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 Post subject: GSB's Whyte PRST-4 build
PostPosted: Sat Sep 28, 2013 7:13 pm 
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Joined: Sat Jan 22, 2011 11:22 pm
Posts: 358
Location: London
My new (old) toy...

Picked up this afternoon. It has a history that's common to so many bikes; bought, used extensively, broken, left in shed until it can be fixed later. Cue several months/ years of playing host to spider webs and gathering dust, and then a house move, new job, or other life change comes along and suddenly it's no longer a valued possession, but a bloody nuisance that's in the way...

Still, his loss is undoubtedly my gain :lol: , I've had a hankering for one of these ever since I saw one in the window of Action Bikes on London's Victoria Embankment back in 2000 or 2001. Sadly the price tag at the time time was way beyond my reach. I think a fully spec'd one was well over £3000! :shock:

So, here it is. A Whyte PRST-4.
Image
Image

This was the final evolution of Jon Whyte's "Plus Fours" linkage front suspension system, a system he came up with after he stopped messing about designing Championship winning F1 cars for Michael Schumacher, but which was sadly discontinued sometime in the mid 2000's.
As a self confessed Engineering geek extraordinaire, it's always been a very intriguing beast to me. Mechanically pure, and visually more exciting than any set of tele-forks. It always brings to mind John Brittens V1000, a racing motorbike he developed from first principles in his shed, completely rethinking the way a motorcycle should be built. He designed and built the thousands of unique parts in his garage, including the engine :shock: , and the linkage suspension systems that are echoed here in the PRST. He took it to Daytona and giving the works Ducati team a massive fright, and an exceedingly bloody nose... Sadly, he died in 1995 from skin cancer, he was only 45...

Anyway, enough of "GSB's Engineering Heroes", back the bike. Considering the furore I recall around the PRST-1 when it came out, & then around the PRST-4 that succeeded it, there is practically no info at all to be found online about them, and in particular, on how to maintain them, tune them, and perhaps even improve them. I'm proposing to rectify this, and this page will play host to a detailed stripdown and rebuild photographic log of every last component, and will detail the technical specs of all the parts that are unique to the PRST, including part numbers and current suppliers, something that I've struggled with today...

This particular bike has been well used, but its straight, crack free and looks like a liberal dose of T-Cut and new fasteners will have it looking 99% factory fresh in no time. If not, then it's going to the powder coaters to be refinished in White, a colour that I think would suit this particular frame. (Or perhaps the colour of Marmite, on account if its love it-hate it reaction from most people!)

It's currently suffering from the usual PRST gremlin, as the spherical bearing in the front suspension appears to have comprehensively shat itself on its last outing. All the other bearings appear fine, but will be replaced anyway to provide peace of mind, a factory fresh frame, and of course to provide a record of how to do it here...

Should any other readers have experience with these bikes, and have any tips, please feel free to post them here.

Much more to follow, including the sale of this bikes predecessor in my stable, my much loved Schwinn 4 Banger. :(


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PostPosted: Sat Sep 28, 2013 7:29 pm 
MacRetro rider
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Location: Inverness
a full set of bearings inc the spherical is around £150 from ATB Sales through your local Whyte dealer, but fresh bearings make a huge difference to these things! also you'll find a drop or two of a high strength thread-lock is a good thing when rebuilding the bearing linkages! :D


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PostPosted: Sat Sep 28, 2013 8:56 pm 
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Joined: Mon Jun 14, 2010 9:06 pm
Posts: 4431
Location: Herts UK
I had prst1 thst I sold purely becausr the l frame felt cramped it has reltively short tt compared to modetn bikrs. Also the fork stem is quite long meaning thr bsrs sit quite high and it isn't an easy task to lower them... and lower too much the forkd will foul thr bars or so I eas told.


Apart from that it wad great.

Re sphericsl bearingd surely they will be standard ooff the shrlf size so you should be ablr to source a set for a fraction of the price you mention.


Trirled simply bearingd websitr?


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PostPosted: Sat Sep 28, 2013 11:06 pm 
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Location: London
Thanks for that VeloFrog. do you still have your PRST?

