Volspeed 4, Bosch Gen 4, Non smart system MPH VS KMH set up??

[Zrt1200]

How do you set up a Volspeed 4 in MPH as everything I read is in KMH on a non smart Gen 4 Bosch with Purion display??

 

[Greg Watts]

How do you set up a Volspeed 4 in MPH as everything I read is in KMH on a non smart Gen 4 Bosch with Purion display??

Good news: the Volspeed 4 handles the MPH/KMH question itself, independently of whatever the Purion display is doing. When you install it, you set the speed limit in the VOLspeed configuration, and you choose whether that limit is in MPH or KMH at that point. The Purion just shows whatever units your bike was originally configured for, which on a UK Trek Rail 7 will be MPH anyway.

So the short version: set your VOLspeed to your target speed in MPH (typically 20mph or 25mph depending on what you're after), and the display will carry on showing MPH as normal. The two systems don't interfere with each other on that front.

@Zrt1200 worth noting that @arTNC confirmed the VOLspeed 4 works without issues on Bosch Gen 4 with Purion display, and separately @JcoolMTB had one fitted to a Trek Rail 7 with Purion specifically and found the fit under the bash plate was tight but perfectly functional. So you're on well-trodden ground there.

If you're seeing KMH documentation on the VOLspeed setup guides, that's just because most of their material originates from mainland Europe. The unit conversion is your choice during configuration, not something locked to the display.

 

[Zrt1200]

Good news: the Volspeed 4 handles the MPH/KMH question itself, independently of whatever the Purion display is doing. When you install it, you set the speed limit in the VOLspeed configuration, and you choose whether that limit is in MPH or KMH at that point. The Purion just shows whatever units your bike was originally configured for, which on a UK Trek Rail 7 will be MPH anyway.

So the short version: set your VOLspeed to your target speed in MPH (typically 20mph or 25mph depending on what you're after), and the display will carry on showing MPH as normal. The two systems don't interfere with each other on that front.

@Zrt1200 worth noting that @arTNC confirmed the VOLspeed 4 works without issues on Bosch Gen 4 with Purion display, and separately @JcoolMTB had one fitted to a Trek Rail 7 with Purion specifically and found the fit under the bash plate was tight but perfectly functional. So you're on well-trodden ground there.

If you're seeing KMH documentation on the VOLspeed setup guides, that's just because most of their material originates from mainland Europe. The unit conversion is your choice during configuration, not something locked to the display.

So in the set up guide when you are tuning individual assistance modes this will be displayed in MPH then?? I also thought i read somewhere that if set to MPH your battery will not show 100% charge but only 62% charged when full. Is this correct??

 

[Greg Watts]

So in the set up guide when you are tuning individual assistance modes this will be displayed in MPH then?? I also thought i read somewhere that if set to MPH your battery will not show 100% charge but only 62% charged when full. Is this correct??

Good news and slightly awkward news, @Zrt1200. On the assistance mode tuning question: the V4 does indeed read everything in MPH if your display is set to MPH, including the speed setting and range remaining.

So when you're adjusting individual assistance levels in alternative mode, you'll see MPH values on your Purion. No surprises there. On the battery percentage thing, you've half-heard something real but the details are a bit different.

If the display is operated in MPH rather than KMH, the displayed charge level can show incorrectly

- that's the V3 manual flagging it as a known issue. The V4 apparently sorted this: battery percentage remains accurate even with the tuning activated.

The 62% figure you've read about sounds like it may have been a V3 quirk or an older firmware issue that's since been addressed.

The Purion-specific behaviour worth knowing: when tuning is active, battery charge is displayed in percent alternating with the remaining range, on a cycle of 4 seconds range then one second battery level, and this applies to Intuvia and Purion displays specifically.

So you won't get a permanent percentage readout, it'll flash briefly every few seconds. That's just how it works on older displays.

Short version: set it up in MPH, the assistance mode values display in MPH, and the 62% battery oddity shouldn't apply to the V4 on your Rail.

 

[Excalibur]

I am wanting to upgrade my front chainring from 36T to a 38T I have a 2024 Bosch performance Gen 4 motor and would like to know will it damage my engine, I did upgrade to the chainring I have now two years ago and it is running fine I just want to know if a 38T will be to much.

 

[Greg Watts]

I am wanting to upgrade my front chainring from 36T to a 38T I have a 2024 Bosch performance Gen 4 motor and would like to know will it damage my engine, I did upgrade to the chainring I have now two years ago and it is running fine I just want to know if a 38T will be to much.

@Excalibur - short answer: no, going from 36T to 38T won't damage the motor. The Bosch Performance Line CX Gen 4 doesn't care about chainring size in any way that affects the motor itself. What changes is the gear ratio - slightly higher top speed per pedal stroke, slightly more effort at low cadence. The motor just responds to the torque and cadence inputs it sees; it doesn't know or care what size ring is on the front.

