Purplemeanie

Hi All, I'm sorry but I'm having to cancel the July meeting. And as a reminder, I'm away August, September and October - so the next meeting will be November. If you have any questions or things to share about EV's then please post to this Area's forum here. Other area members will be able to help and I'll be checking in as we drive to Greece and back again this summer (in case you're wondering, not in an EV, but also not in a Seven)! John

Hi All, For those of you that can make it, we have our monthly meeting, June 11th at 8:00pm (see Calendar Event here for Zoom details). NOTE: There will no longer be an email containing the Zoom details, they will only be available in the Calendar Event. The best way to keep up to date with meetings is to subscribe to the calendar feed (see details below) Before we get into topics. Can I ask that, if you've haven't, then you all do the following on the new Website: Join the EV Special Interest Group here Follow the News Section here Follow the Forums here You can also subscribe to the EV SIG Calendar and have the meeting dates always on your Phone/Tablet/PC by going to this page and selecting the "Subscribe" drop-down and then "Subscribe to iCalendar Feed" (follow the instructions), best done on a phone/tablet. I'm asking you to do this because these links will enable you to get EV SIG updates as we move away from LoveAdmin. Tentative Topics: John's EV Project updates Progress has been a bit slow this month again. I'm waiting for details on a new motor and inverter option. Hopefully I'll have all the details for the call. Hopefully see you on Tuesday 11th. The background for the interest group and info about the meetings can be found here: https://www.caterhamlotus7.club/forum/chitchat/announcing-interest-area-electric-vehicles The minutes of the meetings can be found here: https://www.caterhamlotus7.club/blogs/blog/359-electric-vehicles-news/ If you’ve not joined the calls before then in brief the agenda will be: Introductions by new attendees What’s new in the EV world Thoughts on Caterham’s plans for EV Seven Conversions AOB Participation in the meetings is, of course, optional. John

Hmm. I had some of those assumptions in my spreadsheet but not all. For me, being within two significant figures was all I was aiming at. 🤣 When I get some time I’ll update my spreadsheet with your factors included. This is why it’s so good to share thoughts, somebody (often you James! 😉) has something to teach me! 🙏 Thanks!

Here's a quick screen grab of my spreadsheet...

While I’m on a role here. I thought it would be worth showing some images I’ve shared on socials recently. I’m looking at a motor option now that would have to sit in the transmission tunnel of a Seven… where the gearbox normally sits. The prospective motor would be an extremely tight fit… with just a mm or two on each side as clearance. [ Has anyone got experience fitting anything into a car with that tight a tolerance? And I appreciate a regular 5 or 6 speed box probably only has that sort of clearance… at least the 5 speed in my car is only a mm away from the chassis in places! ] To this end, I spent a lot of time in CAD (first image) trying to determine if the motor will fit in my scanned digital twin. But it’s so close that I wanted to get a better physical confirmation… so I 3D printed the motor. I’d like the motor to sit further back in the chassis than shown in that picture, but as we all know the transmission tunnel tapers and so my concern about it fitting where I want it. This is a 320bhp motor that I’ll probably only be able to realize about 60% of that in my first iteration of this project. The second image below is of the 3D print of this motor. I don’t create the model, that’s from the motor supplier. If you’re wondering, then the perforations in the stator casing (pink bit) are for two reasons. Firstly so I can “see through” it when I put it into the chassis and secondly to save a bit of print time and filament (though that may have not been the case in the end - infill density vs number of wall layers). I just need some time, space on the drive and good weather to take the cover off the project car and offer up this motor to see what works. John

I agree with your calculations James. I think I posted speed vs drag (and power??) in a Low Flying article last year. It’s surprising how closely my graphs correlated with max speed at max power. with my drag calculations showing drag power equal engine max power. 🤔😉

James, also picking up on your point about drag. It's probably also something I should try and graph... motor power output vs power required to overcome drag. As you are well aware, but I'll state for others reading, it's important get a balance of peak motor power and drag. Though TBH... in my first incarnation of this project I'm almost certainly going to be current limited (current determines torque -> T ~ BI). I'm going to struggle to get enough current to the motors AND enough voltage to also get past the knee of the torque curve (which equates to needing increased voltage due to field weakening). Though I do have a cunning plan (cue Baldrick voice) to cheat a statically set current and voltage tradeoff that you normally get with BEV battery packs. John

Following some tinkering last night and more pondering today, I've updated the code and created a new plot. The plot in the first post was a statically pasted image to this site. The image below is a link to the file on GitHub, which should therefore change as the code and image changes on GitHub...