Of course ATB sales is the owner of Whyte bikes these days, but as you point out, they don't deal directly with the public! Even though they provide an email address for public enquiries on their webpage, when you use it, you get this;

ATB Sales wrote:
Please be advised that this public enquiry facility is no longer in operation.

We would request that you contact your local authorised dealer with any enquiries and they should be able to assist you with our full support.

To find your nearest dealer you can use the dealer locator on each of our brands websites.

http://www.whytebikes.com
http://www.marin.co.uk
http://www.wilierbikes.co.uk
http://www.mezzobikes.co.uk

Yours faithfully,

ATB Sales Ltd


Fair enough. So try Whyte then. However, in a bewildering display of logic, Whyte tell you to get in touch with either your local dealer, or ATB Sales... :facepalm:

Whyte Bikes wrote:
For additional information about products and services please contact your Local Dealer.
Visit the Dealer Locator to find your dealer's contact information.

If your dealer is unable to assist then ATB Sales Ltd can be contacted by the following methods.

By post:
ATB Sales Ltd
Whitworth Road
St Leonards on Sea
East Sussex
TN37 7PZ
Reg in England No. : 2131034 - VAT No. : 472 6541 37

By telephone: 01424 753566 - By fax: 01424 751415


Of course, I'm all for supporting the dealer network, those are the guys who sell us the bikes in the first place, and have to scrape a living in doing so. But when you just have a simple question you want answered, like, "Do you still have any decal kits for a PRST-4" which merely requires a yes/no answer, followed by a price, I don't want to go bothering the poor sod at my LBS with it. He's busier than an Italian PM at a Bunga Bunga party without me wasting an hour of his time and running up his phone bill for the mouth watering prospect of a 15p profit margin on some stickers....

Anyway, the bearing situation is definitely one I want to look into. £150 for 15 fairly common bearings "seems" a little pricey to me, especially when you consider that the problematic spherical bearing that Whyte supply (PBA10) is the cheapest, nastiest, die cast bearing they can get their hands on. The rest of the bearings are generic aircraft control bearings developed decades ago. I'm almost positive that there are either savings to be had, or improvements in quality and longevity available for that sort of money. The spherical bearing in particular is an area that can definitely be improved upon.

O2gf74

The top tube length on my PRST is identical to that on my 4 Banger, so it seems ok to me. It's got a long steerer which puts the bar quite high (and for some reason the previous owner has augmented this with a set of Easton carbon riser bars - soon to be in for sale thread near you...) and it's got a 100mm 5 deg rise stem on it, but you're quite right, clearance is the issue. Its a 4" travel front end, and the non-telescopic design means it needs 4 and a bit inches clearance between the top of the fork and the stem when extended, since when compressed it does get mighty close;
Image

That's unusual, but ok. What frightens me is the prospect of bearing wear putting the front tyre into the lower wishbone if you bottom it out. Imagine the picture below with a bit of brake induced flex :shock: I think a superman impression would almost certainly be on the cards...
Image

Better stay away from fat tyres on a PRST then.


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PostPosted: Sun Sep 29, 2013 7:51 pm 
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Joined: Sat Jan 22, 2011 11:22 pm
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I decided to look a little closer at the spherical bearing and front suspension today. I had an odd incident with one of the shock absorbers too...

The front suspension system relies on the spherical bearing to provide a transmission path for all the suspension forces into the front shock in its axial plane, and all the braking forces in the bikes frame in its radial plane, all whilst maintaining enough movement to allow steering to take place, and the suspension to move. That's a huge amount to ask from one little bearing...

The bearing lives in a machined recess in the lower wishbone, the recess is shown here with the bike upside down. Note the flange against which the bearing rests, and through which the suspension forces are transmitted;
Image
The bearing that lives in there should be a plain spherical bearing type PBA10, that looks like this;
ImageImage

What I found in my bike though, was a tiny bit different...
Image

This is an odd bearing indeed. Its a compound bearing, one inside the other. The two parts on the extreme left and right are the parts that Whyte provide to connect the fork to the inner element of the spherical bearing. The bolt goes through the middle and clamps it all together. In the middle is the spherical plain bearing, as expected, but with a far larger centre bore. The pieces either side of that are inner race halves of a normal angular contact single row ball bearing. The balls should live between the groove that these parts make when joined together, and a matching groove that lives inside the spherical part of the main bearing... So what we have is a high speed - low load ball bearing sat in the middle of a low speed - high load bearing.

