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Shorter Stopping Distance for ultralight vehicles? 
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#1
May2008, 08:48 PM

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The concept of ultralight vehicles is intended to allow greater fuel efficiencies in part by the use of composite structures to reduce mass by 2 or 3x. In several discussions of these vehicles I have seen and heard mention of the supposed additional safety benefit of shorter stopping distances, but I have not found any elaboration on why this is so, implying I fear that I missing something obvious.
Of course I reached for the standard stopping distance derivation: the kinetic energy of the vehicle and the work done by friction are both linearly related to mass, so that stopping distance is independent of mass as shown here: http://hyperphysics.phyastr.gsu.edu/HBASE/crstp.html giving the familiar distance = velocity^2/(2*Cf*gravity) So is there some other mass related factor here that is, say, a practical result of chassis, suspension, tires, or brake design? Reduced sway? The ultralight vehicle article is here: http://www.rmi.org/images/PDFs/Trans...nStategies.pdf Is lengthy covering several disciplines and I do not mean to introduce it all here. I am referring to the safety section on pg 14: 


#2
May2108, 10:38 AM

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Bump.
Am I missing the obvious? 


#3
May2108, 10:57 AM

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While lighter vehicles are easier to stop, technically stopping time has more to do with the capacity of the brakes, and the contact patch of the tires. You can make a relatively heavy vehicle stop very fast with big enough brakes; but big brakes are expensive, require more clearance (larger wheels on the car), and more maintinence.
Really the largest benefit of lightening a vehicle is the kinetic energy required to get it moving, reducing fuel consumption when accelerating. In the case of race cars, reducing weight increases acceleration with a set amount of power generation. 


#4
May2108, 11:12 AM

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Shorter Stopping Distance for ultralight vehicles?
In racing you can also use softer tires with a lighter car which improves the coefficient of friction. Lighter cars don't need the same amount brake ventilation (also power related) and lighter cars benefit more from aerodynamic downforce.
For suspension, lighter cars will have less unsprung weight, but heavier cars may have proportionally less, and I'm not sure which end comes out favored there. 


#5
May2108, 11:17 AM

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#6
May2108, 12:41 PM

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#7
May2108, 03:36 PM

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The advertising claim that superlight cars stop faster than heavy ones is really just trying to sell them; it isn't necessarily based in fact. It could be argued that it is easier and cheaper to make a light car stop quickly, but that's about it (and it's easier and cheaper to do most anything performancebased in a lightweight car). 


#8
May2708, 09:50 AM

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An ultralight aircraft can fly much slower and land at a steeper angle. The slow speed means less energy to dissipate with brakes. The steep approach angle means a more precise touchdown and better obstacle clearance at the end of the runway.
This is not the same subject but I have always thought it was interesting. A lightly loaded airplane will not glide as far as the same airplane heavily loaded. 


#9
May2708, 04:03 PM

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Did a bit of surfing and collected stopping distance specs as tested by Edmunds
Stopping distance from 60mph BMW M3: 3726 lbs, 19" tires, 100 ft (best any vehicle Edmunds tested) Jaguar XF: 4200lbs, 20" wheels: 108 ft Pontiac G8 GT: 4000lbs, 109 ft Audi A6: 114ft Lexus LS 400: 4500lbs, 120 ft VW Golf GTI (1998): 2800lbs, 139 ft Jeep Wrangler Rubicon (year?): 165 ft 1997 Wrangler: 184 feet (rear drums) 2003 Wrangler: 167.4 feet (rear disc) 2007 Wrangler (4 door): 4592lbs 148 feet (rear disc, larger front disc) So distance is all over the place, with little correlation to mass. I conclude then that this is all braking system and wheel/tire related, which further detracts from the claim by car ultralighters that they have an intrinsic advantage in stopping distance. 


#10
May2808, 02:11 AM

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The UK driving test has a big list of stopping distances that you have to memorise. Of course you aren't asked what 73m looks like on the road  you just have to recite "18m reacting and 55m stopping at 60mph"
Many of the accidents on UK roads are presumably caused by drivers trying to use a theodolite to measure the distance to the car in front while driving. A UK car show just found that the shortest stopping distance was for small sporty hatchbacks, typically < 25m from 70mph or a 1/3 the official distance. This site lists the typical distances for lots of cars (100mkh = 62mph) http://www.movit.de/rahmen/stoptbl.htm 


#11
May2808, 08:57 AM

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thanks mgb_phys, very interesting as it lists the same vehicle 'empty' and 'fully loaded'. The fully loaded cases looks to be on average 34M longer and in some cases 1520M longer!! This then supports the case of the ultralight vehicle designers: they can stop shorter. I'm at a loss to explain why!



