Paul Turner
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| posted on 9/9/15 at 01:44 PM |
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quote:
Originally posted by alfas
so we are back at the beginninmg: overgeared
My first Caterham had the ubiquitous 1.7 X-Flow with about 130 BHP. Originally it was built as a road car and fitted with a 3.6:1 diff and I used 185
70 13 tyres, gave about 18mph/1000rpm in 4th. When I took to the tracks more first I fitted 185 60 13 tyres dropping the gearing to nearer
17mph/1000rpm in 4th and when I got more serious I changed the 3.6:1 diff to a 4.1:1 diff dropping the gearing to just below 15mph/1000rpm in 4th.
This was useless on the road but I simply fitted the 185/70 13 tyres which made it bearable (just).
The more I reduced the gearing the pick up definitely improved but not by a huge amount, put simply it was faster but it did not make the car drive
like it had 50% more power. My lap times improved but not significantly, every little helps.
Before spending on the transmission I would suggest you get the engine set up and actually ascertain its power and power band. Get what you currently
have working correctly. No amount of internet discussion will sort this, you need to take the car to a good rolling road.
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bumpy
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| posted on 9/9/15 at 01:53 PM |
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Thanks Paul.
For me the rolling road is a last resort. Most of the pleasure I get from owning a kit car is the fun of teasing out solutions to a variety of
problems and the joy of fixing them.
I personally would see the use of a rolling road as an admission of failure.
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bumpy
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| posted on 9/9/15 at 01:55 PM |
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quote:
Originally posted by Irony
Easy answer is to borrow a set of wheels that are smaller and yours. Fit them and see if it goes like stink. Surely someone on here lives near you.
A friend has offered me the use of his wheels, so this is what I will do.
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Paul Turner
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| posted on 9/9/15 at 02:57 PM |
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quote:
Originally posted by bumpy
Thanks Paul.
For me the rolling road is a last resort. Most of the pleasure I get from owning a kit car is the fun of teasing out solutions to a variety of
problems and the joy of fixing them.
I personally would see the use of a rolling road as an admission of failure.
Its not a failure at all. I enjoy tinkering as much as the next person but I want a car that works not one that I am unhappy with. The rolling road
operator can control the engine and make changes and monitor them in a controlled environment. I am not saying its not possible to do this on the road
but you still need experience, jets and many hours of testing. This can be done much quicker on a RR so ultimately get to enjoy your car more.
quote:
Originally posted by bumpy
A friend has offered me the use of his wheels, so this is what I will do.
Just remember that its not just the wheel size its the overall diameter of the wheel/tyre package. For example, a 185/70 13 has an overall diameter of
about 585mm, a 195/50 15 about 575mm which is smaller despite being on a bigger wheel. You have 17" wheels but do not give the tyre size, with
really low profiles they will have a similar diameter.
If the 13" wheels you are borrowing have low profile tyres beware that your sump and other parts of the car may ground and get damaged.
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bumpy
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| posted on 9/9/15 at 03:07 PM |
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quote:
Originally posted by Paul Turner
quote:
Originally posted by bumpy
Thanks Paul.
For me the rolling road is a last resort. Most of the pleasure I get from owning a kit car is the fun of teasing out solutions to a variety of
problems and the joy of fixing them.
I personally would see the use of a rolling road as an admission of failure.
Its not a failure at all. I enjoy tinkering as much as the next person but I want a car that works not one that I am unhappy with. The rolling road
operator can control the engine and make changes and monitor them in a controlled environment. I am not saying its not possible to do this on the road
but you still need experience, jets and many hours of testing. This can be done much quicker on a RR so ultimately get to enjoy your car more.
quote:
Originally posted by bumpy
A friend has offered me the use of his wheels, so this is what I will do.
Just remember that its not just the wheel size its the overall diameter of the wheel/tyre package. For example, a 185/70 13 has an overall diameter of
about 585mm, a 195/50 15 about 575mm which is smaller despite being on a bigger wheel. You have 17" wheels but do not give the tyre size, with
really low profiles they will have a similar diameter.
If the 13" wheels you are borrowing have low profile tyres beware that your sump and other parts of the car may ground and get damaged.
Thanks Paul for the additional info.
IF I ever changed my wheels it would be to lower the gearing and to improve drive comfort, which is always an issue with light (fibreglass)
cars and low profile tyres.
I would head for profiles of 70 or even 80, and yes I know this will affect cornering at the limits, but that's not important to me.
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Paul Turner
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| posted on 9/9/15 at 03:26 PM |
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quote:
Originally posted by bumpy
IF I ever changed my wheels it would be to lower the gearing and to improve drive comfort, which is always an issue with light (fibreglass)
cars and low profile tyres.
I would head for profiles of 70 or even 80, and yes I know this will affect cornering at the limits, but that's not important to me.
What size tyres are on your car now. We can then advise what you would need to fit to lower the gearing substantially.
Fitting 70 or more especially 80 profiles will not lower your gearing, but it will be more comfortable and easier to drive.
