Why does car understeer when going downhill? - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

hpde_addict · 12-14-2017, 05:05 PM

Intuitively, I thought that when the car is going downhill, more weight is transferred to the front tires, leading to the front tires gaining grip and rear tires losing grip. However at the Thunderhill skid pad, I noticed that when going downhill, it is much more difficult to get the car to oversteer compared to level and uphill. And when I'm already drifting in the downhill portion, my car tends to go outward like in understeer. The test was just driving around in a constant-ish circle, progressively adding throttle. What's mechanics are happening that's leading to this behavior?

Shark_Bait88 · 12-14-2017, 05:32 PM

Is your speed the same in the uphill and downhill portions?

If you're going faster downhill it could be that you're going in too hot, which can be why it's pushing.

Lex_K · 12-14-2017, 05:55 PM

Quote:

Originally Posted by hpde_addict

Intuitively, I thought that when the car is going downhill, more weight is transferred to the front tires, leading to the front tires gaining grip and rear tires losing grip. However at the Thunderhill skid pad, I noticed that when going downhill, it is much more difficult to get the car to oversteer compared to level and uphill. And when I'm already drifting in the downhill portion, my car tends to go outward like in understeer. The test was just driving around in a constant-ish circle, progressively adding throttle. What's mechanics are happening that's leading to this behavior?

Technically you are off camber when you are turning on a downhill.

hpde_addict · 12-14-2017, 06:25 PM

Quote:

Originally Posted by Shark_Bait88

Is your speed the same in the uphill and downhill portions?

If you're going faster downhill it could be that you're going in too hot, which can be why it's pushing.

I don't think I'm scrubbing any speed on the downhill portions, if that's what you mean by "too hot". I drive around in a circle below the limit and then add throttle to start the drift.

Hades · 12-14-2017, 06:35 PM

Wouldn't it be harder to get the frontend to move due to weight transfer? i.e. the front weighs more, so it would take longer to react to direction changes

TommyW · 12-14-2017, 06:41 PM

Is your trac control on or off?

DandoX · 12-14-2017, 06:51 PM

I do not claim to know the exact reason or be an expert but I watched this video today and thought of this thread.

Could it be your applying more steering rotation when going down hill like in the beginning of this video? Since that guy was gradually gaining speed then turning the steering wheel more aggressively which pushed the car vs allowing the back end to slide. Up hill you may be just turning less allowing the car to slide more easily?

Just a thought.

Video: https://jalopnik.com/a-donut-is-actu...ift-1821270890

CSG Mike · 12-15-2017, 04:00 PM

Quote:

Originally Posted by hpde_addict

Intuitively, I thought that when the car is going downhill, more weight is transferred to the front tires, leading to the front tires gaining grip and rear tires losing grip. However at the Thunderhill skid pad, I noticed that when going downhill, it is much more difficult to get the car to oversteer compared to level and uphill. And when I'm already drifting in the downhill portion, my car tends to go outward like in understeer. The test was just driving around in a constant-ish circle, progressively adding throttle. What's mechanics are happening that's leading to this behavior?

Weight transfer rate is different.

Ultramaroon · 12-15-2017, 05:00 PM

Quote:

Originally Posted by Lex_K

Technically you are off camber when you are turning on a downhill.

+1

CSG David · 12-18-2017, 07:55 PM

Weight transfer.

TommyW · 12-18-2017, 08:38 PM

You're probably asking too much of your front tires

rice_classic · 01-04-2018, 06:06 PM

Physics

Going down hill means gravity is adding to your car's momentum and increased momentum means a greater force upon the thing trying to resist or change that momentum (turning or slowing).

Therefore your car is understeering because the tires are overburdened. Either the moment of directional change needs to be reduced (turn in slower or with less radius) or your momentum needs to be reduced by further lowering your speed prior to turn in.

How a car handles directional changes in regards to track slope or pitch is what makes driving (or racing) on tracks with elevation change so enjoyable.

ROADRACER · 01-13-2018, 02:39 PM

I have noticed the same phenomenon on track. You have made a correct observation. I get push on down hill turns and oversteer on uphill turns. This makes getting the car setup for perfect balance in all the turns basically impossible. There may be some way to lessen the effect but I have not discovered it.

TommyW · 01-13-2018, 03:06 PM

Quote:

Originally Posted by ROADRACER

I have noticed the same phenomenon on track. You have made a correct observation. I get push on down hill turns and oversteer on uphill turns. This makes getting the car setup for perfect balance in all the turns basically impossible. There may be some way to lessen the effect but I have not discovered it.

