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RYates
USA
68 Posts |
 Posted - 10/04/2008 : 03:34:18
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Question for the group: I have read John Harris' 1996 Yearbook article and looked through the archives on this topic, but I am not certain about the meaning of the toe-in specs. Factory spec toe-in for N types is 3/16in. If I am reading correctly, the Harris article calls for 3/16 TOTAL - - meaning 3/32 toe-in for each wheel. In another exchange, Gerhard refers to a post-War Service Memo recommending 5/16in. or 5/32 per wheel. I have always been under the impression that toe-in was per wheel. Is it really a total number encompassing both wheels? I am constructing my gauge this weekend to check both of my cars.
Reed Yates |
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John Reid
United Kingdom
704 Posts |
Posted - 10/04/2008 : 09:48:58
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Reed,
With the wheels set straight ahead (i.e. ensure distance from rim to chassis is equal both sides), toe-in is the difference between the distance measured between the wheel rims at the rear and then at the front, both readings taken at wheel centre height.
So it's a difference measurement, encompassing both wheels. I think what Gerhard refers to is for the K3. Also have a look at the topic "P Type toe-in" on 9th March 08.
John R
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RYates
USA
68 Posts |
Posted - 11/04/2008 : 02:49:49
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Thanks, John. I will check both cars this weekend. The steering on the N type 2 seater just seems so much "friendlier" than the N special. Could it be the bulkier Blockleys on the Special?
Reed |
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John Reid
United Kingdom
704 Posts |
Posted - 11/04/2008 : 14:16:17
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Apart from toe-in, there are quite a few things which can affect how the steering feels, for example the castor angle (this is where the wedges come in), the wheel offset, the tyre section, and as you say, the tyre itself. One of the bits of geometry which has a noticeable effect on how the steering feels to the driver is how far away the centre of the tyre contact patch is from the point where the extended central axis of the king pin meets the road. In order to reduce the turning moment this can cause, a designer will usually minimise this distance, and when the points coincide this is "centre-point steering". Changing the tyre section and wheel offset may change this distance and therefore affect how the steering responds.
This is in no way a rigorous reply to your comment, just a few jottings - it would be an interesting exercise to look at the geometry of Triple-M steering as it was designed (anybody already done it?).
John R |
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Peter Scott
United Kingdom
1240 Posts |
Posted - 11/04/2008 : 19:29:33
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quote:
Originally posted by John Reid
............ One of the bits of geometry which has a noticeable effect on how the steering feels to the driver is how far away the centre of the tyre contact patch is from the point where the extended central axis of the king pin meets the road. In order to reduce the turning moment this can cause, a designer will usually minimise this distance, and when the points coincide this is "centre-point steering". Changing the tyre section and wheel offset may change this distance and therefore affect how the steering responds. .........
John R
John - A very interesting post. I have not read about 'centre-point steering' before. However I did find a very intersting explanation of this concept here http://www.eland.org.uk/steer_intro.html
The forum has had some very good topics on it recently after a rather quiet patch a few months back.
Peter |
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John Reid
United Kingdom
704 Posts |
Posted - 12/04/2008 : 08:31:44
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Peter,
Thanks for the helpful link. Understanding steering geometry and its interrelated variables can certainly keep the brain active!
John R |
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Peter Scott
United Kingdom
1240 Posts |
Posted - 12/04/2008 : 20:41:48
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John
Yes steering geometry is a fascinating subject and getting it right really make a difference to how the car drives. I boned up on all the data in Blower when I rebuilt my front suspension a couple of years ago. I also checked up on the logic of camber and caster angles and toe-in but did not appreciate the logic behind king-pin inclination. However Mike Allison did check the alignment of the axle for me whilst a worn king-pin eye was being sorted.
So the forum has added yet more to my understanding.
There is an interesting article in the March Automobile on the subject.
Thanks, Peter |
Edited by - Peter Scott on 12/04/2008 20:43:41 |
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David Allison
United Kingdom
665 Posts |
Posted - 14/04/2008 : 14:58:30
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Reed
From memory your N special has off-set rims and Blockley Racing Tyres and your red N road car I drove had standard rims and road tyres.
Racing tyres do have a lot more drag - but I think it might be that you have over compensated and are trying to run too much toe in.
I always take the car to full ahead position on clean flat concrete.
Adjust the track rods so they are equal and there is no toe-in or toe out.
I always check the tie rod as well to make sure there is no tendency to push one way or the other in the straight ahead.
Then measure the distance wheel rim to wheel rim in the straight ahead, adjusting the toe in to 3/16".
