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Spitfire chassis set-up issues...!


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Hi, I'm a complete newbie to this but I'm hoping some knowledgeable person among you may be willing or brave enough to help me:) I'm using a Spitfire 1500 chassis as the basis for a motorcycle engined road car & have inevitably come up against several issues the latest of which relates to handling problems 😞

I wonder if there is anyone within striking distance of RH16 who would be able to cast an eye over it for me, as I'm running out of ideas?

Front end issues may have been mostly overcome, but the back end certainly isn't behaving as it should, or indeed needs to!

As a follow up I'll see if I can post any photos that may help - that will probably have most of you sending round the men in white coats to sedate me & lead me off to the funny farm! 

I suspect someone with experience of competition build & use may well be able to assist, but I'm open to all offers!

Many thanks, Jerry M.

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  • Ben Hutchings changed the title to Spitfire chassis set-up issues...!

Hey Jerry, I'm sure someone knowledgable about Spit chassis and handling will be along shortly, but it might also be an idea to see if your local group is up and running again. It looks like the Gatwick Group meet close to you in Crawley. I believe the organiser is @Lesley Connaughton so hopefully they'll see this and confirm! 

It sounds like an interesting project, we'd definitely like to see some photos 😁

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Hi, thanks for the response - briefly I have excess negative camber due to lack of weight - (picture stock '78 spit 1500 rolling chassis with Morris Minor Pick-up cab - don't ask! - Triumph motorcycle powerunit - stock diff flipped 180, alloys with 185 x 15's front & 205 x 15 rear - to try & improve contact patch/grip! - exhibiting excess suspension movement during any cornering, certainly not fast!

The weight reduction from stock is extreme, near enough 0 weight on back suspension. Can't add too much weight due to being very wary of overloading the bike clutch.

Currently working on ver. 2 of rear spring mount before further roadtest - all ideas very welcome:)

Could I call you? - I'm very slow at typing:(

regards,

Jerry.

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17 minutes ago, Jerry Mitchell said:

I have excess negative camber

I suspect you mean positive camber (wheels in at bottom, out at top)

If you've rotated the diff, how and where are you mounting the spring? The easiest way to adjust ride height (and hence camber) is to move the spring, which is why some people fit "lowering blocks" between spring and diff on Spitfires. Also, with that little weight on the back you may need to remove a leaf or two to get a more appropriate spring rate.

That said, you're using Carlos Fandango super-wide wheels. Bad idea. The swing-axle setup really doesn't like them and works FAR FAR better on the original 155/80 profile. There will always be significant camber change on a swing axle and that's really bad news on wide tyres. The only practical way to avoid that camber change is to reduce the suspension travel to near zero, by fitting a virtually solid spring, but then you get something that bounces wildly on every pebble or pea.

If you want good handling, you actually need some weight. Formula one cars weigh almost nothing and grip really well on wide tyres, it's true, but that's only because their aerodynamics produce huge down-force (virtual weight) once up to race speed, and their tyres are designed to turn to glue. At mere mortal speeds they have no grip at all.

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No! def neg camber initially tending to 0 with CF wheels now fitted, but was neg before with original wheels -  due to weight loss presumably? - currently worsening on acceleration from standstill - Spring mounting on plate attached by short vertical lugs to 3 diff case bolts at front & 2 at rear to the 2 conveniently located holes in chassis rear x-member - bolted through but spaced to allow some 'give' - ver 2  plate now being made lower to get spring to sit as low to diff case as possible!

Last time out tried with first long leaf spring removed, but not conclusive with other issues to cover first!

Still have new set of stock steel wheels & 155's on hand to refit if necessary..................

Have thought about the hardtail idea – would be willing to give it a try to assess outcome – may even consider living with it if makes a BIG improvement, would just need an inner tube to sit on though

Aerodynamics non-existent! – would go with adding weight if it becomes necessary – did try before & may have been going the right way – just have to consider clutch if needing to add quite a lot more though!

