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Learning how to build Lotus Seven replicas...together!
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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.

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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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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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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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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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PostPosted: October 19, 2021, 3:50 pm 
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Darxus wrote:
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.


Could you fit a 255/45-20? We have a set of Pilot Sport AS3+ on our Flex & they've been fantastic (for an AS tire). Haven't looked to see if there's any decent summer options in that size.

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PostPosted: October 19, 2021, 5:29 pm 
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JAMADOR wrote:
Could you fit a 255/45-20? We have a set of Pilot Sport AS3+ on our Flex & they've been fantastic (for an AS tire). Haven't looked to see if there's any decent summer options in that size.

No reason I couldn't. But honestly, I'm looking for less traction. And I know that 234/40-17's give me plenty of traction for my taste at about twice the car weight, on a '98 M3. I just seem to really like the aesthetics of tall tires.

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PostPosted: October 20, 2021, 10:37 am 
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Darxus wrote:
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.


No, the front gearsets are "reverse rotation" so they are specific to the front IRSs.

Ford's LSD for the front axle as used in the Raptor is here:
https://www.levittownfordparts.com/sku/ ... 150ls.html

Quote:
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.


I think I mentioned this before, but don't get so hung up on sidewall size or height or ratios. Every tire is different, and what worked on one car can feel completely different on another.

Quote:
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.


Wheel offset affects your suspension calculations and scrub ratio. You'll want to know what your offset is when you are running those numbers, but you can generally pick a wheel that you like and that fits the brakes that you want and work from there.

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PostPosted: October 20, 2021, 10:43 am 
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Darxus wrote:
I was planning to get a big brake kit from Brembo or something. Brake size is very roughly modeled at this point.


Another point here - on a light car on the street, big brakes will never get up to temperature. Heck, even on a 3000lb road car, simple 12" rotors and single piston calipers are more than enough for moderate track duty. With half the weight, a big brake kit only increases your unsprung weight while never ever getting into the optimal temperature for good stopping performance.

Darxus wrote:
I at least want to model the '95/'98 M3's tire size, since I know that traction is overkill for this weight.


Another point on weight and heat - if your tire never gets up to temperature, you'll never have good traction. A Miata on 205-width tires can outhandle a Corvette on 335s simply because it can get its tires up to temperature.

I also have an E36 M3 so I get what you're talking about, but almost none of the data for that car translates to something like you're designing.

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PostPosted: October 23, 2021, 2:03 am 
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Early 914 and 911 racks were interchangeable I believe.

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PostPosted: October 23, 2021, 9:36 pm 
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Laminar wrote:
Darxus wrote:
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.

No, the front gearsets are "reverse rotation" so they are specific to the front IRSs.

Ford's LSD for the front axle as used in the Raptor is here:
https://www.levittownfordparts.com/sku/ ... 150ls.html

Damn, would've been too easy.

Yeah, it looks like many front diffs are substantially more assymetrical than rear diffs. But maybe that's with some kind of stub shaft, that can be removed?

This is a pre-Tacoma Toyota truck IFS diff, available with LSD, and a gear ratio that'll work with this motor, and not as asymmetrical as many, so that's at least an option.

Laminar wrote:
I think I mentioned this before, but don't get so hung up on sidewall size or height or ratios. Every tire is different, and what worked on one car can feel completely different on another.

Thanks.

Laminar wrote:
Wheel offset affects your suspension calculations and scrub ratio. You'll want to know what your offset is when you are running those numbers, but you can generally pick a wheel that you like and that fits the brakes that you want and work from there.

Yup. It's about time I start learning how to calculate steering and suspension geometry. Any recommendations?

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Last edited by Darxus on October 23, 2021, 10:36 pm, edited 1 time in total.

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PostPosted: October 23, 2021, 9:39 pm 
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Laminar wrote:
Another point here - on a light car on the street, big brakes will never get up to temperature. Heck, even on a 3000lb road car, simple 12" rotors and single piston calipers are more than enough for moderate track duty. With half the weight, a big brake kit only increases your unsprung weight while never ever getting into the optimal temperature for good stopping performance.

Another point on weight and heat - if your tire never gets up to temperature, you'll never have good traction. A Miata on 205-width tires can outhandle a Corvette on 335s simply because it can get its tires up to temperature.

I also have an E36 M3 so I get what you're talking about, but almost none of the data for that car translates to something like you're designing.

Damn, I didn't think about things getting up to temperature, thanks. The entire reason I wanted a big brake kit is aesthetics. Any suggestions for brakes that won't look tiny in 20" wheels? (I realized the unsprung weight would probably be... counterproductive.)

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PostPosted: October 23, 2021, 9:41 pm 
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Kinetic Research wrote:
Early 914 and 911 racks were interchangeable I believe.

Beautiful, thank you so much. Looks like it'll fit great! So a ball joint connects to those ends, so the pivot point would be several cm wider than that 200mm?

200mm wide grey cylinder in the middle, view from the bottom:
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PostPosted: October 23, 2021, 10:47 pm 
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Added grills (front, and behind radiator). Batteries and controller are liquid cooled, I'd like to switch to a motor that is. I'm not sure how exactly the rear grills, frame, and suspension are going to interact. Probably switch to louvers, like the Ford Model A, and farther back. And I want to separate the batteries, so they don't unnecessarily get exposed.

I've had a little lingering fear that I'm using very crude shapes because I'm bad at curves. So today I played with curves some, and I'm satisfied that I can do curves fine, and I just really do like the crude angular shapes.

Big 4k 60fps video loop is posted to my website as usual: http://www.eternalmachinery.com/ecar/eb ... fps60.webm
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PostPosted: October 24, 2021, 11:30 pm 
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Laminar wrote:
Another point on weight and heat - if your tire never gets up to temperature, you'll never have good traction. A Miata on 205-width tires can outhandle a Corvette on 335s simply because it can get its tires up to temperature.

I just watched season 1 episode 3 of James May's Cars of the People, and he also raved about the Miata's "skinny little tires". Those cars also weigh well over what I expect this thing to weigh, so I expect those tires to be entirely adequate. So, I broke down and tried modelling at least the diameter of them.
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Yeah, I don't think I'll be going with the most practical tire size. I'll still try more in between.

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