Help with Mid Engine Design

[culver]

I'm not sure there is a "perfect". It depends greatly on the tires. The Panoz front engined sports racer of about a decade back was front engined in part because they figured the rules allowed more front tire than most rear engined race cars (mid engine in street car parlance) needed yet less rear tire than they wanted. In order to offer a weight balance that was more appropriate for the tires weight was moved forward. Conversely, the old 1970s F1 cars as well as the proposed Delta racer are very rear heavy. If you are expected to use equal tires all around I would say you would likely want something closer to 50:50 vs a car with staggered tire sizes.

If you have a FWD you don't want 50:50, you want forward weight and possibly larger front tires.

[RacingManiac]

If you have a FWD you don't want 50:50, you want forward weight and possibly larger front tires.

Essentially you'll want the tires balanced to your weight distribution....regardless of the drive wheel, since that will dictate the amount of the available grip at that given axle. With some margin to the amount of abuse the axle might see.

Most RWD sporty cars(front engine) features staggered tire width with width biased to the rear despite them being possibly closer to 50/50 distribution. But you rarely see that FWD car does the opposite. I think that is mostly due to the nature of many of those cars. FWD car are more typically seen in cost conscious platform and that for an OEM, a staggered design means more cost on wheels and tires. Where as on the sportier car their higher price will mean better margin for them to "do it right"....Interesting to see though I think now the new RS3 from Audi(front engine, AWD, based on a FWD type design), will actually feature wider front tire....a concept that'll be more suited to its weight distribution...

[Matra et Alpine]

IN racing saloon cars with same wheel sizes all round you'll see the tyre pressures radically different front to back to get the same control over grip you seek with weight balance or tyre footprint

[RacingManiac]

I often see 47:53 being referred to as perfect, I think the idea being that you have more weight over the rear for acceleration then when you brake the shift forwards tends to even things out. I think that was in regards to FR cars though.

I think that can apply to all rear drive cars....and its possible to see the rational to it also. Most cars are capable of much better braking performance than they are accelerating forward. Given rearward bias will allow for the drive wheel to have more advantageous setup. I'd also imagine that in the past when you might be more limited to braking power, a 50/50 car is fine for not over stressing the front brake/tire as your weight transfer might be limited dynamically. But as modern tire and braking system are capable of more, more severe weight transfer to the front will mean that if you can get the rear to do more work under braking, will help you in those situation. 911s typically are pretty good in braking situation since in those cars the they have more work done by the rear wheel due to their wacky layout....So if you can get a more rearward bias car you can help out the cause...

Interesting to note that nowadays in even modern racing we are seeing a shift to more forward weight distribution. Most LMPs now are probably closer to 45/55 if not 50/50....and with them also using the front tire that are much closer to size to the rear. The same shift was happening to F1 also as Bridgestone(before the last season where they changed the balance of their front to rear) and the current Pirelli both produce much more front "biased" tire to give them that more "positive" front end. My guess is that since you will be quite severely traction limited in those formula due to the tire regulation that a shift in design focus toward a car that is more balance in cornering and braking(more so, not to say LMP/F1 car were not before) means more gain overall...

[culver]

Essentially you'll want the tires balanced to your weight distribution....regardless of the drive wheel, since that will dictate the amount of the available grip at that given axle. With some margin to the amount of abuse the axle might see.

Most RWD sporty cars(front engine) features staggered tire width with width biased to the rear despite them being possibly closer to 50/50 distribution. But you rarely see that FWD car does the opposite. I think that is mostly due to the nature of many of those cars. FWD car are more typically seen in cost conscious platform and that for an OEM, a staggered design means more cost on wheels and tires. Where as on the sportier car their higher price will mean better margin for them to "do it right"....Interesting to see though I think now the new RS3 from Audi(front engine, AWD, based on a FWD type design), will actually feature wider front tire....a concept that'll be more suited to its weight distribution...

GM did larger fronts with the last gen V8 Grand Prix. One of the auto rags noted that it did help the handling.

[Ferrer]

This was also the case on the front wheel drive V8-engined Impala SS, wasn't it?

[Simon_Tuman]

Great info everyone, I really appreciate it.