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Surprised to see negative camber with that setup. It’s almost like the spring is too short but I think the early and late springs are the same length, just different leaf counts. I’ll duck out again here as there are many with way more knowledge! 

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Agreed! - that's the ver 1 mount - ver 2 will be a fair bit lower, just clear of the diff casing - it looks in theory as though the rear diff mount could be lowered by about 1" but there may be issues with propshaft  alignment, so that's on hold & yes bike motor although originally shaft drive very annoyingly rotates the wrong way - if only it was a 2-stroke................!

I'll get the ver 2 fitted ASAP & see what that does & then post again.

N.B. in case you hadn't noticed, stock chassis is totally unmutilated & everything I've done around it has been done via the original body & other major component mounting  points - thus it could be restored as a stock 1500 anytime in the future if my proposals don't work out:)

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8 hours ago, Jerry Mitchell said:

it looks in theory as though the rear diff mount could be lowered

Moving the diff down will make negative camber worse, unless the spring also moves down in which case it will make no difference.

Some of those photos appear to show positive camber - it's only the "from behind" view that's very obviously negative - so were the others taken after it had been jacked up or does it lift and squat depending on motion? If the latter, tracking can cause that, especially severe toe-out, which can also really upset handling.

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2 hours ago, RobPearce said:

Moving the diff down will make negative camber worse

I couldn't get my head around that statement after looking at Jerry's pics. I think that's true when the diff is already inline with the wheel centreline, but in this case the Diff is high. I searched around and found this site with these animated gifs that seem to clear things up - for me at least if I'm reading it properly: http://auskellian.com/paul/links_files/performance_enhancements.htm#rear (if that link doesn't take you straight to the 'Optimise the rear' section, click on the 'Rear Suspension' link at the top of the page.

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20 minutes ago, Ben Hutchings said:

I think that's true when the diff is already inline with the wheel centreline, but in this case the Diff is high.

The diff is always in line with the shafts by definition. If you move the diff down relative to the ground (or spring, which has that effect) then the inboard end of the shaft must, by definition, move down, and the wheel camber must, by definition, become more negative. If the diff is "high" (relative to the wheel centres) then you have, by definition, positive camber.

If you look at Jerry's pictures, most appear to show the diff high, as you say, but if you look closely all of those pictures also show positive camber. The one photo with blatant negative camber doesn't really show the diff at all but looking at the ride height of the chassis the diff is really low to the ground in that one.

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Although you say the chassis is stock Spit 1500, the rear spring is an earlier fixed leaf type.  The 1500 should have had a "swing spring" with the floating centre leaf.  This may not ultimately cure your camber conundrum but would be far superior in terms of handling. 

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38 minutes ago, mikeyb said:

the rear spring is an earlier fixed leaf type

No, it's not. It has the four stud fixing and pivot box of a swing spring. See the first and fifth photos.

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I'm assuming you have the standard specs to hand? How do the new measurements for the datum points measure up to the original? 

Photos won't always help as the angle and focal length changing can be misleading. 

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9 hours ago, RobPearce said:

No, it's not. It has the four stud fixing and pivot box of a swing spring. See the first and fifth photos.

Sorry, I didn't look at the earlier pictures carefully enough, you are right!

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Hi all & thanks for the very welcome input so far - there are certainly things to try out moving ahead - firstly I'll address the spring mount as mentioned earlier, then check through tracking & camber again to see where that takes it - front radius arm locations may need looking at again, even hardtailing & adjustable height uprights are worthy of further investigation too!

I'll come back with a progress report asap - btw does anyone happen to live remotely near W. Sx.? Thanks🙂

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  • 2 weeks later...

The new spring mount has now been fitted & I'm thinking about adjustable radius arms as my forward end mounts to the body may be some way off compared to the original, so I wonder if anyone could help me out by giving measurements for reference points to get me as close as possible - namely ground to front end mountings & also mountings to chassis - if I can get it pretty close in these 2 planes (vertical & horizontal) then I might be in with a better chance:) Thanks.

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