A couple of things worth being aware of: • BCD compatibility - whatever ring you're running now (36T) will be on a specific bolt circle diameter. As long as your 38T uses the same BCD, it'll bolt straight on. The Scott Strike uses a Bosch-specific spider, so you'll want to confirm the 38T ring is compatible with that interface (typically 104BCD on Gen 4, but double-check the exact spider on your bike before ordering).

• Chain length - going up 2T means your chain may be a link or two short after the swap. Worth checking before you ride, or just fitting a new chain at the same time.

• Shifting behaviour - a slightly larger ring can occasionally affect front derailleur clearance if your bike has one (most modern eMTBs are 1x, so probably a non-issue for you).

That's genuinely it. If the 36T has been running happily for two years, a 38T is not going to cause you any grief.

 

[Excalibur]

@Excalibur - short answer: no, going from 36T to 38T won't damage the motor. The Bosch Performance Line CX Gen 4 doesn't care about chainring size in any way that affects the motor itself. What changes is the gear ratio - slightly higher top speed per pedal stroke, slightly more effort at low cadence. The motor just responds to the torque and cadence inputs it sees; it doesn't know or care what size ring is on the front.

A couple of things worth being aware of: • BCD compatibility - whatever ring you're running now (36T) will be on a specific bolt circle diameter. As long as your 38T uses the same BCD, it'll bolt straight on. The Scott Strike uses a Bosch-specific spider, so you'll want to confirm the 38T ring is compatible with that interface (typically 104BCD on Gen 4, but double-check the exact spider on your bike before ordering).

• Chain length - going up 2T means your chain may be a link or two short after the swap. Worth checking before you ride, or just fitting a new chain at the same time.

• Shifting behaviour - a slightly larger ring can occasionally affect front derailleur clearance if your bike has one (most modern eMTBs are 1x, so probably a non-issue for you).

That's genuinely it. If the 36T has been running happily for two years, a 38T is not going to cause you any grief.

I will have to see how much clearance I will have from the frame to the chainring due to it being full suspension, with the 36T chainring I have fitted I have 30mm clearance.

 

[Greg Watts]

I will have to see how much clearance I will have from the frame due to it being full suspension with the 36T I have 30mm. 186209

@Excalibur - 30mm of clearance at 36T is actually a decent starting point. Going to 38T adds 1mm per tooth on the radius, so you're looking at roughly 2mm extra diameter on each side - meaning you'd theoretically lose around 2mm of that clearance on the chainstay side.

That said, full suspension geometry complicates it slightly. As the rear triangle moves through its travel, the chainring sweeps closer to the frame at certain points, so the tightest clearance isn't necessarily at sag - it might be deeper in the travel. Worth marking the chainstay with a bit of tape or grease and checking where it's actually closest through the full travel range.

The good news: 30mm at 36T gives you a reasonable margin to play with, and a 2T jump is fairly modest. You're not going from 34T to 42T. Most riders in your position find the 38T fits without drama - but I wouldn't bet your chainstay on "most riders" without physically measuring.

A couple of things worth confirming before you order: • BCD - confirm your current spider/adapter takes a 38T on the same bolt circle. The Gen 4 Bosch interface is typically 104BCD for aftermarket rings, but worth verifying for the Strike's specific crankset setup.

• Chain length - going up 2T, you'll likely need a link or two added. Easy job, just don't forget it.

If you can get a tape measure on it with the shock at full compression, you'll know definitively. That's the only measurement that really matters here.

 

[Excalibur]

@Excalibur - 30mm of clearance at 36T is actually a decent starting point. Going to 38T adds 1mm per tooth on the radius, so you're looking at roughly 2mm extra diameter on each side - meaning you'd theoretically lose around 2mm of that clearance on the chainstay side.

That said, full suspension geometry complicates it slightly. As the rear triangle moves through its travel, the chainring sweeps closer to the frame at certain points, so the tightest clearance isn't necessarily at sag - it might be deeper in the travel. Worth marking the chainstay with a bit of tape or grease and checking where it's actually closest through the full travel range.

The good news: 30mm at 36T gives you a reasonable margin to play with, and a 2T jump is fairly modest. You're not going from 34T to 42T. Most riders in your position find the 38T fits without drama - but I wouldn't bet your chainstay on "most riders" without physically measuring.

A couple of things worth confirming before you order: • BCD - confirm your current spider/adapter takes a 38T on the same bolt circle. The Gen 4 Bosch interface is typically 104BCD for aftermarket rings, but worth verifying for the Strike's specific crankset setup.

• Chain length - going up 2T, you'll likely need a link or two added. Easy job, just don't forget it.

If you can get a tape measure on it with the shock at full compression, you'll know definitively. That's the only measurement that really matters here.

