I just have to learn to brake very early and carefully. It just seems strange to me that when I watch some videos on youtube where M08 cars are racing with M07, the M08 doesn't seem to brake much earlier. Are those M08 drivers just super skilled?

I had this XV-01 as an FWD car before changing it to RWD, and I could brake much harder and later with the FWD. Because of that, I am very much slower around the track with the RWD. Maybe I just need to practice much more.

I hoped that the RWD XV-01 would be as fun to drive as the RWD Slash is. I didn't realize the driving characteristics would be so much different.

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I see at least two reasons:

XV-01 was designed as 4WD car with front motor. If you made such a big change, weight bias probably changed a lot. 

FWD can brake much harder, because all weight during braking is on front wheels. All you have to do, is to control car to not rotate.

Currently, probably you have a lot of weight on rear. It is same story as with Prosche 911. It has a lot of grip and drivers perfectly, but line between full grip and no grip at all is very narrow. It just runs smooth and at some point starts to rotate, because of heavy rear.

@Twinfan mentioned also weight transfer, which is very important.

Be aware that also chassis geometry has to be PERFECTLY fine. If both sides are symmetric, there is no much weight transfer, there is no factor to start rotation.

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I don't know if it works on the over-run/breaking, but have you tried a receiver with a built-in giro? Apparently the Flysky GT5 receivers (BS6) have that facility built in, so that if the rear end steps out, it counter-steers to help correct.

Whether that is allowable in your race series I'm not sure of course, but it might be a consideration?

Jx

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As that car has a front diff i guess you could try connecting a brake to it, paralleling the throttle channel to both the ESC and a servo that will actuate the brake when giving it reverse signal.

I dunno what would be best at this scale.

Maybe a disc or drum on the diff input and having the servo push something against it to emulate regular front brakes.

You would probably want a relatively stiff front diff so braking is applied to both wheels relatively evenly.

Barring that sort of customization you might try laying the servo flat on the top deck and mounting the battery as far forward as possible to shift the weight bias.

Or maybe making 4wd again and installing a one way diff backwards in the front, that way it would drive like a RWD on power but when you let off the throttle or give a reverse/brake signal the front wheels would also be applying drag/braking.

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Always run additive on the rear tires, and heaters if you have them.  Not sure how similar touring car is to F1, but I find running a tighter rear diff helps and don't have issues with braking hard on carpet or asphalt in the F104.  I only have experience with the F1 cars as far as RWD, but never had an issue with properly prepped tires.  I do see lots of other racers having issues when the rear tires are not properly prepped (car will spin itself around under braking). 

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If you are trying to get the braking to feel like a 4WD car, it just won't. 

This is why a lot of races use to be in classes.  2wd and 4wd were not in the same class. 

Do you have other 2wd cars?  How well does it break when compared to those? 

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10 hours ago, Verskis said:

I tried to reduce rear droop to allow for less weight transfer to the front, but that didn't seem to have much effect.

Cool project and a drastic change from the original XV-01!

I have spent quite some time and energy on TT-02 RWD conversions over the past 4 years. I am not an expert but what I think I have learnt is this:

- Tires and track grip is what has the biggest effect on drivability

- When spinning out under either acceleration or braking we tend to focus on the rear but the front of the car is often at fault!

- Experimenting with droop is what makes the biggest difference

- Generally speaking more power causes more problems :(

- You have to have reasonable expectations as it will never be as forgiving as an AWD car

In particular, at some point I wasn't running enough front droop (probably close to 0) and the car would spin out all the time. I then adjusted it to 2mm of front droop and it became much more tame, improving both the ability to put the power down as well as applying more and more braking.

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4 hours ago, JennyMo said:

I don't know if it works on the over-run/breaking, but have you tried a receiver with a built-in giro? Apparently the Flysky GT5 receivers (BS6) have that facility built in, so that if the rear end steps out, it counter-steers to help correct.

Whether that is allowable in your race series I'm not sure of course, but it might be a consideration?