So my next question would be in regards to the cooling system and the stuff placed up front. Im still keeping the fuel tank, and the battery up front, but should I still keep the radiator up front. I think from an airflow standpoint the stock location is best, but it creates other issues like having all that coolant flow from the front of the car all the way to the back. I think its fairly straightforward, but it creates possible problems with air in the system and I imagine I would need a 2nd pump up front to push all the coolant to the back. For packaging reasons it would be nice in the back, but I just dont see enough airflow on the sides of the car without substantial body modifications (like the renaults) to draw air into the radiator. One guy mounted the radiator in front of the motor (behind the driver) and wanted to draw air up from underneath the car, which I think is a bad idea.

What ideas do you guys have for this piece of the puzzle?

[culver]

This was also the case on the front wheel drive V8-engined Impala SS, wasn't it?

I believe the Impala didn't use the staggered sized tires. I might be confusing which used them but I recall the article specifically mentioning the larger fronts made the cornering more neutral.

[culver]

Great info everyone, I really appreciate it.

So my next question would be in regards to the cooling system and the stuff placed up front. Im still keeping the fuel tank, and the battery up front, but should I still keep the radiator up front. I think from an airflow standpoint the stock location is best, but it creates other issues like having all that coolant flow from the front of the car all the way to the back. I think its fairly straightforward, but it creates possible problems with air in the system and I imagine I would need a 2nd pump up front to push all the coolant to the back. For packaging reasons it would be nice in the back, but I just dont see enough airflow on the sides of the car without substantial body modifications (like the renaults) to draw air into the radiator. One guy mounted the radiator in front of the motor (behind the driver) and wanted to draw air up from underneath the car, which I think is a bad idea.

What ideas do you guys have for this piece of the puzzle?

No great idea there but many old race cars used from mounted radiators. Perhaps you could post the question on Apexspeed where someone might know about old front cooled Formula Fords.

[Matra et Alpine]

Matra used the front rad, rear engine in road cars fine
Alpine did it in all the competition race/rally cars.
So did imps and most cars with high power/engine-size.
Just use nice big bore pipes. One pump is sufficient.
If you were trying to get >200hp/litre then you MIGHT benefit from an additional pump to assis, but electric is simple to add.

[Magnum9987]

All that tire size/weight distribution stuff is some good stuff. It makes for some great reading. I can't help with that but I will provide some input with the radiator placement.

No, an additional pump shouldn't be required. Air in the system shouldn't be a problem as long as you do a proper coolant system bleed. Once you got the coolant in, run the engine until the water pump turns on, and let all the air out, and add coolant until bubbling stops.

[Matra et Alpine]

^^^ self-run piping - esp larger bore - can be a pain to avoid any pockets where air can remain trapped.

Even if so, dont panic, an adjunct to Magnum's plan is to use what became standard for the Matras..

First have a fill point manifold at the engine side and as high as you can get it. THEN make up a "header tank" that screws on the fill cap and is basically 1m of 2" copper pipe and a small tank on top. Fit and fill and then follow magnum's procedure and now run the engine till very hot and rev it so the mechanical pump is working hard. The "header tank" of fluid means there is always a positive pressure to fill any voids, reduced aeriation and saves you having to keep an eye on it.

Based on experience, the Matra you could bleed "normal" and STILL find it gurgling in the paddock after a hard session Never had it after I'd made one of these - copied from other Matra owners

However for just street use a small air isn't going to be a problem

[Badsight]

I'm glad you caught on to what I was saying. Many people read the marketing stuff and assume 50:50 is ideal. At one point I believe most of the marketing stuff as well (Hp/L, 50:50, etc). Once you start asking WHY you find that often it isn't what we think.

the Ultima GTR is an excellent example . it was a race winner in the 80's/90's before they started banning it

its got a 65-35 distro - far from the supposed ideal but with excellent handeling

btw - isnt the coil-over swap for Corvettes the way to go to improve their handeling ?

[Matra et Alpine]

It was the Mk2 a differnet chassis that was the winner and Ultima love to cite "we were banned because we were too fast" when in reality they were "banned" ( along with lots of other prototypes ) because they didn't confirm to the emerging production, safety and weight standards set to make the sport safer.
Look at some of the spaceframe crashes of the 80s and you'll see why

[Badsight]

yeah its one thing to hang steel pipe together for suspension forces, quite a step up again to have a properly deforming crash structure

the GTR kit they sell now is a beast all the same.