Then roll the car back and forward a couple of times on the flat concrete before checking again - if the measurement is still 3/16" then tighten everything up and try the car up the road.
With the the off-set you have on the racing tyred wheels it might be that slighlty less than 3/16" will give better feel - but I have never run anything else and I think that very few others have done either.
Regards David |
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RYates
USA
68 Posts |
Posted - 15/04/2008 : 01:54:40
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David,
Thanks for the useful thoughts. Your memory regarding the wheels is half correct. Both cars are running on 18in. center-laced wheels. The 2 Str got new shoes after an adventure on the M25 at speed on the way to Silverstone in the early 90s. Following a lorry that lost half a dozen curb stones right onto the motorway in front of us. Managed to dodge all but one!! One of those slow motion things where time slows down and your life flashes before your eyes. Anyway, the 2 str is on those wheels and 4.75/5.00 Dunlops.
The only thing I don't quite follow is your mention of checking the track rod to make sure it doesn't have a bias either right or left. Are you just checking for excessive play at either end? Or do you mean something else?
One other factor: both Bishop Cam steering boxes were thoroughly rebuilt by George Edney. On the 2 Str he fitted one of his top plates incorporating the spring-loaded ball pressing on the sector shaft. On the Special he merely fitted a new standard top plate with the appropriate shims.This top plate showed only minor scuffing,but it would still be producing some drag. I thought perhaps this was the difference, so I got one of Richard Hardy's new tops which are the identical principal. No discernable difference whatsoever. I have been thinking about transferring the Edney top to the Special, but I'm reluctant to mess up a known quantity (and very good thing) with the 2 Str. Any further ideas?
Reed |
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David Allison
United Kingdom
665 Posts |
Posted - 15/04/2008 : 08:50:52
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Reed
I was going to answer direct but then thought that maybe my answer would be of use to others too.
The drag link connecting the two wheels together can alter the straight ahead position in a number of ways - mostly driver perception.
It is very easy to adversley effect the steering of the car even with the tracking apparently correctly set up.
If the drag link is set too long - the passenger side wheel will try to turn right - too short and the wheel will try to turn left.
This is perception of the driver trying to hold the steering wheel straight.
I normally disconnect the drag link (connecting the steering box to the passenger side front wheel) before setting the tracking to prevent any influence.
The dimension in John Harris' article is measured between the wheel rims front and rear of the axle line - do not try to adjust the tracking by measuring the distance between the wheel and chassis frame (the chassis are not that accurately made on the side rails) there is too much margin for error.
I normally use string marked off with magic marker - measure the inside dimension behind the axle line first (it needs two people) between the wheel rims - then transfer the string to the front edges and measure the difference - this is a less than ideal method it has to be said - I prefer to get the tracking checked at the local tyre bay.
The proffesionals use a callibrated system which records from the outer edges of the wheel rims.
This sight gauge measures off in degrees and minutes and they have a conversion chart to take the measurement for older cars.
I dont think that the problem you have between the two cars is to do with the steering box top - The NA has a standard steering box and has always steered better than the NO racer.
That was until last year when Oliver totally rebuilt the steering box on the NO and made better again this winter by fitting 18" wheels instead of the heavily off set 16" wheels we used previously.
It could be that your special has a higher ratio steering worm - this will make the steering feel heavier.
It could also be that like the NO the worm and sector in the box is a BADUN!
I still think the tracking is worth getting checked and that 3/16" is the best start point.
Also check the tyres are wearing correctly - too much toe in and the tyres will scuff the outside edges - too much toe out and they scuff the inner edges.
On my Midget we always used clean white paper on the floor and pushe the car forward across the paper to check the tyre scrub - a clean painted garage floor does just as well on MMM cars (the Midget had 9" wide tyres).
Good luck with your tracking - if you need help give me a bell at the weekend.
Regards David |
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tholden
United Kingdom
1638 Posts |
Posted - 15/04/2008 : 12:42:25
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Hi David
Your length of string marked with a magic marker sounds a bit Heath Robinson and potentially unreliable! I agree that the local garages equipment is the best but you can also get an accurate result using a simple gauge which can be made out of box section steel with two uprights welded each end so that the outside edge of these is just within the wheel rim dimension. These uprights can then be drilled and tapped at wheel centre height and 5/16 inch bolts used to accurately measure to the rim edge.
I set the tracking the same way as you suggest with toe in as per the works information sheet, and usually check the measurement in several different places around the rim to allow for any slight run out.
Works a treat !
Terry Holden |
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David Allison
United Kingdom
665 Posts |
Posted - 15/04/2008 : 13:32:59
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| the secret is vernier string (sic)! |
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