Jx

I'm not going to race the car as RWD, as there are no racing classes that allow such an abomination. This is just for fun. The main target is to drive it on gravel as a rally car in the summer, but now I lowered and stiffened it up for some carpet track action. 

Anyway, I don't want to have a gyro. Seems like cheating to me, even if I'm driving all by myself :D Where's the fun if the car drives by itself?

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3 hours ago, Ziddan said:

As that car has a front diff i guess you could try connecting a brake to it, paralleling the throttle channel to both the ESC and a servo that will actuate the brake when giving it reverse signal.

I dunno what would be best at this scale.

Maybe a disc or drum on the diff input and having the servo push something against it to emulate regular front brakes.

You would probably want a relatively stiff front diff so braking is applied to both wheels relatively evenly.

Barring that sort of customization you might try laying the servo flat on the top deck and mounting the battery as far forward as possible to shift the weight bias.

Or maybe making 4wd again and installing a one way diff backwards in the front, that way it would drive like a RWD on power but when you let off the throttle or give a reverse/brake signal the front wheels would also be applying drag/braking.

No front diff on this car. Well, I could add one, as there is still stock XV-01 rear gearbox working as a front shock tower holder, but I'd rather like to keep it simple.

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3 hours ago, 87lc2 said:

Always run additive on the rear tires, and heaters if you have them.  Not sure how similar touring car is to F1, but I find running a tighter rear diff helps and don't have issues with braking hard on carpet or asphalt in the F104.  I only have experience with the F1 cars as far as RWD, but never had an issue with properly prepped tires.  I do see lots of other racers having issues when the rear tires are not properly prepped (car will spin itself around under braking). 

Maybe next time I try to sauce the rear tires much more than the fronts. Or maybe no prepping at all for the fronts.

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3 hours ago, DTSCB said:

If you are trying to get the braking to feel like a 4WD car, it just won't. 

This is why a lot of races use to be in classes.  2wd and 4wd were not in the same class. 

Do you have other 2wd cars?  How well does it break when compared to those? 

I have a couple of 2wd Traxxas Slashes. They are quite a lot easier to brake, although of course not nearly as easy as 4wd or fwd.

I know 4wd braking feel cannot be achieved, I just would like to hear about the braking techniques and setup tips of M08, M06 and other onroad RWD car drivers. 

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1 hour ago, Pylon80 said:

Cool project and a drastic change from the original XV-01!

I have spent quite some time and energy on TT-02 RWD conversions over the past 4 years. I am not an expert but what I think I have learnt is this:

- Tires and track grip is what has the biggest effect on drivability

- When spinning out under either acceleration or braking we tend to focus on the rear but the front of the car is often at fault!

- Experimenting with droop is what makes the biggest difference

- Generally speaking more power causes more problems :(

- You have to have reasonable expectations as it will never be as forgiving as an AWD car

In particular, at some point I wasn't running enough front droop (probably close to 0) and the car would spin out all the time. I then adjusted it to 2mm of front droop and it became much more tame, improving both the ability to put the power down as well as applying more and more braking.

Thanks, nice to hear somebody else is also converting perfectly fine 4wd cars to be less able :)

The last point, about the front droop affecting braking, leaves me scratching my head. How can the droop have an effect, when the front suspension is actually compressed during braking? Could it actually be affecting the settling of the car, once you release the brakes?

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Do you run stock rear toe-in? Iirc, it's 1,5° on XV-01, while M-06 and M08 have around 3°. That might be the root cause.

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8 minutes ago, Honza said:

Do you run stock rear toe-in? Iirc, it's 1,5° on XV-01, while M-06 and M08 have around 3°. That might be the root cause.

Yes, stock rear toe. I actually thought that it would be 2 degrees on the XV-01, buy anyway, more would be good.

Does anybody know, how much rear toe could I achieve, if I changed the rear suspension pin holder to this?

https://www.tamiyausa.com/shop/option-parts/rc-xv-02-alum-adjust-sus-mount/

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4 hours ago, Ziddan said:

Or maybe making 4wd again and installing a one way diff backwards in the front, that way it would drive like a RWD on power but when you let off the throttle or give a reverse/brake signal the front wheels would also be applying drag/braking.

