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  • Maths question: HT/SA angles and offset
  • mrdestructo
    Full Member

    Hopefully a temporary situation.

    I’m not too good at maths with more variables than I’ve got hands, so hoping someone can provide some rough figures. Or if there is an online calculator that’ll accept all these inputs:

    Existing angles are:
    65deg HA
    75deg SA

    Existing fork:
    2021 RS Revelation 130mm
    522mm A2C
    37mm offset

    Replacement fork:
    2020 SR Suntour Durolux R2C2 EQ 160mm
    558mm A2C
    44mm offset

    How would the angles change? (offset is different, that throws my limited knowledge)

    How closer and higher would the grips be?

    I’m a bit oversize for the frame as it is, so feeling my saddle could go further back where I need it with the SA changed (good point), and I could fit lower rise, quality bars instead of the budget Hail-Mary ones on it that I have nightmares of snapping and sticking in my face.

    Eventually I hope to get a replacement air spring part (150mm) for the Durolux to reduce the A2C. Supplies of exact components are limited where I live, and I simply cannot find the parts to upgrade my Revelation (150mm air spring, and Charger 2.1 or similar).

    NOTE: even when parts for the Revelation were available here, we are talking £390. The Durolux costs £340.

    SirHC
    Full Member

    130mm @ 20% = 496mm a2c

    160mm @ 20% = 526mm a2c

    The offset isn’t going to make a big difference in axle height, I’ll also doubt you could tell the difference in offset if everything else were the same and thats the only thing which was changed.

    As a rough estimate, for every 10mm travel, the static head angle will change by 0.5degrees. So you’ll be 1.5 degrees slacker, ish.

    As for bar height, 10mm higher at an estimate and they will be a fair bit closer 10-15mm from memory.

    https://bikegeo.muha.cc/

    mrdestructo
    Full Member

    Thanks. I can’t swap the bars then. There’s a lot of difference between the bar choices I have, more than 10mm for sure.

    Bringing the grips up and closer won’t be the end of the world. I’m running an 80mm 7deg stem and “could” look at flipping it if necessary, but I think I have some leeway.

    Altogether, the distance changes aren’t too extreme. I just couldn’t get a 150mm fork right now. Stock is not there. I look forward to having a fork that doesn’t give me arm pump (I feel that the Revelation is a bike park fork, not for technical terrain riding)

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    zerocool
    Full Member

    You could fit a -2 headset to slightly minimise the change in SA and bar height.

    honourablegeorge
    Full Member

    SirHC

    As for bar height, 10mm higher at an estimate

    If the fork is 30mm longer, the bars are going to come up a bit more than 10mm, I’d have thought?

    As for offset – I just swapped a 140 Fox 34 51mm for a 150 Lyrik 42mm. Lot of variables there, I know, but man, the bike feels miles better.

    mrdestructo
    Full Member

    Well, first thing is to get it fitted, and see how things work out.

    Unsure if angle changing headsets are available here. A number of bikes come fitted retail with 57E headsets, yet you simply can’t get them aftermarket here…… Plenty of basic stuff, drive train, etc. But specialised stuff you take for granted as available aren’t.

    People fit 150mm forks to my frame no problem. The odd nutter has fitted 160mm. I have read though that the Durolux is a bit longer A2C than competitor’s forks to start with, so I’m hoping, if needed, I can get hold of a shorter after-market air spring given that I’m over in East Asia where Suntour might be easier to get hold of. The LBSs here simply can’t help me with parts for the Revelation, most couldn’t even adjust a hydraulic brake hose, and the usual online places only have pre-Debonair parts now.

    appltn
    Full Member

    I think https://geo.syn.bike/ is the calculator you’re looking for. It lets you enter the bike’s geometry and then make a number of changes (fork length, HA adjust, bar height, stem length etc) and gives you the final geometry.

    mrdestructo
    Full Member

    Unfortuely I couldn’t figure out how to use that calculator. I never could get both overlays to show wheels, and actually had one overlay at 90degs to the other at one point 🤔

    Fork is fitted now. Same steerer length, same stem, used a 57mm riser instead of 90 to feel roughly the same place. Pumped it up for about 30% sag (no measurements printed) And cautiously read the yellow sticker “Not for extreme freeride or DH”. Visually the bike doesn’t look silly.

    Front end wanders a bit, but isn’t swinging wildly now like the 130mm Revelation did. This A2C length actually feels better than the retail fork (San Quentin 3). It’s fairly silent unlike what people reported about the pre-EQ model.

    Only spun it around the block, rode some stairs (up and down) and jumped a few high kerbs, not a real ride. Having so much adjustability, and of course longer travel, is loads better. It does seem to sit lower (midstroke support less so going through a lot of travel even at the right psi)

    intheborders
    Free Member

    If you’re already running a 80mm stem and feel that you need to move the saddle back, the frame is too small. Raising the front end (headtube) will make it feel smaller.

    I wouldn’t be looking at spending money on a new fork, but swapping out the frame for a bigger one.

    mrdestructo
    Full Member

    It’s the largest one they do already, an XL. Reach is 484mm. Polygons are 450mm. Dartmoor 420mm. Given the limited range of retail options in China, I get what I can get off the shelf. Pretty much only Marin, Polygon and Giant. There are loads of Dartmoor frames for sale here, covering all years and models. Bit lacking in XL options. Standing in the attack position is good before and now.

    Actually, I think the recent replacement saddle I got has shorter rails (retail one was too wide). Hence why I wanted it rearwards. It seems to be in the right place now. Bonus!

    zerocool
    Full Member

    Can you get one of the Chinese companies to custom make you something? My thinking is that if some people here in the UK think that some of the Chinese custom companies are a bargain then it might work out even better if you’re already over there.

    appltn
    Full Member

    Unfortuely I couldn’t figure out how to use that calculator. I never could get both overlays to show wheels, and actually had one overlay at 90degs to the other at one point 🤔

    Ahh yes, the classic -90 degree angleset.

    mrdestructo
    Full Member

    @zerocool all those budget carbon full sussers (airwolf, etc?) Still have the problem that Caucasian sizes are batch made, and batch exported. Anything XL size remaining here “could” be a QC fail that the Western buyers passed over on.

    Not only the quality is lower than needed, but here local builders are more designing for local size riders. In context, women here are seen to be dietary problematic if they reach 55kg in body mass

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