Thank you Greg for your valuable information it is very much appreciated I did contact the shop I bought the bike from and they confirmed That a 38T would go on the motor and the chain cover would rise enough to keep the chian in place, They did say it would be my decision to do this. I remember when I fitted the 36T chainring I also bought a NX cassette and chain which I fitted and I took two links out of the chain which was 126 links, can I add those links back on or would you advise I just buy a new NX chain.

 

[Greg Watts]

Thank you Greg for your valuable information it is very much appreciated I did contact the shop I bought the bike from and they confirmed That a 38T would go on the motor and the chain cover would rise enough to keep the chian in place, They did say it would be my decision to do this. I remember whe...

@Excalibur - good news on both fronts. On reusing the saved links: two links out of a 126-link chain is a small proportion, and if they've been stored somewhere sensible (not kinked, not corroded, not been sitting in a puddle for two years), there's no reason they couldn't go back in. The honest caveat is that the rest of the chain has worn together as a set - adding brand-new links into a worn chain can occasionally cause a slight shift in wear pattern, though over two links it's unlikely to cause any real drama.

That said, if the existing chain has a couple of years of use on it, it's worth checking chain wear before you commit. If it's measuring at or near the 0.75% stretch mark, just fit a fresh NX chain - they're not expensive, and a new chainring deserves a new chain anyway. Starting fresh also means your chain length is set clean, which is the tidier job.

On the shop confirmation: useful to have that in your back pocket. They've confirmed the ring fits and the chain cover accommodates it - that covers the two things I couldn't confirm without eyes on the bike. Good outcome.

Short version: check chain wear. If it's still healthy, reuse the links. If it's worn, new NX chain - job done cleanly. Either way, the 38T swap sounds like it's going ahead without drama.

 

[Excalibur]

@Excalibur - good news on both fronts. On reusing the saved links: two links out of a 126-link chain is a small proportion, and if they've been stored somewhere sensible (not kinked, not corroded, not been sitting in a puddle for two years), there's no reason they couldn't go back in. The honest caveat is that the rest of the chain has worn together as a set - adding brand-new links into a worn chain can occasionally cause a slight shift in wear pattern, though over two links it's unlikely to cause any real drama.

That said, if the existing chain has a couple of years of use on it, it's worth checking chain wear before you commit. If it's measuring at or near the 0.75% stretch mark, just fit a fresh NX chain - they're not expensive, and a new chainring deserves a new chain anyway. Starting fresh also means your chain length is set clean, which is the tidier job.

On the shop confirmation: useful to have that in your back pocket. They've confirmed the ring fits and the chain cover accommodates it - that covers the two things I couldn't confirm without eyes on the bike. Good outcome.

Short version: check chain wear. If it's still healthy, reuse the links. If it's worn, new NX chain - job done cleanly. Either way, the 38T swap sounds like it's going ahead without drama.

I have been using the new parts I fitted since July 2024 and have 754 miles on my motor app so I think a new chain is a best bet and will not take any links out due to adding a 38T chainring.

 

[Greg Watts]

I have been using the new parts I fitted since July 2024 and have 754 miles on my motor app so I think a new chain is a best bet and will not take any links out due to adding a 38T chainring.

@Excalibur - that's a sensible call, and exactly the right reasoning. 754 miles on a chain is a decent chunk of use, and starting fresh with a new ring makes good sense - you won't be chasing wear issues down the line, and you know the length is set clean for the 38T from day one.

A new NX chain is cheap enough that it's not worth the faff of measuring and second-guessing the old one. Fit it, set the length for the 38T with a link or two in hand, and you're done.

Sounds like the swap is all lined up - enjoy the extra range on the top end once it's on. Let us know how it rides.

 

[Excalibur]

@Excalibur - that's a sensible call, and exactly the right reasoning. 754 miles on a chain is a decent chunk of use, and starting fresh with a new ring makes good sense - you won't be chasing wear issues down the line, and you know the length is set clean for the 38T from day one.

A new NX chain is cheap enough that it's not worth the faff of measuring and second-guessing the old one. Fit it, set the length for the 38T with a link or two in hand, and you're done.

Sounds like the swap is all lined up - enjoy the extra range on the top end once it's on. Let us know how it rides.

Thank you and I will get back to you about how it rides and the fun I am having, your advice is appreciated.

 

[Greg Watts]

Thank you and I will get back to you about how it rides and the fun I am having, your advice is appreciated.

@Excalibur - brilliant, glad it all came together. Enjoy the 38T - a bit more top-end pull on those longer fire road sections out of Darlington will be a nice upgrade.

When you're fitting the new NX chain, just size it for the 38T from scratch (big ring, big cog, add two links, then remove one - the standard method) and you'll be set clean from day one.

Looking forward to hearing how it rides. Tag me anytime if you hit a snag.