SCRUB RADIUS.

[TOAD]

Hi Chaps,
I wonder if any one has any thoughts on a problem I have. Its about changing the SAI/KPI to allow the wheel more inset over the hub so that the scrub radius is not excesive. The car will have a lower wishbone instead of the normal track control arm.

Regards 

        Roger...................

[ticker131]

Roger
please find attached picture, tell me if it doesnt magnify for reading the text, i can email it to you.
there is a piture some where on the forum members were discussing front hubs, you can see in the picture two mounting holes for the steering joint, i think for swapping ackerman between the wheel offsets ie tarmac and gravel.
i can foward some more info as regards the kpi they are both tied in together in reducing scrub,
hope this helps.
sach   

[TOAD]

Hi Ticker,
  only just seen that picture, as far as I can see, the Ackerman will be about the same because the steering arms will still be at the same angle as before just laying down a little. On the other hand, I suppose the angle as both steering arms approach the rear end will be greater because the hubs and therefore the steering arms are further apart at the front.  We have a Kart here, I'll have to have a play.

       Thanks for the thought

                Regards

                       Roger...............................

[TOAD]

Hi Chaps just to finish this post, well I say that Its up to you chaps really, this is what I'm thinking.? ?( THESE ARE ONLY MY THOUGHTS) don't know if it correct so don't kill me.

The conclusion I am coming to concerning SAI and Scrub amount to this, as far as scrub goes a little bit of Negative will add some steering feel, with out getting excessive kick back. The steering kick back was my starting concern because of braking on the limit, if one wheel brakes loose, the sudden loss of grip on that side, would cause the car to snatch over in the opposite direction. This side way?s yaw would destabilise the car when you need stability the most.
By increasing SAI you would load up the suspension more, changing the Suspension rates in the process, but I feel this is a lesser evil compared with running to much Scrub, this being caused be the increase in rim width. The suspension rates can then be re-adjusted with testing to compensate for the revised suspension loadings.
Ackerman will be increased with the re-positioning of the front top mounts forward of there original position. This being done to give more wheel space and some sort of reasonable steering lock. My understanding of Ackerman is that more is better for tight corners, because with increased slip angles on the inner tyre due to weight transfer, the tyre needs as much help as it can get. Oppositely, for long fast corners, that use less steering angle, Ackerman is not needed as much because the wheels are not turned through such an acute angle that induce large slip angles. Further more, with chassis sitting more balanced on both front wheels, more of the steering load is shared with the inside tyre on these fast turns. I think the reason for this is that on a long bend, body roll builds with increased G, but the dampers are still able to keep control of the suspension movement. On the other hand, in a tight hairpin bend, suspension movement is faster and more dynamic, allowing the inside tyres to momentarily unload some of there down force, and therefore grip on the track. To compensate for this the inside tyre is turned in further to get the last available grip. (Toe out on turn)

Static Toe out, may give to much Toe out when turning with the Ackerman addition if set to much in the first place. Caution needed here.

Set the least static Camber for maximum grip on roll in long corners, and keep this to a minimum for best braking grip.

Adjust Caster to increase Camber in the tight corners to compensate for a lower static Camber angle as set above.

Increased Ackerman is unlickly to be an issue until larger angles of steering are applied. This is only likely on hairpins or in the pits, so no worry's there.

Weight jacking across the car from increasing the SAI is only likely again on larger steering angles so this should not be to much of a problem ether.


If any one has a thought or correction for me to think about please let me know (I expect I have it all wrong anyway but there you go got to start some where ).

? ? ? ? ? ? Regards
? ? ? ? ? ? ? ? ?Roger.........?

[theredx19]

What wheels are you using and what offset is on the rims but a man who would have experience of geometry modifications would be Walezy....

[TOAD]

Although I haven't bought them yet, I would expect something like 11x18 front and 12 1/2ish x18 Rear           285 to 305 Front       305 to 325 Rear  Tyres will be Pirelli because I like them, and they worked well in the past.

As for the offset I was thinking, the Rear hubs need to go in a certain position to get correct strut angles, so once all this has been mocked up I can then predict offset front and rear from then.  I have added, hopefully a picture of the Zakspeed Capri to illustrate my intentions.

   Many Thanks..
       Roger..................

[ticker131]

roger.
hows it going, ive been looking over your scrub radius/sai post and been thinking about it for even longer, just a quick question are you using your original hubs? before i shame myself with my thoughts.

Are you trying to retain the original fiat kpi/sai angle?

sach.

[TOAD]

roger.
hows it going, ive been looking over your scrub radius/sai post and been thinking about it for even longer, just a quick question are you using your original hubs? before i shame myself with my thoughts.

Are you trying to retain the original fiat kpi/sai angle?

sach.

