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 Post subject: Gear Ratio Sanity Check
PostPosted: September 24, 2017, 8:18 pm 
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So, I am in parts acquisition and planning stages on this and trying to figure out my gear ratio information.

Car is a 75 Triumph Spitfire, hacking on an NA miata front and rear suspension (yeah, gonna stick tires out 4 inches past the fenders) and powering with an R1 (year not yet determined)

So far I have the spit (a roller with only minor rust that I have $60 in), Miata rear suspension, Miata front corners. Serious cut and weld stage comes after my Bugeye Sprite Resto moves under its own power (should be months).

This will be for autocross and trackdays.


So, starting at tires, I am planning for American Racer 13's (13x8) with a diameter of 20.3 inches.

Miata rear ends will be either 4.1 or 4.3 (have a 4.1open, have a line on a 4.3 torsen, numbers suggest 4.3, but this is where I need my sanity check.

I found transmission gear ratios for an exemplar 2008 R1 in the following two links.

http://aperaceparts.com/tech/2008yamahayzf-r1.html
http://www.motorcyclespecs.co.za/model/ ... 1%2008.htm

These suggest (looking at 12.5k RPM) that 1st gear will go to 70mph and sixth runs out at 139 (purely theoretical, drag may have a bit to say about it, then again, decent power...)

does this make sense?

I did some thrust calculations using Jason Rhodes sheet and a rough looking dyno chart to source the torque
http://www.rhoadescamaro.com/build/?page_id=591

I am showing that 1st gear motospit vs 2nd gear Lotus Elise has the motospit out-pulling everywhere.



So thats what i have done so far.

Is this all making sense? Most autocross builds would be looking for topping 2nd gear at 70mph, but then again, thats using sane RPM limits. If I ran gears to make that happen (a theoretical set, not one I have found to buy) top speed would be absurdly unusably low.

Is first really that tall?

Does it work out? (car runs ok, not too boggy)

Do most BEC guys put in alternative transmission gears to overcome this or something?


I just need a general, "yeah, you arent missing anything, you are on the right course" from people who have been there.

-Mucho appriciated! (sorry for the ADD thought-streams)


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PostPosted: September 24, 2017, 11:21 pm 
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Sorry, cannot really help with the gear ratio's but I will follow your build with interest.
I'm more a crazy street car guy than Auto-X, but I've been surprised to see so many Spridget builds and none for the Spitfire.
From my perspective the Spit is a superior platform in many ways.

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PostPosted: September 25, 2017, 12:10 am 
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Welcome aboard!
apexcarver wrote:
Is first really that tall?
Yup. Big engined sport bikes commonly have a 1st gear that is half the height of 6th gear.
apexcarver wrote:
Do most BEC guys put in alternative transmission gears to overcome this or something?
Rarely, but it often is possible and would be a good idea if you're doubling the mass that engine has to pull around. Touring bikes generally have broader ratios, and it's not unusual for a manufacturer to make a touring and/or standard street model that shares most of its motor with a sport bike (but with milder cams, different carbs, different exhaust, etc) and sometimes that etc includes different internal gear ratios. I googled the bleep out of the Yamaha MT-10 (a nekked R1 with less hp and a torque peak ~2500 rpm lower than the R1) but haven't been able to find the gear ratios. I'd wager that 1st and 2nd are lower than equivalent R1 parts, and I'd expect them to be interchangeable M10 to R1, and if both those ifs turn out to be true, I am certain they'll cost more than your Spitfire did.

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PostPosted: September 25, 2017, 2:00 am 
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Well first and foremost as Jack says "Welcome aboard!" You are on the right site for the wealth of info and the great guys on this site.

After reading your post, my simple answer to your question(s) would be 'yes' FWIW :cheers: But I would need pictures to confirm my answer, as JD would attest too. :D

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PostPosted: September 25, 2017, 7:46 am 
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RichardSIA wrote:
I've been surprised to see so many Spridget builds and none for the Spitfire.
From my perspective the Spit is a superior platform in many ways.


