Fitting GRP4 Arches

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Fitting GRP4 Arches

Postby burns » 25 Apr 2006, 19:18

I've got a Manta coming my way next week so I can get started on putting a bigger engine in over the summer, probably just the 1.8 for the mo. However, the Manta B axle will be going on at the same time to save chopping my standard prop up and to handle future power hikes. So, I was going to put GRP4 arches on.

However, I'm not sure how this is done and was wondering if someone could help explain. I assume you chop the standard arches back quite a way, then bond the new ones on with some sort of sticky stuff? Do you paint them before or after fitting? Any other stuff I may not have thought of?
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Postby Galgeth » 25 Apr 2006, 20:45

Hi burns,

Standard procedure would be to trim the original outer arches back to a similar size to the new arches then rivit the new arches to the wings. The amount you will need to trim the arches will depend upon the size wheels you want to run, ride height and the difference in size between the originals and the new arches. Ideally they would be cut so there is no 'lip' inside the inner arch where mud will collect. I have never seen these arches fitted first hand so dont know how they offer up. We have an A-box at home (from my fraudulent past) to which we were going to fit the customary 'bubble arches'. This involved fabricating new inner arches for the car due the extent to which the wings needed trimming! I dont think this is needed for the chevette? My brother is building a castrol grp 4 replica at home so you could probably learn from each other. I will be bumming around Southampton in the summer should you want a hand. Would be interested to see how the car you robbed me of develops! :D
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Postby Galgeth » 25 Apr 2006, 21:07

Oh forgot to explain about painting etc. Obviously any cut metal needs to be protected with the armour of your choice. Red oxide stuff is the family favourite, and keeps out the cornish weather! Obviously you dont really need to paint the inside of the grp4 arch and i would advise only painting the outside once they are fitted. After riviting the arch there are a number of options. For a smooth look, the arches can be blended into the wings to cover the rivets etc and the outer lip of the arch. I am unsure of the best product to use, but if its going offroad then you need something with some give in it. If you are not too fussy about the appearance, then just leave the rivets and lip showing. I think you will also need something to stop water running inside the arches from outside - mastic or something similar - again not exactly sure. If you look in the gallery, gallery 51 I think, you can see the rivited arches on 'hsr2.6' 's white chevette from the C-Islands. I should stress, these recommendations are based mainly on theory and are not from my own personal experiences of fitting these arches.
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Postby burns » 26 Apr 2006, 07:50

Cheers, thats given me a bit to think about. I'm trying to keep it as tidy as possible, so I'll be blending the arches in.

I didn't realise people just used rivets normally, I'll have to look into that. The other thing I'm trying to work out is whether I could just run wheels with more of an offset on the back to tuck the rims back under the standard arches. It would mean running missmatched wheels of course, kind of spopiling the "tidy" plan.
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Postby Herbie_Flowers » 26 Apr 2006, 11:42

burns wrote:The other thing I'm trying to work out is whether I could just run wheels with more of an offset on the back to tuck the rims back under the standard arches.

remember Chevettes are ET35 as standard mate :wink:
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Postby burns » 26 Apr 2006, 13:11

Yup. I need to measure the axle and find out exactley what the widths I'm looking at are. Eg, if it's 2" wider, I could fit wheels 175s with a et50 offset on the standard arches. Whether you can get wheels like that is another matter, but you get the idea.
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Postby Shellysowner » 26 Apr 2006, 14:36

Err, well a fair amount of FWD Vauxhalls have wheels that are ET49 so you might be in luck Burns. In fact, Mk3 Cavalier 6-spokes look mighty gangster and 'rally' with the centre caps removed (saw one the other day with a 'cap missing). Of course they'd need spacing out by about 15-20mm at the front though :? :shock:
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Postby burns » 26 Apr 2006, 14:43

Hmm, I'm not sure I like the idea of 20mm of extra wheel stud, seems a bit dodgy. Well I guess it could be useful as a stop gap solution until I get new arches sorted, and just have odd wheels front to back :? .
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Postby Shellysowner » 26 Apr 2006, 14:48

burns wrote:Hmm, I'm not sure I like the idea of 20mm of extra wheel stud, seems a bit dodgy. Well I guess it could be useful as a stop gap solution until I get new arches sorted, and just have odd wheels front to back :? .


Well quite - I'd tend to agree that 20mm of extra spacing can't be good news - not least for the wheel bearings! I know that with steels it is possible to cut out the centres and then re-weld them to a different offest although I very much doubt that this is quite such a stout plan with alloys... it'd probably cost a fortune anyway - and thats if its even possible! Of course the other option would be to use a type of wheel of modern manufacture that the manufcaturer sells in a variety of offsets and buy and odd set.
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Postby burns » 26 Apr 2006, 15:14

I don't want to spend much which is the problem with getting new wheels. If wheels of a different ofset were spacer'd then the load on the bearings should be the same as normal, the centre of the tread would still be in the same place as it should be, thats the point in the spacers. I still don't like the idea of huge wheel studs though.
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Postby Shellysowner » 26 Apr 2006, 16:19

Thankyou for explaing what spacers do Burns :roll: You're right in principle but it's not quite as simple as all that. I don't fancy getting into a heated debate on the subject - partly because I'm no expert but if you are planning on running any wheels with extreme amounts of spacing for long periods of time then I'd recommend doing some reading around first.
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Postby burns » 26 Apr 2006, 18:06

Maybe I should have put a wink after that sentence, I meant it in jest. I've had a look around the net and it seems there are issues other than geometry RE spacers so I think I'll pass on them, thanks for pointing that out. This whole wondering about the offset thing was just a thought as people might want a stronger axle without needing bigger arches.
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