You're overloading the front outside tire downhill and overloading the outside rear tire on uphill. Those are the tires responsible for the most grip in those circumstances

12-14-2017, 05:05 PM	#1
hpde_addict Senior Member Join Date: May 2015 Drives: '18 Camaro V6 1LE Location: SF Bay Area Posts: 161 Thanks: 57 Thanked 47 Times in 31 Posts Mentioned: 2 Post(s) Tagged: 2 Thread(s)	Why does car understeer when going downhill? Intuitively, I thought that when the car is going downhill, more weight is transferred to the front tires, leading to the front tires gaining grip and rear tires losing grip. However at the Thunderhill skid pad, I noticed that when going downhill, it is much more difficult to get the car to oversteer compared to level and uphill. And when I'm already drifting in the downhill portion, my car tends to go outward like in understeer. The test was just driving around in a constant-ish circle, progressively adding throttle. What's mechanics are happening that's leading to this behavior?

12-14-2017, 05:32 PM	#2
Shark_Bait88 Senior Member Join Date: Jun 2014 Drives: 2013 Firestorm FR-S Location: Midwest Posts: 1,350 Thanks: 1,674 Thanked 865 Times in 481 Posts Mentioned: 12 Post(s) Tagged: 2 Thread(s)	Is your speed the same in the uphill and downhill portions? If you're going faster downhill it could be that you're going in too hot, which can be why it's pushing. __________________ "Shark_Bait88, the man who’s spent the most money modding his stock FR-S and it’s still stock." -@jdnguyen

01-04-2018, 06:06 PM	#12
rice_classic Senior Member Join Date: Jun 2012 Drives: Nevermorange FRS Location: Seattle, WA Posts: 4,160 Thanks: 755 Thanked 4,200 Times in 1,803 Posts Mentioned: 77 Post(s) Tagged: 1 Thread(s)	Physics Going down hill means gravity is adding to your car's momentum and increased momentum means a greater force upon the thing trying to resist or change that momentum (turning or slowing). Therefore your car is understeering because the tires are overburdened. Either the moment of directional change needs to be reduced (turn in slower or with less radius) or your momentum needs to be reduced by further lowering your speed prior to turn in. How a car handles directional changes in regards to track slope or pitch is what makes driving (or racing) on tracks with elevation change so enjoyable. __________________ SCCA T4 - FRS Last edited by rice_classic; 01-04-2018 at 06:26 PM.

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12-14-2017, 06:35 PM	#5
Hades Senior Member Join Date: Dec 2017 Drives: 2013 Scion FRS Location: Socal Posts: 214 Thanks: 521 Thanked 100 Times in 64 Posts Mentioned: 0 Post(s) Tagged: 0 Thread(s)	Wouldn't it be harder to get the frontend to move due to weight transfer? i.e. the front weighs more, so it would take longer to react to direction changes

12-14-2017, 06:41 PM	#6
TommyW Senior Member Join Date: Feb 2017 Drives: '13 Whiteout Location: San Clemente Posts: 1,492 Thanks: 496 Thanked 1,242 Times in 673 Posts Mentioned: 10 Post(s) Tagged: 0 Thread(s)	Is your trac control on or off?

12-18-2017, 07:55 PM	#10
CSG David Join Date: Aug 2012 Drives: S2000 CR Location: California Posts: 2,109 Thanks: 537 Thanked 1,721 Times in 956 Posts Mentioned: 173 Post(s) Tagged: 3 Thread(s)	Weight transfer.

12-18-2017, 08:38 PM	#11
TommyW Senior Member Join Date: Feb 2017 Drives: '13 Whiteout Location: San Clemente Posts: 1,492 Thanks: 496 Thanked 1,242 Times in 673 Posts Mentioned: 10 Post(s) Tagged: 0 Thread(s)	You're probably asking too much of your front tires

01-13-2018, 02:39 PM	#13
ROADRACER Senior Member Join Date: May 2015 Drives: unlimited cheap build N/A track car Location: TN Posts: 101 Thanks: 12 Thanked 55 Times in 32 Posts Mentioned: 3 Post(s) Tagged: 0 Thread(s)	I have noticed the same phenomenon on track. You have made a correct observation. I get push on down hill turns and oversteer on uphill turns. This makes getting the car setup for perfect balance in all the turns basically impossible. There may be some way to lessen the effect but I have not discovered it.

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