Range Hit From Bike Rack — Data Point

[ForeverFree]

My wife’s 3LR RWD.

18-inch aeros ... but aero covers forgotten at home.

74 mph most of the way. Temps in the 70s leaving LA, near 100 across the Mojave and up the Owens Valley, then about 70 upon arrival in Mammoth.

Normal burn for this car at that temp and speed is about 250 Wh/mile or just under 1.1 RM/mile. (RM = “Range Mile”)

With a Kurt Sherpa hitch mounted rack and two 26-inch mountain bikes, we paid roughly a 50% range penalty. On level ground, at 74 mph, usage averaged 375 Wh/mi or about 1.65 RM/mile.

There were a few slower stretches (I-405 rush hour + Owens Valley town speed traps). However, there was also a net elevation gain of nearly 7000 feet. All told, we used 527 RM to cover 298 actual miles.

 

[Zoomit]

What were the winds like?

 

[derotam]

Normal burn for this car at that temp and speed is about 250 Wh/mile or just under 1.1 RM/mile. (RM = “Range Mile”)

Yeah, but that doesn't say whether you have done that trip before...without the bike rack. abetterrouteplanner shows 347Wh/mile consumption for a LA to Mammoth trip, so you aren't much different there.

i don't think the bike rack did too much.

 

[rickdogg82]

50% seems alike a lot. But a loaded bike rack will make a much greater difference that aero wheel covers which give 5-10% improvement(?).

 

[AlanSubie4Life]

I think @Daniel in SD sees over 400Wh/mi at 80mph with a rack (with one bike) mounted on the rear of the vehicle.

He could comment on how much worse it is than normal (typically he is at about 330Wh/mi at 80mph I think - has 9.5 inch 265 width PS4S).

 

[Daniel in SD]

I think @Daniel in SD sees over 400Wh/mi at 80mph with a rack (with one bike) mounted on the rear of the vehicle.

He could comment on how much worse it is than normal (typically he is at about 330Wh/mi at 80mph I think - has 9.5 inch 265 width PS4S).

I’d have to do a real test some time. I do get over 400Wh/mi with the bike on but I never take the same trips with the bike off so it’s hard to compare.

 

[Uncle Paul]

As the speed goes up the drag increases exponentially. At 50 MPH the rack might not be a big deal, but at 70-80 there is going to be a penalty.

 

[Daniel in SD]

As the speed goes up the drag increases exponentially. At 50 MPH the rack might not be a big deal, but at 70-80 there is going to be a penalty.

Quadratically, energy usage to overcome drag goes up with the square of speed.
At 80mph you're using twice as much energy to overcome drag as at 56.5mph (80/sqrt(2))

 

[ForeverFree]

What were the winds like?

Nothing remarkable.

 

[ForeverFree]

Yeah, but that doesn't say whether you have done that trip before...without the bike rack. abetterrouteplanner shows 347Wh/mile consumption for a LA to Mammoth trip, so you aren't much different there.

i don't think the bike rack did too much.

Without the bikes, I used 370 range miles for this trip. Roughly 285 Wh/mi.

With rack and bikes, 527 range miles. Huge hit.

 

[acoste]

Quadratically, energy usage to overcome drag goes up with the square of speed.
At 80mph you're using twice as much energy to overcome drag as at 56.5mph (80/sqrt(2))

That is correct, however drag resistance is only a portion of the total energy consumption. In Tesla Bjorn's tests the consumption increase from 55mph to 75mph is almost linear with the speed. Which shows that the constant and the linear consumption are still the dominant ones at these speeds.

Based on the above, 50% penalty for the bikes sounds too much. That would mean, the drag coeff x area would increase ~2 times.
20% penalty would be more realistic.

 

[AlanSubie4Life]

That is correct, however drag resistance is only a portion of the total energy consumption. In Tesla Bjorn's tests the consumption increase from 55mph to 75mph is almost linear with the speed. Which shows that the constant and the linear consumption are still the dominant ones at these speeds.

Based on the above, 50% penalty for the bikes sounds too much. That would mean, the drag coeff x area would increase ~2 times.
20% penalty would be more realistic.

As far as I can tell the energy due to drag is somewhere on the order of 140Wh/mi at 65mph. Just to put a number on it. That’s probably +/-10% just based on a fit to my 3 test speeds that I did recently.

Sort of aligns with your data assuming ... 100Wh/mi other losses...works out to 200Wh/mi @55mph and 285Wh/mi @ 75mph which is a ratio pretty close to the speed ratio; 1.43 vs 1.36. But would depend on whether you are talking RWD/AWD/P3D...fixed losses vary significantly between them (not aero, very much - aero covers, vehicle height, and rear spoiler are the only variables there).

I think the coefficient of drag could be impacted pretty severely - the back of the vehicle is a pretty important place, from what I understand - the turbulence there really matters.

For a 330Wh/mi at 80mph, that is 212Wh/mi aero and 120Wh/mi fixed (higher, due to the sticky wide tires mentioned above).

Anyway, to get to 400Wh/mi you’d “only” need the Cd to be 33% higher. 282/212 =1.33.

All rough order of magnitude numbers here. Obviously being off by 10% on the baseline number matters a lot! So these numbers above could be off by a bit. But they aren’t WAY off.

 

[derotam]

Without the bikes, I used 370 range miles for this trip. Roughly 285 Wh/mi.

With rack and bikes, 527 range miles. Huge hit.

I understand what you are saying and I see your statements, i just don't believe that the bike rack and bikes alone caused that difference... just my opinion.