Mister Towed on the road...

[christinedmc]

Hello Scottie,

I will use a different speedo. Not Triumph. I have it recalibrated as we speak. My car has 165/80/15 tyres. I had to measure the turns of the cable while pushing the car over a distance of 50 metres. They change the gears in the speedo. It will cost me about 100 pounds or so.

Michiel

[Scottie22]

That's interesting, which speedo did you use to do that?

[christinedmc]

Hi Scottie,
I beleive it is a BSA motor cycle speedo. Very appealing. it runs anti clockwise.

Michiel

[Scottie22]

Wow! I am impressed!
I will face similar problems later in my build, as I will use Jaguar dials, which will be miles out by default.
When I get to that point , I may contact you for some advice, as you seem to have it all under control!
As a relative newbie, is it "correct" to discuss this on Mr Towed's site?
Or should I be posting these type of questions on your build thread?
It is not my intention to offend anyone!!

[Viatron]

Fpr speedo calibration try ETB Instrents

[Mister Towed]

Originally Posted by Scottie22

Wow! I am impressed!
I will face similar problems later in my build, as I will use Jaguar dials, which will be miles out by default.
When I get to that point , I may contact you for some advice, as you seem to have it all under control!
As a relative newbie, is it "correct" to discuss this on Mr Towed's site?
Or should I be posting these type of questions on your build thread?
It is not my intention to offend anyone!!

No offence taken Scottie, I'm not precious about this being my thread, I'm perfectly happy for it to be used as a chat room to discuss issues that are of interest to others when I change something during my tinkering phase.

As for how to compensate for the different rolling diameter of the larger tyres, a change to 175/80/14 tyres represents a 10% increase in rolling diameter over the 155/80/13's originally fitted to a Herald or Vitesse donor (not sure what Spitfires had).

As most speedos over read by around 8 -10% (a feature deliberately built in by manufacturers to ensure they comply with construction and use regulations) fitting bigger tyres on an otherwise standard drivetrain just takes out that error so your speedo is likely to be more accurate than when it was new.

I also fitted a 3.27:1 diff from a GT6 which obviously changes the gearing aft of the gearbox, where the speedo takes its drive from. As this would have made the Vitesse speedo under read by almost 20% I sourced a non-overdrive GT6 speedo to correct that error.

The result of those changes is that, when I put my car on the dyno with 175/70/14's, and the GT6 diff and speedo fitted, the speedo was spot on across the entire speed range, reading 110mph at a true wheel speed of 110mph.

My recent change to 175/80/14's will increase the rolling diameter by just under 6% over the 175/70/14's. This means that when my speedo says 60mph, I will actually be going 63.5mph. That's close enough not to get a speeding ticket

As for the acceleration being 'snail like' mine only has 79bhp but it's so light - lighter than a Lotus Elise - that even with big tyres and tall gearing it pulls like a locomotive in any gear across the whole rev range. It'll dribble along happily at 30mph in top and still storm away with a squeeze of the throttle, or you can scream it up to 45mph in first gear, 70 in second or 100 in third (on the Autobahn, of course) without even reaching the rev limit. Best of all is changing into second at 50mph when approaching roundabouts and listening to the spine tingling crackling and popping it does on the overrun

Good luck getting your Jag instruments to work, just remember, where these cars are concerned, as Yoda would say, impossible is nothing.

Here's a couple of useful resources for calculating all of the above -

http://www.kouki.co.uk/utilities/vis...ize-calculator

http://www.mintylamb.co.uk/gearspeed/

Oh, and Phil just followed a modern car along at different speeds agreed with its driver and marked the corrections on the face of the speedo glass with a marker pen. That works too.

[Scottie22]

Thanks so much for such a detailed explanation, really helpful.
When I spoke of "snail like" I was refering to Patoune's 21% plus increase in wheel rolling diameter, something that radical would surely wreck the accelaration? Especially considering it was a race car?

[Scottie22]

I should have added: as far as I am aware the "standard" Spitfire wheel and tyre combo was 155 x 80 x 13. Which would give a rolling diameter of approximately 22.76 inches, if that's of any use.
If you have a true 110 mph at that speed, that has to be better than a new car!! you surely have that sorted.

[Scottie22]

T, I forgot to ask, what does your car weigh please?

[Mister Towed]

Originally Posted by Scottie22

T, I forgot to ask, what does your car weigh please?

It's 780kg's with a full tank. That puts it between the S1 Elise at 725kg's and the S2 at 860.

Even with my unstressed straight six only pushing out 79bhp that still gives me over 100bhp per tonne, and it feels quick enough on the road.

I do have a spare engine sitting in the garage that might just get upgraded when funds allow...

[Scottie22]

Mister T. That's really light for what you have, and what it looks like, no
wonder it pulls well.
When you look at the extra work involved to swap the engine for a few more BHP that you don't really need, is it worth it?

That will be the situation I will be in after my build. My donar is a 1971 Mark IV Spit, which should have had a 1300 engine in it, but has been fitted with a 1500, which looks new and sounds strong.
I am hoping to cut down on weight enough to end up with the car being no more that 600kilos if I get it right.
If I am successful, with the 1500 putting out around 70 BHP, I should
be in the same boat as yourself with over 100BHP per ton.
That should be enough.

[micky1mo]

Speaking of engines look what's just rolled into my workshop.
V12, 6lt, 300+bhp



Just wondering what to put it in or rather what to build around it

[Alpha]

Well, a V-12 is basically the same length as an in line six, which is what the Triumph Vitesse and GT6 had, so it should fit any of the Ribble kits, no?

Baz from Brussels

[micky1mo]

Originally Posted by Alpha

Well, a V-12 is basically the same length as an in line six, which is what the Triumph Vitesse and GT6 had, so it should fit any of the Ribble kits, no?

Baz from Brussels

This is a bl**dy big engine, tall, long and wide plus it weights in at over 300kg so it would need to be sat well back in a Triumph chassis.

A Ribble's out but, one of Mike's Miglia's with that long bonnet and rearward seating

[The Shadow]

I should imagine any Triumph chassis without severel upgrades is not going to be viabal. they are very thin wall chassis and rear axle , breaks etc will take a severe pounding

[Viatron]

first time your floored that in a triumph chassis it would turn into a pretzel!
Would make a nice table though :-)

[oxford1360]

Probably best to install a bullet-proof shield between you and the diff unless you want to be fragged by your own car.

[Mister Towed]

Originally Posted by Scottie22

Mister T. That's really light for what you have, and what it looks like, no
wonder it pulls well.
When you look at the extra work involved to swap the engine for a few more BHP that you don't really need, is it worth it?

That will be the situation I will be in after my build. My donar is a 1971 Mark IV Spit, which should have had a 1300 engine in it, but has been fitted with a 1500, which looks new and sounds strong.
I am hoping to cut down on weight enough to end up with the car being no more that 600kilos if I get it right.
If I am successful, with the 1500 putting out around 70 BHP, I should
be in the same boat as yourself with over 100BHP per ton.
That should be enough.

600kg's shouldn't be too difficult to acheive - your four pot should shave around 100kg's off the total, plus I've fitted easily fifty kilos of fripperies, like a stereo, luggage bay, water injection system, servo, steel spreader plates under the seats, etc. Good luck reaching your target.

[Mister Towed]

Originally Posted by micky1mo

Speaking of engines look what's just rolled into my workshop.
V12, 6lt, 300+bhp



Just wondering what to put it in or rather what to build around it

Haynes Roadster?

[micky1mo]

Originally Posted by Mister Towed

I've fitted easily fifty kilos of fripperies,

Nothing wrong with a few fripperies, I like fripperies