Having a bugeye and a spit in the same garage - it depends.

Spit is body on frame and the factory suspension is terrible. You could get gt6 parts, but heavy and not really great either. The steering on the spit is concerning, radically reverse ackermann, I can barely push the car at full lock it toes in so much. That said, the body on frame can make fab somewhat easier. Cars are stupid cheap though. Saw a complete solid car for $600 the other week, just needed usual things after sitting in a garage for 20 years. Oh yeah, if you used stock rear stuff, breaking threshold is 100HP and the swing axles can also fail mid-corner and deposit you on your roof.

Spridget. If you are OK going solid rear, it's going to be easier. Narrowing a solid rear is cheaper and easier than an irs. I still wonder if I would have been better off this route. That said, if you stick with factory suspension: lever shocks. Road racers tell me that pretty much stop damping after 20-30 min on track because if thermal effects on shock fluid. There are available solutions and you could always fab something. (I was going to spridget first, but had the spit thrown at me)

Initially I was going to narrow the miata rear, but I just want a track toy out of this. Going full width prevents redesign of geometry and dabbing custom parts. Also probably cuts build time in 1/3 and makes car more repairable later. (Whole cross member a factory part). Also opens up just using miata front crossmember.

That said, gonna be a but of an ugly duckling...

I was actually surprised I didn't already have a login, been lurking for years. Had locost plans for almost 15 years. Finally have career to point of having a house with a shop and was going to build a k series powered locost for Scca dm, but decided track days were more my future and a series of ideas led to simpler sturdier and quicker and easier to build. Plus more disposable, because flirting with ideas of possibly hillclimbing.

Been autocrossing 13 years, have engineering degree, work in automotive field. From years of lurking, I know I fit in here!

Also same username on grm, corner carvers, and the sandbox (rr-ax).


Last edited by apexcarver on September 25, 2017, 10:58 am, edited 2 times in total.

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PostPosted: September 25, 2017, 8:15 am 
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apexcarver wrote:

Is this all making sense? Most autocross builds would be looking for topping 2nd gear at 70mph, but then again, thats using sane RPM limits. If I ran gears to make that happen (a theoretical set, not one I have found to buy) top speed would be absurdly unusably low.

Most Nationally competitive Autox cars aim for high 70s to 80mphs in a single gear but that is not necessarily 2nd. The Jeep I drove does in in 4th as does YC3. it really doesn't matter which gear get though there as long as one of them does. The problem with first gear though as you are really going to kill your clutch when trying to launch or your car will bog down. I know the EMod Jeep launches in a single gear. It has a 2 speed Jerico transmission, they use 1st in the pits and 2nd is the race gear.

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PostPosted: September 25, 2017, 10:52 am 
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wrightcomputing wrote:
apexcarver wrote:

Is this all making sense? Most autocross builds would be looking for topping 2nd gear at 70mph, but then again, thats using sane RPM limits. If I ran gears to make that happen (a theoretical set, not one I have found to buy) top speed would be absurdly unusably low.

Most Nationally competitive Autox cars aim for high 70s to 80mphs in a single gear but that is not necessarily 2nd. The Jeep I drove does in in 4th as does YC3. it really doesn't matter which gear get though there as long as one of them does. The problem with first gear though as you are really going to kill your clutch when trying to launch or your car will bog down. I know the EMod Jeep launches in a single gear. It has a 2 speed Jerico transmission, they use 1st in the pits and 2nd is the race gear.



Very true.

But; if I made the redline 7k rpm (more like the motors I (and probably you) am used to thinking about), then max speed first gear is 39mph, 2nd gear 48mph with 6th topping out at 78.

Probably a lot of my sanity check has to do with no being used to the crazy RPM the bike motor goes to.