Hi Sach,
     Toad has the original hubs but will get the bearings enlarged for the load, maybe I will change the stub axles later after its finished.   SAI,  the whole lot will probably be different.  I need to push the wheels along to the front a little, for some tyre clearance, and  I may need to lay the struts down to get the scub some where sensible, to reduce kickback. 
 I have to finish the middle of the car Video before I start on the front though, still need to do the inner floor and parcel shelf openings for the top rear mounts, and the kids tell me I have to do some more cage stuff, slave drivers.
Looks like the under floor is going to be Lexan type stuff.
   Any How catch you later

             Roger...........     

[ticker131]

you would think with runing such a large front wheel there is issue's with geometry shocker angles and scrub, i have the same capri picture and for the given wheel size there doesnt seem to be excessive shocker angle.

looking at a standard 131 hub/shocker arrangment with a larger wheel i think you have made things easier for yourself with the bigger wheel.

my reasoning being ( with my basic drawing from paint) by going from standard coilspring to the reduced diameter 1.9 coil theres extra clearance, with larger diameter wheel the further up the shocker you go you are gaining clearance, so with kpi/sai there is another nasty scrub offset. which i think we also talking about is not going to be to far from standard.

so im trying to say is from the start i dont think you have dug yourself in to hole to far in one direction, with wheel offsets trying to get eveything to fit. if you do struggle making it fit and there is excessive sai ( this is where i start amusing myself).

replace the shocker with a solid link, design a rocker where the top mount used be. mount your shocker parrellel to the inner wing down to the chassis rail incorparating rising rate. haha.
       

[jasonh131]

Gawd  i did all this at collage 24 years ago ,on my auto engineering course,  can't remmeber a thing now 
jason

[TOAD]

you would think with runing such a large front wheel there is issue's with geometry shocker angles and scrub, i have the same capri picture and for the given wheel size there doesnt seem to be excessive shocker angle.

looking at a standard 131 hub/shocker arrangment with a larger wheel i think you have made things easier for yourself with the bigger wheel.

my reasoning being ( with my basic drawing from paint) by going from standard coilspring to the reduced diameter 1.9 coil theres extra clearance, with larger diameter wheel the further up the shocker you go you are gaining clearance, so with kpi/sai there is another nasty scrub offset. which i think we also talking about is not going to be to far from standard.

so im trying to say is from the start i dont think you have dug yourself in to hole to far in one direction, with wheel offsets trying to get eveything to fit. if you do struggle making it fit and there is excessive sai ( this is where i start amusing myself).

replace the shocker with a solid link, design a rocker where the top mount used be. mount your shocker parrellel to the inner wing down to the chassis rail incorparating rising rate. haha.
       

Hi Sach,
  Good point on the strut angle, I think this is going to get down to look see when I start building that bit.   The inboard dampers idea, I have that designed into my retirement project, but on Toad there isn't any room except under the arches. The tyre will come up to the top spring platform and be about 35mm under the top mount, so bump is tight, but there you go, nothing worth doing is comes without problems. I will most likely lift the top mount up to the level of the top of the wing, about 20mm I think. Ho and the springs are 21/4"

Jasonh,
    I know what you mean about College, I am a Technician,  lecturer when needed,   they only touch what these angles are, not what happens when you change them about much as I may be.  I's one thing talking about what it is, and completly another, on all the resoltant changes that happen after you've mesed with them.  The trip start to finish is fun though.  My entry level class just doing oil, tyre and servicing,   have been discussing Camber and arm alignment, I suppose the interest rubs off.  Sorry being proud of them.
    See Ya later

        Roger...............

P.S  Sach have you seen the under bonnet Pix that was Toad?

[ticker131]

have seen under the bonnet, just an idea, you are still using the standard plenum chamber, the first  16v thema i dropped in mine, i cut the the throttle plate off and welded it back on the side which is now the front of the engine, just to tidy the plumbing up.

just something to aspire to this car appeared in the forum a will ago. 
       

[TOAD]

I did consider this Thema plunham but it put the Intercooler and throttle body pipes to close, I will probably make a new plunham to fit later after the car runs. (second season rebuild) I can only see Toad starting on about 350 fw because of the pistons.  I will buy the parts that make power for later, but turn them down to start with so I doesn't blow up. When we have the other parts Pistons,Rods, Crankshaft, then I can turn the boost up without having to buy a second bigger turbo and the like.  It will be I bit laggy too start with but we are only a family team. (No money) 
  I expect Toad will start off something like this;  Gt35/70,   larger exhaust manifold than those you can buy from ?  Tipo race cams,  Pizo cristal type injectors, Emerald computer system. The head I will play with until I can afford a better one, I have some spare heads I can cut up.    Any thoughts?

   Roger...............   

[theredx19]

That is a very tasty job in that engine bay as have you sorted the plenum chamber out yet as one suggestion is an angle before the throttle body which may work OK being forced induction but keep up the beautiful engineering....

[sid131]

any more pics of that engine bay please?