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Learning how to build Lotus Seven replicas...together!
It is currently October 19, 2021, 3:38 pm

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PostPosted: October 13, 2021, 11:40 am 
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It's definitely a cool concept. However, I do have some constructively intended criticisms that jump out at me.

That seating position looks rather... intimate. I don't see how you're going to fit a safe seat, nor securely mount a harness, for the driver. I doubt it's legal in most states to use the passenger as a seat, nor to share a harness.

You can build the hoop to SCCA specs, but...
1) That's a roll bar, not a roll cage. So I'd use the roll bar specs from the Solo rule book, not the roll cage specs from the GCR.
2) It's way too far behind the driver to actually count for SCCA use, but is fine if just using as guidelines for strength though.

Pretty welds are nice, but strong welds are critical. Pretty does not necessarily equal strong, and strong does not necessarily equal pretty. If you want both, your best bet is to enroll in a welding class.

I can find zero 245/40R24 tire options from my normal tire sites, and the handful 245/60R20 tires are all CUV/SUV/Truck tires. Contact patch size won't matter much with those hard rubber compounds and tall/soft sidewalls. Speaking of which, diameter times width has zero to do with contact patch size. Contact patch size can be approximated by corner weight divided by tire pressure. Tire size primarily changes the shape, not the size, of the contact patch. The largest tire diameter I'd go would be ~27" as anything much larger will have crap availability for actual sporting quality tires.

Do you have a plan for what you're going to pull brakes that size from?

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Last edited by Driven5 on October 13, 2021, 3:10 pm, edited 1 time in total.

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PostPosted: October 13, 2021, 2:16 pm 
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Oh man, I missed the part on tires.

Quote:
I want 245/40-24 tires / wheels. That gives me about the same sidewall as OEM 1995 BMW M3, with the tall tires I want for aesthetics. But they seem... hard to find. I'd rather use parts that are easily available. 245/60-20 has about the same outer dimensions, and is commonly available, but that seems like way too much sidewall. These both would give me about 1.4x the contact patches of the M3, if you can do that by just multiplying diameter x width. I'd be happy with less, exploring the limits of traction is more fun that perfect sticking.


Sidewall height means nothing. You could find something with 6" of sidewall that's rough and tough and stiff, and you can find something with 2" of sidewall that's squishy and loose and floppy.

Start with your intended purpose. That gives you an idea of the kind of tire compound you need (full race slick, 200TW sticky autocross tire, general summer tire, hard all-season, etc.). Then go to your suspension geometry. That will be affected by your total tire height.

Unsprung weight is the antithesis of handling and response. A decently lightweight 17" wheel and tire will weigh about 40lbs. A 24" wheel is going to be about 50lbs on its own, add a P255/30ZR24 tire for another 29lbs and you've doubled your unsprung weight, while limiting your compound and treadwear options and unquestioningly made the vehicle worse to drive. Stay in the 17-20" range if you want any decent options for tires and to maintain a reasonable unsprung weight.

The lighter the vehicle, the less tire you need for a given cornering force. A Miata on a 245/40R17 can pull well over 1 G cornering force.

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PostPosted: October 13, 2021, 3:06 pm 
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Joined: November 6, 2020, 6:29 pm
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Location: Spring, TX
I'm not even aware of any common 24" wheels. Seems excessively large. Much less in a sports car application.

Even the higher end SUVs on the market now are only running 20-22" wheels. I can attest that the 255/45-20 wheel/tire on our Flex are quite heavy.

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PostPosted: October 19, 2021, 11:38 am 
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Joined: December 13, 2007, 9:37 pm
Posts: 17
Location: Pelham, NH
Laminar wrote:
Like you found, the F-150 has an 8.8 up front and Ford offers a decent selection of ratios for it. You can also get a Torsen or other locking differential if you want traction for all of those electrons.

Do you know if the gears from a front Ford 8.8" diff will fit in an 8.8" rear diff? Yeah, I need an LSD.

Bent Wrench wrote:
Then you need another controller to control the two controllers and act as your center diff.

Yesssssss, all wheel drive so all the torques can be utilized.

Or you could wire the two motors together and use a single bigger controller. Which I realize would be exciting for some, but not the feel I'm going for.

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I was stressed about 245/40-24 tires not being available. I tried 245/50-20 (in this rendering), and I think I like them even more. The shorter sidewalls provide more than enough road feel for me in the much heavier '98 M3, so I think these taller sidewalls will provide welcome comfort. But mostly I like the way they look, and they're common. Mostly on SUVs, one car so far, the Chrysler 300C.

Wheel widths are also a more correct 8" wide. I'm a little lost on picking an offset. I don't want to do 0mm. Maybe pick something from a production vehicle, like the Acura MDX, 55mm offset.

Shortened fenders to 180°, so they're not entirely awful to remove.

Still constantly giving me shivers.

I have so many things to learn and choose.

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PostPosted: October 19, 2021, 11:56 am 
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Joined: December 13, 2007, 9:37 pm
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Location: Pelham, NH
Driven5 wrote:
It's definitely a cool concept. However, I do have some constructively intended criticisms that jump out at me.

That seating position looks rather... intimate. I don't see how you're going to fit a safe seat, nor securely mount a harness, for the driver. I doubt it's legal in most states to use the passenger as a seat, nor to share a harness.

Yeah, it's a problem. And I may very well end up substantially changing dimensions for it. I still need to mock it up and see if my girlfriend and I fit in it at all. I'm thinking two fairly slim racing seats.

Driven5 wrote:
Do you have a plan for what you're going to pull brakes that size from?

I was planning to get a big brake kit from Brembo or something. Brake size is very roughly modeled at this point.

Laminar wrote:
Unsprung weight is the antithesis of handling and response.

Yeah, I know it would perform better with smaller tires. And aerodynamics. I am working on finding smaller tires that I like, for that reason. I at least want to model the '95/'98 M3's tire size, since I know that traction is overkill for this weight.

But performance isn't my primary goal. Shivers are. Irrational fun stuff.

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