I think for the soup I am trying to make (parts bins I am using) the 4.3 rear with the 20.3inch tire is the beast I am looking for. 12.5k is peak power when I look further. Apparently in a pinch (and i need to confirm this, plus I will use a stock engine management) it could spin closer to 14k which equates to reaching 78mph in that gear. (and 156mph top speed in 6th, which I am betting drag will prevent it from reaching)

I did some time in FSAE in college, I remember what it was like to launch that... Probably going to stockpile clutches for it and make sure they are easy to change in the car.


Remember, I want to do trackdays with this as well.


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PostPosted: September 25, 2017, 11:28 am 
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apexcarver wrote:
RichardSIA wrote:
I've been surprised to see so many Spridget builds and none for the Spitfire.
From my perspective the Spit is a superior platform in many ways.


Having a bugeye and a spit in the same garage - it depends.

Spit is body on frame and the factory suspension is terrible. You could get gt6 parts, but heavy and not really great either. The steering on the spit is concerning, radically reverse ackermann, I can barely push the car at full lock it toes in so much. That said, the body on frame can make fab somewhat easier. Cars are stupid cheap though. Saw a complete solid car for $600 the other week, just needed usual things after sitting in a garage for 20 years. Oh yeah, if you used stock rear stuff, breaking threshold is 100HP and the swing axles can also fail mid-corner and deposit you on your roof.

Spridget. If you are OK going solid rear, it's going to be easier. Narrowing a solid rear is cheaper and easier than an irs. I still wonder if I would have been better off this route. That said, if you stick with factory suspension: lever shocks. Road racers tell me that pretty much stop damping after 20-30 min on track because if thermal effects on shock fluid. There are available solutions and you could always fab something. (I was going to spridget first, but had the spit thrown at me)

Initially I was going to narrow the miata rear, but I just want a track toy out of this. Going full width prevents redesign of geometry and dabbing custom parts. Also probably cuts build time in 1/3 and makes car more repairable later. (Whole cross member a factory part). Also opens up just using miata front crossmember.

That said, gonna be a but of an ugly duckling...

I was actually surprised I didn't already have a login, been lurking for years. Had locost plans for almost 15 years. Finally have career to point of having a house with a shop and was going to build a k series powered locost for Scca dm, but decided track days were more my future and a series of ideas led to simpler sturdier and quicker and easier to build. Plus more disposable, because flirting with ideas of possibly hillclimbing.

Been autocrossing 13 years, have engineering degree, work in automotive field. From years of lurking, I know I fit in here!

Also same username on grm, corner carvers, and the sandbox (rr-ax).


Yup. Welcome aboard.

There was a user here about 8 yrs ago called Apex. in Toronto area. was that you?

Narrowing IRS axles is cheaply done. It requires you to to design your own of control arms and pickups. JD did this as did I. I had over 6k miles and JD autocrosses his with a big old 'mericun V8.

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PostPosted: September 25, 2017, 11:29 am 
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While the numbers would make it sound feasible, I think the lack of torque and the weight of your car will be the challenge. My experience, none of it autoX, is that if you fall of the powerband ever so slightly, you need to grab another gear. Very much like riding a 125cc dirt bike. Between the car's higher weight and sticky tires it will be tough to avoid that. And if you are already only in 1. gear, there is no fall back. I would rather find a final ratio that would put your desired speed in 2nd or 3rd, and maybe invest in a speed shifter, so you can go back and forth.
Weight reduction will be your best friend.
Edit:
A quick google search: http://www.proshift.com/paddleshift_gal ... /1553/t/10

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PostPosted: September 25, 2017, 2:38 pm 
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rx7locost wrote:

There was a user here about 8 yrs ago called Apex. in Toronto area. was that you?

Narrowing IRS axles is cheaply done. It requires you to to design your own of control arms and pickups. JD did this as did I. I had over 6k miles and JD autocrosses his with a big old 'mericun V8.



I Carved him :twisted:

:lol: nah, no idea, not related. Been using this name across multiple forums for more than a decade.