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bumpy
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| posted on 9/9/15 at 03:30 PM |
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quote:
Originally posted by Paul Turner
quote:
Originally posted by bumpy
IF I ever changed my wheels it would be to lower the gearing and to improve drive comfort, which is always an issue with light (fibreglass)
cars and low profile tyres.
I would head for profiles of 70 or even 80, and yes I know this will affect cornering at the limits, but that's not important to me.
What size tyres are on your car now. We can then advise what you would need to fit to lower the gearing substantially.
Fitting 70 or more especially 80 profiles will not lower your gearing, but it will be more comfortable and easier to drive.
Hi Paul
Tyre sizes are 205 50/R17
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Paul Turner
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| posted on 9/9/15 at 03:49 PM |
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quote:
Originally posted by bumpy
Tyre sizes are 205 50/R17
You quoted in an earlier post that you are doing about 2500 rpm at 70mph using 17" wheels, 3.62 diff and a 5 speed type 9, that does not work
out unfortunately.
The combination you quote including tyres should result in about 2850rpm at 70 mph, I suspect you have don't have a 3.62 diff.
Changing to 185/70 13 would drop the gearing by approx. 8%. In comparison changing from a 3.62 to a 3.92 drops the gearing by 8%, exactly the same.
Changing to an 80 profile will result in very little change.
Changing to a 185/70 13 would drop the car about 1", have you the ground clearance?
But I suspect you have a very high ratio diesel diff in the car and that could be a factor. If that is a fact a change to lower profile tyres and a
3.92 diff will make quite a difference but will not cure any engine set up issues.
You need to sort both.
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alfas
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| posted on 9/9/15 at 05:41 PM |
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who said that the tachometer does work correctly?
did the builder, former owner or actual owner ever cared that the tacho-drive (gearbox-side) does match with the diff / tyre size??
or was the given speed (70mph) determined by a sat-nav?
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bumpy
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| posted on 9/9/15 at 06:37 PM |
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quote:
Originally posted by alfas
who said that the tachometer does work correctly?
did the builder, former owner or actual owner ever cared that the tacho-drive (gearbox-side) does match with the diff / tyre size??
or was the given speed (70mph) determined by a sat-nav?
Yep, checked on a Sat Nav.
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bumpy
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| posted on 9/9/15 at 07:23 PM |
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quote:
Originally posted by Paul Turner
quote:
Originally posted by bumpy
Tyre sizes are 205 50/R17
You quoted in an earlier post that you are doing about 2500 rpm at 70mph using 17" wheels, 3.62 diff and a 5 speed type 9, that does not work
out unfortunately.
The combination you quote including tyres should result in about 2850rpm at 70 mph, I suspect you have don't have a 3.62 diff.
Changing to 185/70 13 would drop the gearing by approx. 8%. In comparison changing from a 3.62 to a 3.92 drops the gearing by 8%, exactly the same.
Changing to an 80 profile will result in very little change.
Changing to a 185/70 13 would drop the car about 1", have you the ground clearance?
But I suspect you have a very high ratio diesel diff in the car and that could be a factor. If that is a fact a change to lower profile tyres and a
3.92 diff will make quite a difference but will not cure any engine set up issues.
You need to sort both.
Hi Paul
I cant seem to get the same figures that you got for my current set up. Have you taken into account that the 5th gear on a type 9 box introduces a
factor of 0.82 between engine revs and prop shaft revs.?
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bumpy
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| posted on 9/9/15 at 08:15 PM |
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NEW INFORMATION
I have just tracked down the VIN plate for my Sierra donor car as lots of Google reading insists this identifies the diff ratio fitted to that car.
The VIN plate box labelled Axle has a V in it. This apparently identifies the diff as a 3.33 
[Edited on 9/9/15 by bumpy]
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adithorp
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| posted on 9/9/15 at 08:20 PM |
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V code is 3.38:1
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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bumpy
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| posted on 9/9/15 at 08:23 PM |
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quote:
Originally posted by adithorp
V code is 3.38:1
Thanks.
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Paul Turner
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| posted on 10/9/15 at 05:58 AM |
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quote:
Originally posted by bumpy
Hi Paul
I cant seem to get the same figures that you got for my current set up. Have you taken into account that the 5th gear on a type 9 box introduces a
factor of 0.82 between engine revs and prop shaft revs.?
Will keep it simple.
My set up is 3.92 diff, 0.84 5th (BGH type 9) and 185 60 14 tyres - 575mm diameter.
Yours is 3.38 diff, 0.82 5th and 205 50 17 tyres - 637mm diameter.
Mine does 3500rpm approx at 70mph. Checked with a Garmin and the tach is a Stack that has been verified as accurate in the past.
Thus pro rata your rpm would be 2660 at 70 mph.
To be doing 2500rpm at 70 it would need to have a 3.18 approx diff ratio.
