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PostPosted: May 1, 2016, 6:25 pm 
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I have just finished moving my fuel cell and battery to the rear of the car which should give me right between 57 and 58% of the weight on the rear which I think is about ideal.

Removing the fuel cell from the front, leaves plenty of space for some upgrades, including inboard suspension and improved steering geometry, possibly with a new faster rack and maybe power steering.
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The big question is should I take the opportunity to shorten the car by up to 10".

The problem with making the car shorter is that it will effectively move the engine forward which will make the weight distribution worse and also increase the polar momentum. Would the cornering advantages gained from having a shorter wheel base be more then the penalty of having more weight on the front.

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PostPosted: May 1, 2016, 6:40 pm 
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Worth it? Yes, as well as lighter, stiffer, and better traction.

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PostPosted: May 1, 2016, 6:42 pm 
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PostPosted: May 1, 2016, 7:52 pm 
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Miatav8,MstrASE,A&P,F wrote:
Worth it? Yes, as well as lighter, stiffer, and better traction.

The car will not really be any lighter. I will save a few <20lbs more like 10 just in the redesign. However the inboard shocks are heavier as there are more components so the weight will probably be a wash. I will just be moving the wheels back, the frame will stay the same length to allow a nose cone and splitter.
I never thought about the frame being more rigid but I guess it will be as there is less leverage ~10% on the existing frame.

Anyone know a way to calculate what the new weight distribution will be if I moved the wheels. Currently the car is 1760lbs with 58% on the rear and 106"wheel base. What would the weight distribution be if front the wheels were moved back 10 inches giving a 98" wheelbase.

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PostPosted: May 1, 2016, 7:55 pm 
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FlightService wrote:
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I am doing the work anyway as I need to move the steering rack and redesign the lower control arms to fix other issues, as well as lowering the car another inch. I will also be doing the inboard suspension as the rear end is just cool. At which point moving the wheels back 10 inches is not really much extra work.

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PostPosted: May 1, 2016, 11:35 pm 
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wrightcomputing wrote:
Anyone know a way to calculate what the new weight distribution will be if I moved the wheels. Currently the car is 1760lbs with 58% on the rear and 106"wheel base. What would the weight distribution be if front the wheels were moved back 10 inches giving a 98" wheelbase.


10" back would be a 96" wheelbase, not 98" correct?

I've had a couple beers, but by my math:
96" WB should round to about 54% rear bias.
98" WB should round to about 55% rear bias.

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PostPosted: May 2, 2016, 7:00 am 
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Do you have the equation you used to calculate the new weight distribution. I am really struggling with it. I am thinking that I need to do separate calculations for sprung and unsprung as the unsprung mass will move the sprung will not. The unsprung mass is about 80lbs per side.

You are correct that 106 - 10 = 96. You can see why I am struggling with the formula.

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PostPosted: May 2, 2016, 7:47 am 
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A fun exercise on weight distribution can be found here. Start with a model based on your best guesses and then start moving things around. It should get you close.

http://www.racingaspirations.com/apps/w ... calculator

We discussed it a few years back. It has some faults but is pretty cool and the calculations are done for you, provided the model is right. viewtopic.php?f=26&t=15055&p=163940&hilit=weight+distribution#p163940

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PostPosted: May 2, 2016, 8:38 am 
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I have used that tool a few times and it is really cool. However is does not work for what I need. I you move the wheels back it does not recalculate the weight distribution it just acts like your moving the mass of the tire towards the rear.
I think the best way to do it may just be to weigh the car and put the front on jack stands and move the jack stands backwards and see how it affects weight. Maybe move the tires too as a representation of the moved unsprung mass.

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PostPosted: May 2, 2016, 9:14 am 
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In your situation, I suggest you try this: Define the wheelbase, track etc. Place a single weight, that represents your current car that gets the current weight distribution. Then look at the distance that weight is from the rear wheels. Shorten the wheelbase and relocate the weight to be the same distance from the rear wheels. Violla! Yeah, not exact. You could improve the results by reducing the weight by the amount saved by the shortened frame. it should work for ballpark numbers. Unless I misunderstand your needs.

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PostPosted: May 2, 2016, 10:18 am 
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When I was figuring out the weight distribution of my car I actually balanced the car on the back side of a piece of angle iron. I then measured from the rear axle to the balance point and figured out the percentage of wheelbase that gave me. You already know the current weight distribution, so you would just have to work backwards to find how far the balance point is ahead of the rear axle, then knock 10" off the wheelbase and figure out the new percentage. It won't be exactly right because of the wheels moving back, but probably close enough for now.

Your current 106" wheelbase and 58% rear bias gives the balance point as 61.48" ahead of the rear tire. So now with the new 96" wheelbase, you would be down to about 56% on the rear if the cg stays in the same spot.
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Last edited by turbo_bird on May 2, 2016, 11:08 am, edited 1 time in total.

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PostPosted: May 2, 2016, 10:36 am 
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And don't forget that moving your front wheels and brakes back will move the CG back some too. If they're 1/20 of the total weight and you move them back 10" it'll bring the CG back half an inch.

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PostPosted: May 2, 2016, 1:17 pm 
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Definitely some good points from everyone and I think shortening the car will offer some benefit. I am looking forward to driving it to see how the 58% on the rear compares to the previous 54% and 50% that I have run previously. That will also provide more information on weight distribution vs feel and performance.

I think re-buying some vehicle scales will give me all the information I need prior to the front end redesign.

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PostPosted: May 2, 2016, 1:25 pm 
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turbo_bird wrote:
Your current 106" wheelbase and 58% rear bias gives the balance point as 61.48" ahead of the rear tire. So now with the new 96" wheelbase, you would be down to about 56% on the rear if the cg stays in the same spot.

I think you might have that backwards. 58% rear weight bias means the CG is 42% of the wheelbase from the rear tire, or 44.52" from the centre of the rear axle. If there was no CG movement, moving the front wheels back puts the CG 46.3% from the rear axle, for a 53.7% rear bias. However, as jack points out, the amount of unsprung weight in the front wheels/brakes/suspension is significant.

Wrightcomputing, do you have the overall weight of your car and the unsprung weight on the front? I'm guessing you have light wheels and such, but let's assume an average 100 lb unsprung weight on a 1500 lb car for example, using method of moments.
A car weighing 1500 lbs with a 58% rear bias has the CG 44.52" from the rear axle, which puts the moment about the rear axle at 66780 in-lbs (1500*44.52). Subtract the moment produced by your unsprung weight at 106" (100 lb * 106" = 10600) and add back the moment of your unsprung weight at 96" (100lb * 96" = 9600 in-lb), or alternatively simply subtract the change in moment (100 lb * 10in = 1000 in-lb). The new moment is 65780 in-lbs, for a new CG of 65780/1500 = 43.85" from the rear axle. This corresponds to 45.7% of your 96" wheelbase, which puts you at a 54.3% rear bias.


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PostPosted: May 2, 2016, 2:58 pm 
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wrightcomputing wrote:
The problem with making the car shorter is that it will effectively move the engine forward...
As well as everything else...Well, except the front wheels.

wrightcomputing wrote:
...which will make the weight distribution worse...
The significance of which is debatable.

wrightcomputing wrote:
...and also increase the polar momentum.
Bringing the front wheels closer to the center of gravity will actually improve this.

wrightcomputing wrote:
Would the cornering advantages gained from having a shorter wheel base be more then the penalty of having more weight on the front.
Would a SCCA Solo II National Champion ever lengthen the wheelbase on their car by ~10%, in an effort to improve weight distribution by just a few percent?

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