The axles arent what really made me change from narrowing. It was more "this is a lot of work, vs, just make it ugly and driving that much sooner"

13-1400lbs shouldnt be all that out of the question for a BEC, right? I mean, DSR's are ~900-1k lbs, they run great. I thought 1200lbs was doing fairly alright for a BEC Locost (although, was it Gene Young what had one under 1k lbs? in BM for awhile after then yanked the plug on BEC's in DM?)


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PostPosted: September 25, 2017, 2:49 pm 
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mgkluft wrote:
I think the lack of torque and the weight of your car will be the challenge. My experience, none of it autoX, is that if you fall of the powerband ever so slightly, you need to grab another gear. Very much like riding a 125cc dirt bike. Between the car's higher weight and sticky tires it will be tough to avoid that. And if you are already only in 1. gear, there is no fall back. I would rather find a final ratio that would put your desired speed in 2nd or 3rd, and maybe invest in a speed shifter, so you can go back and forth.
Weight reduction will be your best friend.
Edit:
A quick google search: http://www.proshift.com/paddleshift_gal ... /1553/t/10



I agree that the lack of torque will kinda suck down low.

Also, weight may not be as bad as you think. Stock engine is cast iron, trans weighs a ok amount, I am removing sheetmetal, swapping aluminium carrier Miata diff in place of cast iron spit stuff, the subframes add a bit, but they are light. I can also get a fiberglass front clip/etc for less than $1k


All steel/stock drivetrain weight example http://www.britishracecar.com/PatRyan-T ... itfire.htm

"This brick of lead ballast probably weighs fifty pounds. It was evidently required to bring Pat's Spitfire
up to the minimum weight requirement for Spitfire's in their racing class, which is 1432 pounds plus
an additional twenty pounds for any racer who chooses to omit their passenger seat."

1300lbs is a reasonable expectation.


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PostPosted: September 25, 2017, 11:43 pm 
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So...have you called your local Yamaha dealer about 1st and 2nd gear ratios for the M10? Or is splitting the case and all too much work?

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PostPosted: September 26, 2017, 8:01 am 
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I owned Gene Young autocross car for a few years (915 lbs),it was geared to top out at about 88MPH. I ran most autocrosses in 3rd or 4th gear. I now own Mike Stieff's chain drive street car (936 lbs.) and 1st is good for maybe 30-35MPH in 1st (no autocrosses in this car yet,I just got it),the car is at approx. 6k at 70MPH and it has a 3.10 rearend sprocket ratio.. This is a Miata differential that he opened up and added the sprocket to (still has LSD). You DO NOT want 1st gear good for 70MPH,it will be too slow and you can't downshift it to get it going-do some more research on rear end ratios.... Here is some pics of mine... Good luck Rich


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PostPosted: September 26, 2017, 10:07 am 
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The gene young car was a solid rear front engine shaft to the rear setup, right? What ratio was the rear end?

I have done a bit more diving and it looks like I could set up a spare rear with a 5.38 (aftermarket setup for a Kia sportage that allegedly works for the miata rear, more research pending. ~$1k investment in a spare rear) and swap for auto-x. This tops out at ~55mph in 1st and the thrust calcs put it right in line with my 97 Cobra in first gear.

A friend was telling me something (he wasnt sure of this) that there is some laygear or something in the R1 case that may about half the RPM making all my computations bunk. I have no data backing this up and looking at R1's at drag I am seeing shifting out of first at close to 80mph. So with his doubt and that evidence I am doubtful.

4.3 seems ok for track work, but yeah, autocross would kinda suck. was trying it out on a simulator (GT6 with the fireblade, you can approximate an autocross course with the GTP arena) and it just doesnt want to go under 30mph.


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PostPosted: September 26, 2017, 11:47 am 
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All bike engines that I have ever worked on have a primary gear reduction between the crank and the clutch basket. Your link above shows that to be 1.512:1. did you do your calculations with the transmission ratios or the overall ratios shown in the chart in your link?


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