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Old 12-11-2007, 07:28 AM   #1
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Wheel weight and suspension stiffness

I have been experimenting a little with different wheels lately. I have tried some combinations of rims, tires, wheel diameters etc. Since joining GasSavers, I have also begun trying out Pulse & Glide.

In my quest for longer glides, I'm starting to wonder if using the heaviest possible wheels would be the most optimal. The wheels would store a lot of energy compared to their weight and as long as I don't use the brakes, it will be a win. When I bought new wheels recently, I opted for super lightweight rims and large diameter tyres to bring down rpm. While these wheels go nice with the general characteristics of this rather sporty car, it may not do much for FE and P&G. Am I thinking in the wrong direction here?

I'm also thinking of lowering the car to at least restore original ground clearance. The big wheels raised the car by 1". I have seen conflicting information on what effect this has on aerodynamics. My (stock) front airdam already covers the belly parts pretty well. Would an aircraft have less drag flying very close to the ground? Intuitively, I would say the drag will increase...

Lowering normally means stiffer springs, which will affect the workload of the shock absorbers. I have never seen this being discussed so I'm bringing this up also...
The shock absorbers suck up a lot of energy depending on road conditions. (Desert rally cars with burning wheels from overheated shocks, spring to mind). I haven't done the math but a car with harder suspension should waste less energy than a soft one. You can literally feel this if you have ever tried a bicycle with shock absorbers.
Anyone seen any work on this ?

BTW. - Here is my latest project.
Honda Prelude - generation 4 (with oversize lightweight wheels)

This car looks like it should have very good aerodynamics. I have never seen any figures though...
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Old 12-11-2007, 07:45 AM   #2
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Being in Sweeden, I would think that a bit of extra clearance might be good in the long winter?

What have been your results (fuel mileage) with the different combinations?

What has the RPM drop been at certain speeds? Did you see an increase in fuel mileage?
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Old 12-11-2007, 08:36 AM   #3
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Well, I went from 205/55/15 (when I bought the car) to 195/60/16 (picture). This is a 5% increase in circumference and a corresponding rpm drop. The car is geared low and has plenty of power so this makes it feel much more normal.

Now, the new rims have different aerodynamics and a higher et (deeper in the wheel well). The new tyres are also narrower but are not really low resistance. The conditions are also changing all the time (getting colder) and I have no fuel instrumentation. In addition, my driving style has changed more towards P&G lately. So there are too many parameters to really say for sure. I do drive the exact same 220 km highway strech two times a week and the only external variation is weather. All in all, the FE has improved 5 - 10% since the new tyres despite colder weather and much more rain.

I have just changed tyres again, this time to 185/65/15 spiked winter tyres. These are 2.5% larger circumference than the original rubber (same rims). I'm on my first tank since switching but I think I'll fill up today to satisfy my curiosity. I only do high speed driving (70 - 75 mph average) when doing my weekly commute but I'm going to do an FE optimized 50 - 60 mph run when I go home at the end of the week just to see what the car can do.

I'm desperately trying to sample my injectors (low impedance) so I can order a SuperMID. I have a long list of experiments to try but it feels like a waste of time without instrumentation...
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Old 12-11-2007, 09:15 AM   #4
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Actually I think the best combination is no dampening and very light wheels so that the tire follows the bump up and down resulting in a net zero loss. Rebound dampening is reducing the acceleration force on the down side of the bump and compression dampening increases decelleration force on the up side of the bump. You need dampening because of the weight of the wheel and its tendancy to bounce off the road when going over bumps.
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Old 12-11-2007, 01:04 PM   #5
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Light wheels are of course better for minimizing dampening losses. Running without shock absorbers is not realistic so I guess minimizing "stroke" by using harder suspension is the only way to optimize this unless I'm going to have to design electromagnetic dampening with regenerative functionality...
I'm thinking of trying to do the math and compare dampening losses/gains with improved glide when using heavy wheels but I'm not sure how to quantify this.
Any suggestions ?
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Old 12-11-2007, 02:47 PM   #6
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Drive on smooth roads! My extreem example was the result of putting street slicks on a mountain bike and the rear suspension was not dampened yet tracked extreemly well on bumpy roads due to the light wheel setup. What you need ultimately is properly damped suspension so you don't get tire bounce and that is about all you can do unless you get into the acceleration of the tire when in contact with the ground vs in the air to control the dampening rate electronically.
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Old 12-11-2007, 03:15 PM   #7
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No, you don't want to put heavier materials on the unsprung weight.
Matter of fact you don't want to add weight ever, but never on the unsprung side because it will affect the handling negatively even more than if you added weight above the springs.

Your car has to PUSH those tires onto the surface, with its own weight and that of the springs, when your car hits a bump the entire mechanism works to keep that tire on the road. If the tire is heavier then it will bounce more, and since your car weighs no more than it did before it will have a more difficult time pushing that tire back down.

In the turns it's even worse, the unsprung weight gets drawn outwards due to centrifugal force and in the process drags the entire vehicle with it.

That's just on the handling aspect alone, don't do it.
As for mpg, more weight never wins either.

Because in the battle between acceleration and coasting the heavier vehicle always loses, it's like that 'free' self-generating energy theory all over again.

You want to save fuel then I would consider racing seats and carbon fiber panels such as the hood, get the weight OFF the car.
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Old 12-13-2007, 01:09 AM   #8
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you gotta accelerate those wheels again during the pulse, so you lose. force required to accelerate weight+friction is higher than what you gain with inertia-friction
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