Hi Jonathan, To answer in turn… 1. I had this conversation with someone on WhatsApp last night about this. I started off writing the code to work out a 0-100 (or other arbitrary similar stat) but decided it made more sense for an overall picture to start with wheel-torque vs speed. I may come back to acceleration graphs (including gear change estimates and starting clutch slip). 2. Hmm. I think all performance measures only capture some of the driving experience. I’m also a fan of Autocars more realistic in gear acc.n metrics. The graph I’ve created here first was a stab at putting a nail in the ground on what sort of performance I might expect. I think it shows an EV based on this configuration would be really punchy from 20 to 80 but might not be quite as quick up to 20 as say this 420. I should probably try and create an acceleration graph, that might give a better sense of how it would feel to drive 🤷‍♂️ 3. Hmmm, yes. People often don’t appreciate the careful planning that goes into gear ratios. There’s the basic misunderstanding that with more power you can pull a bigger gear, but fail to appreciate the cube law of power needed to overcome wind resistance. Thee are many many more twists to the gearing conundrum. EDIT: And of course when you get this "increased gearing" really wrong you find that while you might have enough max-power to achieve a higher top speed, you now find that at slightly lower speeds you now sit off the peak power and so don't have enough power to make you go faster and therefore can achieve this new theoretical top speed. Gearing needs careful planning. 4. Electric Porsches. Well it goes back to point 3. Even though EV motors can spin fast, the torque multiplication through the gear sets, power and top motor RPM didn’t give Porsche both the 0-60/100, top speed and range they wanted without needing two gears. Gearboxes in EVs are especially tricky as there’s not the same inertia as in an ICE car (which is a twin edged sword) so the gear change timing has to be made in association with probably a partial powering of the drivetrain to match revs as the gears change. Downshifts also need to be thought of in this context.

Thanks for the comment. As it happens I knew the wheel/tyre circum. was just a guess. If you look at the code now you’ll see it calculated out correctly - and I even went to the garage and wrapped a tape measure around to verify I was making things better! 🤣

Hi All, As part of my EV conversion project I wanted to get a better grip on how an electric motor would perform against an ICE powered car. Lots of people have commented on how any Seven with an electric motor will probably be a crazy accelerating beast. But while that could be the case, the amount of space in a Seven chassis (even an SV) may be prohibitive. And what matters is torque at the wheels. As we all know "power" is just torque multiplied by RPM. So its torque that determines the performance of your vehicle. Then you have to take into account any gearing between the motor and wheels. Any gear reduction will "amplify" the torque created at the output (and gearing up will decrease output torque of course). But in general a Seven has a gearbox going one stage of gear reduction followed by a differential doing a second stage. And then the differential also splits the torque in two to each wheel. Therefore, the torque to each rear driven wheel at any given motor speed is: wheel_torque = motor_torque * gearbox_ratio * differential_ratio / 2. So it should be simple to come up with a Wheel Torque vs Speed graph, showing the torque produced by the motor. I tried to come up with a way of doing this in a spreadsheet but found it was a bit of a blunt instrument. So, I wrote a script. I made the script so it can be reasonably easily modified to provide different "motors" and "gearboxes". The script runs through each motor configuration and plots each gear it finds in the gearbox on a chart. THIS CODE IS NOT PRODUCTION GRADE: I threw this together for my own benefit. If I'd have wanted to get paid for the code then it would have looked quite different. It also evolved and could probably do with a refactor and prune. Anyway, here's a plot form the code as it stands at posting date. I created a benchmark set of in-gear plots for a Caterham Seven 420 (I found the data at the link repeated in the code). I then created two motor variants based around a motor I'm thinking of using in my conversion project. The two variants vary by the current that can be supplied to the motor and by a gear reduction I'll need to include in the design. You can see from the graph that for the variants I've configured, the 420 beats the EV configurations in 1st gear but the EVs then take the lead from 2nd gear onwards. If you have any comments or suggestions then please let me know below. The link to the code is here: https://github.com/Purplemeanie/TorqueAndPowerComparisons John