Panic braking

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canuckjc

Active member
Joined
Jun 29, 2014
Messages
34
Location
Los Altos, CA
Has anyone had the "opportunity" to experience the panic braking performance of their i3?
It happened today during rush hour on the interstate courtesy of a driver who cut me off and proceeded to come to a stop as they examined their lane options (There were a few choice words shared). Anyway, the stopping ability was not super confidence inspiring. I thought I was going to exercise my collision insurance and ultimately came within inches of the car in front of me.
Conditions:
70F, sunny, dry road. ~35mph to 0mph, full braking, ABS functioning as intended. The tires were the obvious weak link, unable to hold traction under the heavy deceleration. Limited contact patch is the likely cause.
Granted, I'm coming from several performance cars that stopped like they were hitting a wall, so my frame of reference is skewed. But I was surprised.

I tried to look up braking performance numbers but was unable to find any. If anyone has the info, please share.
 
BMW i3 BEV owner Peder Norby (InsideEVs contributor) tipped us off to a Motor Trend test article of the battery-electric version of the BMW i3.
http://r.search.yahoo.com/_ylt=A0LEVr4WzsxUQWQAXIEnnIlQ;_ylu=X3oDMTEzYXFlZGtyBHNlYwNzcgRwb3MDNwRjb2xvA2JmMQR2dGlkA1lIUzAwMl8x/RV=2/RE=1422737046/RO=10/RU=http%3a%2f%2finsideevs.com%2fmotor-trend-bmw-i3-test-0-60-mph-6-4-seconds-braking-unmatched%2f/RK=0/RS=5ZvzG5nd.zq.ehmUz8m5MABZr9A-

Of particular interest are a few of the test figures:

0 to 60 MPH in 6.4 seconds
Braking 60 to 0 MPH 108 feet

The 6.4-second time is one-tenth of a second quicker than what Car & Driver posted, but a full six-tenths quicker than BMW’s officially posted time of 7.0 seconds.

Even more impressive is the 60 to 0 MPH braking figure of 108 feet. As Norby states, “The closest to the BMW i3 was the Alpha Romeo 4c at 111 feet. Most of the others were in the 120-130 range.”

Indeed, the BMW i3 is unmatched in this category. Why the impressive braking performance? Some think the skinny tires would hold in the i3 back here, but the truth of the matter is that the low curb weight of the i3 (combined with a solid braking system) allows it to excel in this category, despite those tires. Wider tires could improve this figure slightly (a few feet), but they would add weight, which would negate some of the gain.
 
I have had two panic stops so far - both for deer at night. First was dry pavement at 60 mph - I thought for sure we were going to hit them (wife screamed)- stopped a good 30' short. (reference, last two cars wer MINI cooper S and subaru baja). What also helped was that for me, the car felt fully in control so I was able to take a hand off the wheel and lay on the horn. The second time was last week on snow covered roads - about 40-45 mph. The antilocks kicked in smoothly and kept the car in a straight line.

That fired a neuron - having the high beams low on the vehicle is absolutely awesome when its snowing!
 
Reassuring that the i3 performs well in braking tests, just not much consolation if the less effectively braked following vehicle with an inattentive driver slams you from the rear. :roll:
 
Thanks for the replies.

I'll add that I concur on the stability of the car under panic braking. It was tracking straight and true for sure.
From personal experience, I've owned a few cars with bonafide 60-0mph stopping distances in the 100ft range and this did not feel like that. Perhaps it's attributable to imperfect pavement conditions -- the section of interstate was grooved -- and some interaction with the skinny tires.
Maybe I'll find some nice piece of asphalt and give it some test runs.
 
I drive down English country lanes every day and have had several emergency braking situations to not hit dear jumping out onto the road. Having gone through it about 4-5 times, the thing I have realized about the i3 is that it doesn't FEEL like it braking very well but the reality is that it is in fact braking exceptionally well. Physics dictates that because the car is significantly lighter than most any other car anyone has driven, the energy required to bring that weight to a stop is much less. We (humans) feel that required energy when braking. Hauling a massive, heavy car requires lots of energy and that's what we feel.

All of the performance numbers speak volumes to demonstrate how impressive the stopping performance is, it just doesn't feel that way when happening.
 
@elptex
Perhaps you're right. I certainly wasn't calibrated to the feeling. I'll give it a go on some secluded road and see if it feels the same.
Thanks.
 
Also, speaking of tire contact patch, consider the cross section. We typically only think of wide tires as having a fantastic contact patch. That is only half of the equation, accounting for X-axis of contact patch. The other half is the radius, which accounts for the Y-axis of the contact patch. For example, a very wide tire with a small radius may have (to use simple number) a contact patch that is 1 by 10. Another tire may be narrow but have a large radius, with a contact patch of 2 x 5. In both cases, the total contact patch size is 10, just realized in different dimensions.

