shifting the (351W) engine forward

[Martin]

We're doing a LHD to RHD conversion (more specifically, Mark Dougherty is doing it for me). While we're at it, I'm considering shifting the engine forward a bit to help with the weight distribution.

Anybody know a good way of mathematically (or more scientifically) figuring out how much to move it forward by?

So far, I have "a couple of inches", "6 inches" and "a bit" should make it better balanced.

Any ideas?

I haven't committed to doing it yet, but am figuring that I might as well if it it helps.

Martin

[AJ Roadster NJ]

I would've thought that moving it forward would further tilt the distribution AWAY from 50-50, and that to approach the holy grail of 50-50 would require moving it back a couple more inches. Do I have this wrong?

[Martin]

I would tend to agree with you. I thought the same thing just looking at it, but apparently, the engine needs to move forward. Four or five people with a lot more knowlege than me have all said to move it forward.

Hopefully this thread will work it out once and for all.

I know it depends on battery location, and where you place a lot of other things, but there are a lot of smart people around here. I'd like to at least understand the reasoning behind it and not just have a "couple of inches" which could really mess things up.

Martin

[johngeorge]

I dont think you want 50-50 weight balance front to back.. http://en.wikipedia.org/wiki/Automobile_handling

from article (Center of mass):

The rearward weight bias preferred by sports and racing cars results from handling effects during the transition from straight-ahead to cornering. During corner entry the front tires, in addition to generating part of the lateral force required to accelerate the car's center of mass into the turn, also generate a torque about the car's vertical axis that starts the car rotating into the turn. However, the lateral force being generated by the rear tires is acting in the opposite torsional sense, trying to rotate the car out of the turn. For this reason, a car with "50/50" weight distribution will understeer on initial corner entry. To avoid this problem, sports and racing cars often have a more rearward weight distribution. In the case of pure racing cars, this is typically between "40/60" and "35/65".[citation needed] This gives the front tires an advantage in overcoming the car's moment of inertia (yaw angular inertia), thus reducing corner-entry understeer.
Using wheels and tires of different sizes (proportional to the weight carried by each end) is a lever automakers can use to fine tune the resulting over/understeer characteristics.

[KC Wildcat]

Do some searching here and on the other forum on "corner weight". I believe there are some stats on FFR website about front to rear weight percentages. It is really a crap shoot (a guess) unless you can do some measurement and observation (hard on a car that is not completed - but a completed car would make the mods harder to perform).

Here is what might be practical to do. First consider if you want 50/50 in a dry car, a wet car or a loaded car (dry - no gas or fluids; wet - add weight for gas and fluids -but do you want a full tank or 1/2 tank of gas/; loaded would be wet plus weight placed to compensate for the drivers weight). I know that may sound crazy to get that detailed but, if your car weights 2200#, a 220# driver is adding another 10% to the mix!! 18 gallons of gas (at 7#/gal) is adding another 125# or 5% - so you see how greatly that can affect things. This is why some orgs weighs cars with drivers now (because Danica Patrick is 30-50 lighter than her peers) and race cars handle differently at different times between pit stops (the rear gets lighter as the tank is closer to empty (not too mention they use baffles to control slosh, especially on road courses).

Ok, now that some of the considerations are in the open, what you can do is build the car as far as you can without engine in place, put it on 4 scales (on a very level surface) and drop your motor and transmission in place. Then put as many components in close to their final place, include extra weight for fluid in the radiator, gas in the tank, drivers but in the seat, battery in place, etc. Place windsheild in place and anything else of weight that you can. Add the front scales and compare to the total of the back scales. Lift and slightly move the engine/tanny forward (I assume this is the direction you will need to go and set it back down...keep doing this till front and rear scales are equal. Again, this will not be 100% accurate...for that you need to fully build the car, fill the tank and radiator, undo the drive shaft, motor & trans mounts and move the engine while the car is on scales...that would give you an accurate reading, but you would have to cut and move the mounts and make other adjustments on the completed car. If someone has moved their engine (351W) already, that might give you a starting point, still each car will vary, so it is a guess, but a slightly educated guess.

[Jim Schenck]

Martin,

You can calculate the distance needed knowing the weight of the drivetrain and the total car and knowing that most small block cars end up around 53% rear bias. However, by doing this you have to remember you are compromising accelleration (which loves rear weight bias), comromising braking (which also loves rear weight bias), and making the optimum handling tire sizes likely the same front to rear (so no more running the nice 315 rear tires). When we design a car we are trying to get more bias to the rear just because even with the same size tires on all four corners the car spends little of it life in constant high speed corners, and in the corners where you are accellerating or braking the rear bias car will work better. The amount of power you can have with a 351 in such a light car may make it frustrating if it doesn't grip from a stop as well as it should, and that is where most people building these cars find them to be so much fun.

[Martin]

After reading this thread and the linked articles, I think I'll just leave it as it is and just trust that ffr has got it about right. I'll get a proper alignment done with cross weighting sorted out.

[CraigS]

Glad you decided to leave it as is. The 3-5% extra rear weight works well w/ the usual 275 fronts and 315 rear tires. While not directly applicable,when you look around at purpose built racecars,ie: F-1,Indy,LeMans, etc. you find that they all have extra rear weight and extra rear tire width. Seems a natural thing to me.