Yep, I'm scratching my head too... The high speed capability isn't required. Possible reasons for fitting it would be to reduce friction when steering, but it comes at the expense of an even more fragile bearing that fails in an even more spectacular manner than the type I was expecting to find. There were a number of balls I didn't find in there, so at least a few have escaped. The amount of clearance the failure of this as yet unknown bearing type generates when it fails is spectacular, the bike would be utterly unrideable.

So what sort of bearing is it? According the the dust seals I salvaged from the greasy mess inside the bearing cover boot, its a;
Image

6800RSA From PEEL in China. Except it isn't. The 6800series bearing in peels catalog looks rather different to this one.

Un petit mystery, n'est-ce pas?

Whilst I had the suspension in bits I took a look at the other bearings as well. The upper wishbone, which is pretty much kept out if the filth, spray, mud and muck, was buttery smooth in operation. No issues there at all. The 2 Bearings in the lower wishbone have had a harder life though, as they are very notchy, feeling like they are full of sand...

The last thing I did, since everything was in bits anyway, was pull the shocks so I could give the air cans a clean. Its a simple task, that I've done dozens of times before without issue. The PRST-4 is fitted with two identical Fox Float R shocks. The rear one was fine. Vent the air, cycle the shock, vent the air again, unscrew air can, clean out filth. Easy...

The front shock was much the same. Vent, cycle, vent, unscrew. The can came away easy enough, and then the bastard exploded... :shock:

After the ringing in my ears had subsided, I had ended up with a perplexed expression, and my face, shirt, and my patio doors covered, and I do mean covered in shock oil... I never realised that there was so much oil in that tiny little damper. It would appear that the damper rod has sheared away from the piston and my maintenance was just the excuse it was looking for to make a break for freedom.

Take from that whatever safety tips you wish...


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PostPosted: Thu Oct 03, 2013 11:18 am 
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Joined: Tue Sep 21, 2010 8:08 pm
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Location: East Lothian
Love the look of this and suscribing to your build.

Do you know what the leverage ration for the front and rear is please - trying really to work out if they are suitable for the larger sized?


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PostPosted: Thu Oct 03, 2013 12:40 pm 
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Joined: Sat Jan 22, 2011 11:22 pm
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Location: London
How large is large? I'm a bit of a lump (6'2" - 6'3" and about 16 stone) and it's fine for me.
Both front and rear are a 4" travel using a 1.5" stroke shock. The base leverage ratio then is 2.66 : 1.
Obviously that's a vastly over simplified view considering the complexities of the suspension systems, but it should answer your question.
The rear end is identical to the Marin quad frames of the same period, so that might be a useful guide?


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PostPosted: Thu Oct 03, 2013 1:33 pm 
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Thanks for that I will look into it further. Are they 165mm shocks?

Turning to your potential clearance issue, how about running a 24" wheel with oversized tyre?


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PostPosted: Thu Oct 03, 2013 2:06 pm 
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They are 165mm shocks with a 38mm stroke, or 6.5" x 1.5" in old English and 'Merkin.

The use of 24" wheels on PRST's isn't that uncommon. There are several photos of PRST's so equipped on the web. I think the most common reason for fitting was to lower the handlebars to a more "racey" position and quicken up the steering a bit.

I've just installed a new spherical bearing on my bike, so I'll have a look to see what that's done for clearance.


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PostPosted: Sun Oct 06, 2013 10:18 am 
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Joined: Sun Aug 11, 2013 11:35 pm
Posts: 8
Hi, I just re-built 2 PRST-1s for my son and I. Got them from a guy off the Isle of Wight. There is a technical manual online for the prst4 and prst1 at whyte bikes. It helped me a little. I used my lbs to supply the small kp6a bearings for the frame 5 quid each, apparently they are the same as the marin quad link. My local bearing shop does the spherical for 13 quid so that helped keep the cost down. I find mine to be a comfortable and willing xc / trail bike although in Peterborough there ain't too many hills to use.
Image
I was a bit nervous with my shocks as I'd never taken one apart before glad mine didn't go bang :shock:


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