This now makes the obvious question of what ratio is good for what. Wide (1 by 10) is great for lateral (eg. handling), while narrow is great from front-back (eg. acceleration and braking). In this regard, the i3 has tires of exceptional radius. While 19" and 20" rims have been around for ages, they are very commonly adorned with teeny-tiny sidewalls, meaning that the rubber isn't contributing much to the radius. The i3's wheels, however, still have a fairly decent sidewall, with the rubber contributing a greater proportion to radius than typical 19/20" rims. The result being a "2 by 5" contact patch vs a "1 by 10" contact patch.
 
Agreed, it is tire patch, and as oriented on the i3's tall narrow wheels, it optimizes line of travel traction. The tire patch on the i3 is equivalent to that of the mini on performance tire/wheel package. IOW, it is not limited on tire contact patch. I've only had one emergency stop and it was at low speed - an idiot had gone through a red arrow seconds after it had turned red while I was trying to go straight through - I had to stop quite short to avoid hitting him - the car operated fine.
 
Actually, physics dictates that the tyre width and rolling radius have no effect on the size of the contact patch. Tyre stiffness ignored, there are only 2 things that dictate the size of the contact patch: vehicle weight and tyre pressure. Contact patch = vehicle weight divided by pressure.
 
PhilH said:
Actually, physics dictates that the tyre width and rolling radius have no effect on the size of the contact patch. Tyre stiffness ignored, there are only 2 things that dictate the size of the contact patch: vehicle weight and tyre pressure. Contact patch = vehicle weight divided by pressure.

What you say is correct though used to try and make an incorrect point. Tire pressure and vehicle weight do indeed affect contact patch, with those variables of what that contact patch can be in the first place being dependent on two factors (statistically constants): tire width and tire radius. It is simple geometry.
 
elptex said:
What you say is correct though used to try and make an incorrect point. Tire pressure and vehicle weight do indeed affect contact patch, with those variables of what that contact patch can be in the first place being dependent on two factors (statistically constants): tire width and tire radius. It is simple geometry.

Not sure I was trying to make a point. I was simply adding some facts to a discussion that seemed to need it. The size of the contact patch is completely independent of tyre width and radius. The shape of the contact patch is affected by both.
 
PhilH said:
elptex said:
What you say is correct though used to try and make an incorrect point. Tire pressure and vehicle weight do indeed affect contact patch, with those variables of what that contact patch can be in the first place being dependent on two factors (statistically constants): tire width and tire radius. It is simple geometry.

Not sure I was trying to make a point. I was simply adding some facts to a discussion that seemed to need it. The size of the contact patch is completely independent of tyre width and radius. The shape of the contact patch is affected by both.

With 100% positive intent (I'm really not one of those people must always be right; I'm specifically interested in ensuring only correct information is understood by people), that's not correct. To not seem/appear argumentative, I'll refer to the wikipedia page on contact page which explains things very well: http://en.wikipedia.org/wiki/Contact_patch
 
elptex said:
With 100% positive intent (I'm really not one of those people must always be right; I'm specifically interested in ensuring only correct information is understood by people), that's not correct. To not seem/appear argumentative, I'll refer to the wikipedia page on contact page which explains things very well: http://en.wikipedia.org/wiki/Contact_patch

Ditto. Just trying to get some facts into the discussion. My original statement was that, ignoring tyre stiffness and construction, then the formula for contact patch is what I gave. The wikipedia page must be one of the worst there: it's self-contradictory. Following the link to the page with some test results says the same thing: the formula would hold true for a balloon. However, given that tyres have stiffness, then it does not hold true for a real tyre, and so the conclusion is that the contact patch variation is driven mostly by tyre construction (sidewall stiffness, tread stiffness, etc.) rather than by geometry.
 
The people that make the tires, and BMW engineers both say that the tire contact patch on the i3 is essentially the same area as that of a mini with performance tires on it. IOW, you are not being limited by tire contact patch on the i3. It is tall and narrow for aerodynamic reasons. Now, the 20" summer performance tires have a wider profile, at least on the rear (but that option is also available on the 19" options). The winter tire/wheel package (as it is on the MEga trim level, I think), though is square verses offset like on many of the option packages.
 
jadnashuanh said:
It is tall and narrow for aerodynamic reasons.

Interesting. I'd always assumed (with no evidence whatever) that the tyre diameter and width reduced rolling resistance.
 
I think the high pressures are the biggest component of any range increase due to the tires. IIRC 33 and 41 PSI are much higher than most cars I have owned and are probably the very highest OEM tire pressures of any car on the road.
 
Really, I am honestly surprised! I had always considered pressures in the forties and above to be something reserved for truck and trailer tires..... Consider me corrected and informed, Thanks :p
 
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