@Verskis What Ziddan said above is the only way to achieve 4WD-type braking in a 2WD car, but then that would make it not truly 2WD B)

As others have mentioned, 2WD needs a totally different driving style than 4WD. Brakes on a 4WD car are like having ABS in a 1:1 vehicle: you can brake a lot harder, even slam on the brake pedal while turning, and you will just slow down in control.

Brakes on a 2WD are the same as the e-brake/parking brake in a 1:1, you have to brake a lot earlier and a lot softer, otherwise yes, the rear end will spin. 2WD also needs nearly instant countersteer if the rear end kicks out, which sometimes happens faster than our slow human reflexes can react to, so a 2WD will be a lot easier to drive with a gyro, including keeping it in a straight line during braking. A stiffer rear diff will help keep it straighter too.

As you mentioned, the XV01 is more twitchy than the Slash, which makes sense. They have similar motors (maybe 540 vs. 550 but still in the ballpark) however compared to a Slash, the XV is a lot smaller and lighter, so the motor has more of an effect on the handling of the car, particularly braking. So the trigger position for brakes on the Slash would not be the same for the XV. The XV would need lighter brakes throughout the whole range.

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19 minutes ago, Verskis said:

Thanks, nice to hear somebody else is also converting perfectly fine 4wd cars to be less able :)

The last point, about the front droop affecting braking, leaves me scratching my head. How can the droop have an effect, when the front suspension is actually compressed during braking? Could it actually be affecting the settling of the car, once you release the brakes?

Not enough droop simply makes the car unpredictable; it will veer off and swap ends at the slightest amount of turn in, that would have otherwise been controllable or even unnoticed.

You are correct that in a perfectly straight line, on a perfectly smooth track, it would make no difference... But then under these ideal conditions any car will brake correctly (but since these ideal conditions do not exist, etc etc).

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27 minutes ago, El Gecko said:

 

As you mentioned, the XV01 is more twitchy than the Slash, which makes sense. They have similar motors (maybe 540 vs. 550 but still in the ballpark) however compared to a Slash, the XV is a lot smaller and lighter, so the motor has more of an effect on the handling of the car, particularly braking. So the trigger position for brakes on the Slash would not be the same for the XV. The XV would need lighter brakes throughout the whole range.

Good points, and now that I think about it, the Slash should have a bit more rear toe and also some antisquat on the rear suspension, those should help keeping it more stable during the braking.

Antisquat should work as a pro-squat during the braking, right? That should reduce the weight transfer to the front, at least on theory. It would also be quite easy to test, just some spacers more under the front mounts of the rear suspension.

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@Verskis After converting several TT-01E and TT-02 to RWD, FWD, some with and without flipped one-ways (for 4WD braking), I can say that what makes a tremendous difference is the setting of the free-wheeling diff. This is from first-hand experience in parking-lot racing.

In your case, you can keep the front diff and lock it. You can also try just stiffening it, but locking is probably the best starting point. This will keep the XV-01 fully RWD, as the front wheels will still just freely spin, but it will really transform the car. It will improve braking stability, corner entry and also on-power corner exit. The downside is that it will add some understeer, but this can easily be corrected with driving technique.

I would even add that the front diff setting on a RWD car has so much impact, that I am surprised why regular high-end RWD kits do not have a front diff.

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7 hours ago, El Gecko said:

@Verskis What Ziddan said above is the only way to achieve 4WD-type braking in a 2WD car, but then that would make it not truly 2WD B)

Id argue it would be 2WD/4WR+B (reverse and brake) :P

The concept of having front wheel braking on a rear wheel drive car, mimicking real world is actually pretty interesting, ive got a TT02 chassi built, but unrun, which im intending to put a J71 VTA shell on.

The original car is rear wheel drive so i had been thinking id to leave the front diff out but i might try decoupling the front diff from the driveline and designing some sort of a brake instead, might be fun :)

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