Will longer forks affect ride?

General cycling advice ( NOT technical ! )
hoppy58
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Will longer forks affect ride?

Postby hoppy58 » 17 Feb 2017, 11:35am

I've recently built up this old mtb as a tourer/off road adventure etc bike. I bought the bike from new and it has had many guises, but latterly the frame has been hanging up in the workshop having been a donor bike to other projects. It's a Sunn Vertik, made when Sunn ruled XC competition in the mid-late 90s..consequently it has classic xc race geometry and is designed to be ridden rigid or with an optional short travel front suspension (I think 40 - 55cm travel). The frame is lovely, - double butted fuji cromoly about 4.2 lbs as I remember.

The bike handles really well in this latest guise, however due to the low xc stance and short steerer tube, the bars are a bit low. I don't really want to put a steerer extension or an upright stem as I think they're a bit ugly!, however I am thinking along the lines of a new fork with a long steerer, allowing me to tune the set up.

The length of the original forks is 395 from crown to dropouts. Surly troll forks are 455mm, whilst Surly Big dummy forks are 425mm...I'm thinking the Troll forks (which have the advantage of loads of braze-ons) could alter the geometry too much, whereas the Big Dummys could just about work. I'm not too genned up about fork trail etc. but am I correct in thinking that the longer fork will simply make the head angle slacker? Has anybody tried a similar set up? Any info gratefully received...

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Last edited by hoppy58 on 17 Feb 2017, 12:00pm, edited 1 time in total.

Threevok
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Re: Will longer forks affect ride?

Postby Threevok » 17 Feb 2017, 11:43am

In the MTB world, you not would normally go past 20mm of the max travel the frame can handle. It may be an idea to check what the max is for the frame.

As you are putting rigids on and not suspension forks, you may be OK. It will make the front a lot slacker, and you may need to compensate with a change in stem/bar sizes

Norman H
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Re: Will longer forks affect ride?

Postby Norman H » 17 Feb 2017, 12:08pm

You also need to measure the offset of your existing fork . Triton Cycles have a good selection of forks, It might be worth talking to them.

The Surly Cross Check looks to be nearer to what you want or there's this from Identiti.

hoppy58
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Re: Will longer forks affect ride?

Postby hoppy58 » 17 Feb 2017, 12:20pm

..I've just been looking at other forks on the Surly site and it looks like a Straggler 700c disc fork, measures 400mm (with a 44mm offset (?))which should work, albeit running a 26" front disc wheel and brake ?

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531colin
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Re: Will longer forks affect ride?

Postby 531colin » 17 Feb 2017, 12:23pm

If that frame is built to take a suspension fork, I would expect the rigid fork to be "suspension-corrected"......ie a gap above the tyre where the tyre would be when the suspension is fully compressed. So if you have 50mm suspension travel, you might expect less than 20mm "sag" and therefore more than 30mm space above the tyre?.....you don't seem to have that.
So, either your frame isn't "suspension ready" or the existing fork is too short?
In round terms, 20mm fork length change means about a degree change in head angle.....a degree is enough to notice, specially if its in the "wrong" direction.
Thought,,,,,I believe "smart phones" whatever they are, can have an "app" (whatever that is) which lets the user measure an angle (to the horizontal).....this needs a younger man than me, but it could give you somewhere to work from.

hoppy58
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Re: Will longer forks affect ride?

Postby hoppy58 » 17 Feb 2017, 12:24pm

thanks Norman...the identiti fork looks spot on. Can anybody explain 'fork offset' to me?

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531colin
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Re: Will longer forks affect ride?

Postby 531colin » 17 Feb 2017, 12:26pm

hoppy58 wrote:thanks Norman...the identiti fork looks spot on. Can anybody explain 'fork offset' to me?


https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry

hoppy58
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Re: Will longer forks affect ride?

Postby hoppy58 » 17 Feb 2017, 12:28pm

..thanks Colin. The existing rigid fork is the original Sunn fork, however I could have bought the same frame with Sunn's own "Obsys" suspension fork, which was either 40 or 55mm..can't remember. (these were the days when downhill bikes had 75mm travel forks!). The bike is running 26 x 2.15 big apples which fairly fill the forks..I think it originally came with 1.95 mtb tyres.

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531colin
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Re: Will longer forks affect ride?

Postby 531colin » 17 Feb 2017, 6:24pm

If you just want the bars higher and its an inch and eighth steel steerer, its easy to fit an extender like this http://www.wiggle.co.uk/bbb-bhp-2021-stem-extender/?lang=en&curr=GBP&dest=1&sku=5360445134&kpid=5360445134&utm_source=google&utm_term&utm_campaign=UK_PLA_Components&utm_medium=base&utm_content=mkwid%7csLTb3G4no_dc%7cpcrid%7c67090789382%7cpkw%7c%7cpmt%7c%7cprd%7c5360445134uk
...and some extra spacers.....all you will see is a spacer stack, same as if you fit different forks with a long steerer.

elPedro666
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Re: Will longer forks affect ride?

Postby elPedro666 » 18 Feb 2017, 4:09pm

As Threevok said, adding an inch to the forks slackens the head angle by around a degree. Probably not enough to ruin it but I'd be inclined to take 20mm of the stem as well, to combat the added 'floppiness' (tech term).

Personal preference, but I thoroughly dislike stacks of spacers - the strangled chicken look - and think a higher rise stem is a far more elegant solution.

More generally though, as soon as you start forcing a beautiful, sharp frameset like that to be something it isn't, then I feel that you lose its character. Take your weight off the racy front end and it'll just be vague and unhappy. Long, low, fast, sharp is its raison d'être, and it does that absolutely beautifully I'm sure!

A really cheap and easy alternative (if you can live without the bar ends) would be a riser bar. Less money, less hassle, easily reverted and hopefully minimal affect on the handling...

I'm a trendy consumer. Just look at my stupid phone.