S4ordie

So I've been looking at all of the great OCIC Dyno Day videos on Renntube and wondering what my car's output will be when Doc Brown is finished waving the magic wand. As I watched and looked at the output charts I began to wonder -Does the weight of the driver sitting in the car effect the Dyno numbers at the rear wheel?

I weigh 205lbs (93kilos-ish). If a driver weighed 110lbs (50kilos) would the dyno report better performance numbers? If yes, shouldn't we employ professional little people to sit in our cars and press on the accelerator (extension blocks provided)?

Just curious.

Alan

The car is strapped down to keep the tires well in contact with the dyno roller and to stop the car from moving... any weight in the car is incidental in this case... Not so on the street - but employing little people to drive your car instead of you seems to miss out on the enjoyment quota - even if you might get better numbers...

Alan

mark kibort

I was real curious about this as one time the car was strapped down really hard, harder than usual. the next time we did the dyno with minimum tension. no diffeernce in the numbers. FYI, the total cost of rolling friction is about 20hp at 150mph (4th gear) dropping down to 10hp at 80mph (still 4th gear). this is just the rolling of the tires on the rollers with rolling friction and wheel and diff bearings, along with inertia.

mk

RKD in OKC

Dynojet guy I talked with when dynoing my 415 rwhp 951 said that strapping down equals about 3000 lbs total, and that weight is what the dyno is calibrated for. So from that, I assume it affects the corrected numbers somehow.

Vlocity

On a lot of the newer Dyno's they will actually input the data for the car including it's estimated weight.

Ken

Alan

But the point is its how tight its strapped down that really matters most - there is likely more variance in that than the 'variable' weight of the car - and in any case it "should" be compenated for...

Usually the small delta weight of driver & fuel is going to be fairly irrelevant - it can only affect traction if the total down force is insufficient - OR rolling friction (slightly) if its too much. Unlike the usual scenario it doesn't have any effect due to accellerating the weight since only the roller is moving and its weight is fixed...

So depending on how its strapped down a bigger driver may even help...

I'm still intrigued by the idea of the professional Leprechaun stand-in drivers driving on little more than fumes to help maximize performance in the quarter for bragging rights...

Alan

mark kibort

I hope not. the factor is so low, it can only mess things up if it is an added factor. ever see the dyno guys us their foot to move the drums with the car in neutral? thats the rolling force. like i said, all of the variables added together are only 20hp at 150mph down to 10hp at 80mph. If you could change that friction by 10%, which would be HUGE, that would be 2hp at the top and 1hp at the the bottom. not even worth thinking about. Ive tried lowering air pressure, raising air pressure, strap down tight, lose, etc, and nothing changes the out put when we get to the point of overlapping runs. Look no farther than rolling friction changes for just adding weight. Its less than a rounding error in the entire skeme of things.

Ed Scherer

So, what y'all are saying is that even if I get my weight down to, say, jockey weight — 100 lbs. or whatever — my car's not going to get Ott-class numbers?

Crap.

Another potential diet motivation idea goes into the rubbish bin.

S4ordie

That is really really funny. Here in Hawaii we have mythical industrious little people called Menehune. I bet they could turn a quicker 1/4 mile time ...

Thanks for the feedback on this from everyone. I understand now that it is more the strapping tension which equates to weight on the rollers that can really affect output.

So if Louie Ott had put the extra 10 psi in his rear tires and they strapped him down a bit less, he would have blasted through 600 rwhp with ease.

mark kibort

I tried this, and it didnt yeild anything different. 10psi to 40psi vs 30psi and lose rear straps, almost dangerous. no change

dr bob

The ONLY thing that adjusting 'weight' or strap tension is going to do is affect the tire distortion where they contact the roller. Too tight and too many torques means some of your engine effort will be spent bending the rubber around the roller. There is no inertial component since you aren't actually moving the car. The only inertia affecting results is the mass and dimensions of the driveline weights. Heavy tires on heavy rims take more torque to accelerate, effectively adding to the mass of the roller, for instance. That same large spinning weight would also distort any attempted estimate of driveline loading that's based on spindown time with car in neutral.

A dyno operator might want the normal weight of the vehicle to do a soft prediction on street performance, like a 0-60 time or something. Without hard aero drag numbers and frontal area, you wouldn't want to depend on the results for more than an estimate anyway.


Folks should remember that the dyno is a tool for measuring change more than an absolute 'how much does it make' meter.

dr bob

Ed, if stripping 100lbs of driver weight got you down to 595, I'd say go for it just for the health benefits. The President's plan will only go so far...

S4ordie

Interesting comments dr bob. Seems like higher tire pressure would reduce roller friction thereby increasing output potential.

If all you wre interested in was posting highest possible numbers would it be beneficial to put on skinny tires/wheels?

All of this makes me think about the "Myth Busters" episode where the put a plane on a conveyor belt to see if it would take off without forward progress. Amazingly, to me, it did. Does this somehow correlate?

AO

Anything that increases rolling resistance that is not measured will result in decreased dyno numbers. How you measure that is the difficult part. If you strapped down a car super tight, I think it could affect the number negatively. Simillarly if you pumped up your tires super high, that could improve your numbers... theorhetically, of course.

dr bob

Sure. I think...

Consider that, on the dyno, the engine in the car, via the driveline and tires, is trying to accelerate a know mass with a known inertial moment. The mass of the car means nothing except that it's there to hold the tires to the rollers. Period. You aren't movong or lifting the car so its weight is unimportant. What is being measured is the ability to move the rollers. We try to come up with thumbnail 'factors' for friction spent in the driveline so we can estimate what the actual crankshaft horsepower is. Accessories and their drives are power robbers, for instance. That heat in the transmission, for instance, is calories that are wasted there rather tha available to drive the rear wheelws. Tire heat is the same, the result of energy wasted reorganizing rubber molecules.

Skinny tires with lots of air would be 'better' only if the artificially high dyno number is 'better' for you. To find out how YOUR car will push pavement backwards under the car, test with the same gear you use on the road.