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bumpy
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| posted on 10/9/15 at 07:34 AM |
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quote:
Originally posted by Paul Turner
quote:
Originally posted by bumpy
Hi Paul
I cant seem to get the same figures that you got for my current set up. Have you taken into account that the 5th gear on a type 9 box introduces a
factor of 0.82 between engine revs and prop shaft revs.?
Will keep it simple.
My set up is 3.92 diff, 0.84 5th (BGH type 9) and 185 60 14 tyres - 575mm diameter.
Yours is 3.38 diff, 0.82 5th and 205 50 17 tyres - 637mm diameter.
Mine does 3500rpm approx at 70mph. Checked with a Garmin and the tach is a Stack that has been verified as accurate in the past.
Thus pro rata your rpm would be 2660 at 70 mph.
To be doing 2500rpm at 70 it would need to have a 3.18 approx diff ratio.
Thanks Paul for taking the trouble. Since I retired my brain has slowed to a halt.
There may be a number of small reasons why the calculation does not work out exactly, including slight speedo error, slight rev counter error and the
fact I run my tyres at 20 psi for comfort (JBA Motors and club recommendation).
So for now it looks like it could well be a 3.38. In the next day or two I will get a wheel off the ground and check.
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bumpy
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| posted on 10/9/15 at 07:38 AM |
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quote:
Originally posted by gremlin1234
quote:
Originally posted by bumpyI can do a rough calculation assuming the dizzy has a diameter of 3"
Circumference is 7.8" which is 360 degrees. Therefore 0.5" equates to an advance of 23 degrees, which added to the initial setting of 10
degrees BTDC gives a total advance of 33 degrees. This seems about right but I will of course check it.
2.5" not 3" would give circ of 7.8"
remember also that the distributor runs at half the engine speed
Cant get my head around how this would affect that crude calculation I did. 
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bumpy
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| posted on 10/9/15 at 07:41 AM |
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quote:
Originally posted by Paul Turner
If the 13" wheels you are borrowing have low profile tyres beware that your sump and other parts of the car may ground and get damaged.
That's an excellent point Paul, at the moment my road clearance is minimal, and it would not tolerate going lower still.
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alfas
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| posted on 10/9/15 at 10:04 AM |
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this is all too confusing:
its a kitcar...so who knows which parts from the donor have been used or not...or meanwhile replaced or not.
jack-up the rear of the car.
mark your propshaft with a white or yellow pen or sticker, the same you do with one rear wheel (tyre)
than you need to rotate (by hand) both (important is BOTH) rear wheels simultaniously 1 revolution and in the same time count the revolutions your
propshaft is doing.
e.g. diff has a ratio of 3.5
1 rev of the wheels will result in 3.5 rev´s of the prop.
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bumpy
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| posted on 10/9/15 at 01:44 PM |
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I have taken some measurements.
Existing diff on the car measures a ratio of 3.35 (3.38)
Spare diff in back of garage measures 3.92 (3.92).
So everything is looking good for a swap over which will take my revs at 70mph in 5th gear from 2500 to 2900 - much more realistic.
BUT
The push in drive shafts fitted on the car have a diameter at about half distance of 27.6 mm and the push in drive shafts on the spare diff have a
diameter at similar point of 24 mm. Can anyone explain this?
[Edited on 10/9/15 by bumpy]
[Edited on 10/9/15 by bumpy]
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bumpy
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| posted on 10/9/15 at 02:30 PM |
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I can only take measurement of the drive shafts that are not fitted, but at the 26 splines the diameter is 28.5mm
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rusty nuts
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| posted on 10/9/15 at 05:36 PM |
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I suspect that your spare diff and drive shafts are from a 1600 Sierra
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alfas
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| posted on 10/9/15 at 11:54 PM |
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only the facts are intersting:
as the drive-shaft measurement isnt the same the diffs are NOT interchangeable.
i dont knowi fthe internals could be swapped...but imo it makes no sense as a sierra diff can be bought cheaper than all the hazzle swapping
ratios..
so you definately need a new diff....but good news....diffs with a ratio like yours are very welcome within the cobra and v8 guys. so you might make a
profit.
not sure if those guys like the push in drive shaft thing or if they prefer diffs with flanges.
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bumpy
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| posted on 11/9/15 at 07:44 AM |
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Thanks Alfas
The problem seems to be, if I keep my drive shafts that have been refurbished, I need a way of determining if the new diff I buy from say E Bay will
accommodate them.
I am pretty confused by the bewildering options, particularly what seems to be five or six options on the spline numbers.
All I know for certain at present is I need a 7" unit, push in drive shafts and 3.92 ratio.
It doesn't help that sellers just seem to define the ratio and nothing else.
Surely it cant be that complicated.
Should I start a new thread on this?
[Edited on 11/9/15 by bumpy]
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alfas
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| posted on 11/9/15 at 08:42 AM |
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a genuine buyer will answer those questions and will take measurements for you.
it might be that you have a uncommon combination of parts, only used on some unpopular sierra models
unfortunately i´m not that expert with sierra diffs....
do you have a scrapyard nearby?
why not use this forum "market-place" --> "wanted"
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