#12
May2808, 10:13 AM

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Since the brakes at maximum clamping force can only convert a specific amount of kinetic energy per second to heat, having more weight means more kinetic energy which in turn means it takes longer to convert all of the kinetic energy to heat. 


#13
May2808, 10:30 AM

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Edit: Mech_engineer  I see you had been saying essentially this above already; I missed the point because I was too focused on that equation. Thanks. 


#14
May2808, 10:45 AM

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#15
May2808, 11:27 AM

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One of my favorite articles Road and Track has ever published is the August 2003 "Power Trip" where they have a 01000 deathmatch. Basically they haul butt to 100 mph as fast as they possibly can, and then slam on the brakes to get back to zero. Shortest time in each class wins.
Not only is it incredible the amount of time some of the vehicles take to do it, but lots of useful data was recorded about each vehicle, that can be used to compare them in a sort of an apples to apples test. Here is an interesting graph from that article: http://www.roadandtrack.com/article....&page_number=5 The Viper puts down an average of 237 hp getting from 0100 mph, but has an average braking power from 1000 mph of 547 hp. Looking at the graph we can see the braking curve is very linear, so we probably have a good estimate of the braking system's maximum power dissipation (taking into account traction available from the tires as well)... But look at this next graph: http://www.roadandtrack.com/article....&page_number=9 In the "exotic" class, the Saleen S7 is pitted against the Lamborghini Murcielago. The S7 weighs in at 3050 lb, a full 1140 lbs lighter than the Lamborghini. Yet, the Lamborghini stops 70 feet shorter and 0.8 seconds faster from 100mph than the S7. Why? Both cars have the exact same tires fitted (Pirelli P Zero Rosso's, 245/ 35ZR18 front and 335/ 30ZR18 rear), so the answer has to be a combination of more traction available to the Lamborghini because it weighs more, and the fact that the Saleen does not have ABS. The Saleen should have more braking power available, since it has 1" larger discs in the front and 0.8" larger dics in the rear, but its traction is limited by its lighter weight, and its lack of ABS causes the tires to lock up easily... The effects of no ABS can be seen in the graph, where the Lamborghini's braking curve is completely linear all the way to from 100 to 0 mph, while the Saleen's fluctuates wildly since the driver has to modulate the pedal to try and make up for the lack of ABS. Even though the Saleen was much faster to 100 mph, it ironically loses the 01000 because the Lamborghini is HEAVIER (more traction available from the ame set of tires) and has ABS. The Lamborghini puts down an average of 606 braking hp, versus the Saleen's "paltry" 370 braking hp. So there you have it, a case where being heavier means a shorter stopping distance... 


#16
May2808, 11:28 AM

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It may be important to point out that the ultralight concept car proposed in the Moore  Lovins paper not some kind of tiny toy car. It is a fivesix seater roomy design and comparable in passenger room to the Ford Taurus, and thus MooreLevins has room for Taurus sized brakes. However the MooreLovins design is 854kg and the Taurus is 1423kg.



#17
May2808, 11:50 AM

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#18
May2808, 01:07 PM

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So I tried charting the results of the Road & Track tests to see if there were any correlations...
It appears that available braking power linearly correlates to vehicle weight (regardless of brake size, and ignoring the S7 due to lack of ABS), perhaps due to additional friction available with the ground? It looks like this is because all of the braking systems are powerful enough to lock up the tires given the chance, and so instead braking force depends on tire compound, F/R weight distribution, and vehicle weight. The MercedesBenz SL55 AMG is the heaviest vehicle in the article at 4520lbs, but also puts down the most braking power at 638 hp, and stopping from 1000 in 312 feet. Also, Braking time/distance very slightly correlate to vehicle weight. It is my contention that this is because this is a very narrow sample of vehicles on the market, and they are all performanceoriented vehicles with powerful multipiston 4wheel disc brakes. If a large sample was taken including trucks, SUVs, economy cars, and superlightweights, I suspect the correlation could possibly disappear or at least become less available. 


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